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1. gt true The call executePlan HW Plan is delayed when the status is wait As we saw before when the status is updated an event event EventVar is posted which will activate the plan executor If the status is run the next rule will be tried The call executeStep HW Step moves the disk as required in Step the call get SleepTime CP Time gets the sleeping time from the control panel and cgSleep Time lets the animator sleep for Time milliseconds before it executes the rest of the steps in the plan Notice getEventVar CP EventVar cannot be used in the guard of the first action rule to get the event variable from the control panel since getEventVar is a user defined predicate and no non inline calls are allowed in guards 3 5 Demonstrating a Geometry Theorem on Quadrilaterals This program demonstrates the following geometry theorem on quadrilaterals The quadrilateral formed by the four edges that connect the middle points of the edges of another quadrilateral is always a parallelogram Figure 8 shows two snapshots The user can move any point of the outer quadrilateral by dragging it Wherever the outer point moves the four points of the inner quadrilateral always stay at the middle points of the four edges of the outer quadrilateral This example is taken from 1 Figure 8 Demonstrating a geometry theorem The program generates and shows two quadrilaterals It also generates three agents that handle mouse even2. an event event EventVar which in turn will activate the plan executor see below At a certain time only a part of the components in the control panel are enabled In the beginning before the animator is started the Pause and Resume are disabled After the animator is started the Pause button becomes enabled but the Start button and the two text fields become disabled The Resume button is enabled only after Pause is pressed and the animator is in waiting state There is an event handler for each control button All the event handlers share the object hw ControlPanel Disks Poles Table where Disks is a list of disks Poles is a list of the three poles and Table is a rectangle Disks and Poles are lists of rectangles When a disk is moved from one pole to another the position of the disk is updated From beginning to end the positions of the poles and the table remain unchanged The animator is driven by events When the start button is clicked the animator first finds a plan and then turns to execute it The following is the code for the executor executePlan HW Plan HW hw CP Disks Poles Table CP cp IS IR Reset Start Pause Resume wait EventVar event EventVar gt true executePlan CP HW Step Plan CP cp IS IR Reset Start Pause Resume run EventVar gt executeStep HW Step getSleepTime CP Time cgSleep Time executePlan CP HW Plan executePlan CP HW _
3. are all of the same size The not overlap constraint cgNotOverlap L ensures that no two objects in the list L overlap each other The grid and table constraints are very powerful and flexible for laying out objects in tabular forms The grid constraint cgGrid L PadX PadyY enforces objects in L to be placed on a grid board where L is a list of lists of objects or a two dimensional array of objects and PadX and Pady specify the gap between cells Cells on a grid board are of the same size Each object in L occupies the corresponding cell on the board Objects in L can occur multiple times In that case the object will cover all the corresponding cells For instance let S1 S2 S3 S4 and S5 be the dominos that contain respectively 1 2 3 4 and 5 circles The following is a grid constraint that determines the layout of the dominos shown in Figure 2 cgGrid _ S1 _ S2 S3 S4 _ S5 _ There is no gap between cells The table constraint cgTable L is not as restrictive as the grid constraint It forces objects to be placed in a tabular form where each row or each column may have a different size The tree constraint is useful for arranging components into tree structures A tree constraint takes the following form cgTree Tree Type DisX DisY Centered where Tree is a term in the form node Node Children where Node is a graphical component and Children is a list of sub trees in the same form as Tree
4. other executing q B when the button is clicked Each rule can handle one event To handle multiple events on a component one has to create one agent for each of the events Each time when an event is posted all the agents that are waiting for the event will be activated If there are multiple agents waiting for an event then the agent generated first will be activated first 2 5 Synchronization At the entrance of each predicate the event queue is checked If it is not empty the current predicate will be interrupted and the agents waiting for the events will be activated The interrupted computation is resumed after the events are handled Because agents can be interrupted further by other events the order in which agents are executed is usually unpredictable Agents can communicate with each other through logic or global variables An agent can be delayed until a variable is instantiated or an argument of a structure is set to certain value or a global variable is set to certain value Certain computations are uninterruptible For example it may cause inconsistency in the system when a predicate that updates shared data is interrupted or when unexpected inputs are entered For this purpose CGLIB provides a primitive called critical_region Call which is equivalent to Call but the execution is never interrupted by user s intervention 2 6 Packing and Showing Objects The primitive cgShow O shows O and the primitive cgPack
5. Authoring Graphics Rich and Interactive Documents in CGLIB A Constraint based Graphics Library Neng Fa Zhou CUNY Brooklyn College amp Graduate Center 2900 Bedford Ave New York NY 11210 zhou sci brooklyn cuny edu http Awww sci brooklyn cuny edu zhou ABSTRACT CGLIB is a high level graphics library for B Prolog a constraint logic programming system The library provides primitives for creating and manipulating graphical objects and a set of constraints including non overlap grid table and tree constraints that facilitates the specification of the layouts of objects The library adopts a construct called action rules available in B Prolog for creating agents and programming interactions among agents or between agents and the user The library is a fully working system implemented in B Prolog Java and C It can be used in many areas such as drawing editors interactive user interfaces document authoring animation information visualization intelligent agents and games The high level abstraction of the library and the use of constraints and action rules in the specification of layouts and behaviors can significantly enhance the productivity of the development of graphics We demonstrate through several examples the effectiveness of the library as a tool for developing graphics rich and interactive user interfaces General Terms Documentation Design Languages Keywords Programming languages Constraints Graphics programm
6. O packs O and determines the attribute values of 0 where O can be one object or a list of objects When showing an object O the primitive cgShow 0 calls cgPack O automatically to determine the attribute values of O If O has already been shown before the primitive will clean up the previous image of O automatically The bindings of the attributes of O made by cgPack 0 will be lost upon backtracking Just like the read and write predicates in Prolog however cgShow 0O cannot be undone upon backtracking and the graphical objects displayed by the call will not disappear after the execution backtracks over the call This may result in inconsistencies between graphical objects and their visual appearances To keep the visual appearances up to date the user has to call cgShow O after the state of O changes The primitive cgShow O calls cgPack 0 to pack O before showing it One may wonder if cgShow O packs O automatically why do we need cgPack O cgPack is provided to let the programmers have more control over the order in which objects are packed Packing is an NP hard problem in general and may take a long time if objects are not packed in an appropriate order It is always a good practice to determine the layout areas for objects before fixing the objects For instance recall the dominos shown in Figure 2 It is fast to first pack the squares and then pack the circles inside the squares 2 7 Altering and Moving Objects A s
7. Type indicates how the tree grows It can be one of the following top_down left_right bottom_up and right_left e Centered The value can be either centered or itemized The value centered means that the root of each sub tree is placed in the middle of its children and itemized means that the children are itemized e DisxX is the x distance between two leaves or the x distance between a root and its children depending on the type of the tree e DisyY is the y distance between two leaves or the y distance between a root and its children depending on the type of the tree In addition to mandatory constraints CGLIB also supports limited preferential constraints For example the constraint O preferredWidth 100 tells the constraint solver that the preferable width of O is 100 pixels Figure 2 A layout created by using grid constraints 2 4 Event Handling CGLIB adopts a construct called action rules from B Prolog for programming interactions An action rule takes the following form Agent ConditionSeq Event gt ActionSeq where Agent is an atomic formula that represents a pattern for agents ConditionSeq is a sequence of conditions on the agents Event is a pattern for events that can activate the agents and ActionSeq is a sequence of actions the agents perform when activated In contrast to Prolog where unification is used to select applicable clauses matching is used to select applicable action ru
8. asses 3 3 Building a Calculator In this example we consider how to build a working calculator with the interface as depicted in Figure 6 The calculator consists of a display and a keyboard The display is a text field which is a base component The keyboard consists of several keys The layout of the keys are specified by the grid constraint as given in Figure 6 This example illustrates a nice feature of the grid constraint a component can occupy multiple grid squares In this example the keys labeled Beq Badd and 0 BO are twice as big as the other keys To carry out calculation the calculator has to memorize the accumulated result and the operator to be applied After each operation is applied the display should show the current result When the user continues to type the next operand the display should turn to show the operand For this purpose the calculator should also memorize whether the number on display is a result or an operand We use a structure to represent all the information that has to be memorized cal Display Acc Op OnDisplay where Display is the text field Acc is the accumulated result Op is the operator to be applied next and OnDisplay tells whether the number on display is a result or an operand 2 E cgGrid Bc Bdiv Bmul Bsub B7 B8 B9 B4 B5 B6 Badd HAH 9 or o ae B1 B2 B3 BO BO Bdot Beq Figure 6 A calcual
9. cify what attributes are anchors that do not change There are constraint libraries developed in Java for developing graphical user interfaces or web documents 2 7 There are also constraint logic programming systems such as Minerva from IF Computer developed in Java These libraries and languages developed in Java do not offer as high a level of abstraction as CGLIB and are neither as efficient as CGLIB in terms of the constraint solving power but have the advantage that they are Java applets and thus can run on not only servers but also clients 6 CONCLUDING REMARKS CGLIB is a graphics library based on constraint logic programming that provides both constraints and rules for users to specify sophisticated dynamic and interactive graphics Constraints specify how objects should appear and action rules are used to program event handlers and agents that maintain the consistency of objects The library has been employed in a graduate level course on Declarative Programming and several research projects at CUNY Brooklyn Our early experience tells that the library is much easier to learn and use than the Java s graphics package CGLIB should be improved in the following aspects First the class of built in objects needs to be expanded to include more shapes including three dimensional ones Second a mechanism should be provided to allow the users to create their own object types for which the notation O attr can be used Third the perfo
10. e determines the size and position of the button and shows the button in the default window In our example nothing is mentioned about the color and font of the text the size of the button and the position of the button in the window The system determines the attribute values for us either by using the constraints we provided or by using the default values The action rule in lines 5 8 defines the event handler handleButtonClick B The term enclosed in the pair of braces actionPerformed B is called an event and the right hand side of the rule halt is called an action When an event actionPerformed B is posted i e when the button B is clicked the program terminates In general an action can be any sequence of sub goals 2 2 Objects The library supports the creation and manipulation of a rich class of objects including arcs buttons check boxes circles choices images labels lines lists ovals polygons rectangles round rectangles squares stars text areas text fields text boxes and triangles See Figure 1 For each type of objects there is a primitive for creating objects of the type For instance the call cgButton B in our illustrative example creates a button The primitive can also be used to create a list of objects For example the call cgButton B1 B2 B3 creates three buttons Arc ii text Button Checkbox V iche yi v Choice Circle amp f Image Label Label Line List af z Ea Oval RoundRectang
11. gramming event handlers This is not a surprise if the similarity between constraint propagators and event handlers is considered In B Prolog constraints are compiled into specialized propagators that maintain the consistency of the constraints and therefore constraint propagation can be done very fast Just as for solving constraint satisfaction problems in general the heuristics for ordering variables is of prominent importance in packing objects Our system determines the sizes of objects before determining their positions When determining the sizes of objects it prefers possibly small objects to large ones and when determining the positions of objects it selects largest objects first This heuristics demonstrates very good performance We have developed around 50 application programs including medium sized ones For none of the programs does constraint solving take longer than 100 milliseconds on an 850kHz PC For most programs processing drawing requests takes more time than packing objects 5 RELATED WORK Graphics programming has received a tremendous attention as witnessed by the coverage of the Java s graphics package in textbooks Various languages and tools have been developed for creating graphical user interfaces 9 Object Oriented Programming OOP offers a structured representation of graphical objects Even before OOP was envisioned OOP concepts such as class and instance had been used in Sketchpad the drawing sys
12. he predicate updatePoint O I X Y in line 13 updates the Ith point of Q to lt X Y gt which is defined as follows updatePoint O I X Y O quadrilaterals Q P Selected Q xs I X Q ys I Y The notation Q xs I refers to the Ith element of the attribute xs of O which is an array The predicate recomputeP O in line 14 re computes the points of the quadrilateral P such that the points remain at the middles of the edges of Q After the attributes of an object are updated it is drawn again When drawing an object that is already on a window the system automatically cleans up the object before re drawing it 3 6 A Box and Arrow Editor As the last example we consider how to develop a box and arrow editor as shown in Figure 9 The interface consists of a tool panel and a drawing area The user can add text boxes to the drawing area and add arrows to connect boxes For each arrow that connects two boxes the two ends always reside in the centers of the two boxes Wherever the user moves a box the ends of all the outgoing and incoming arrows of the box will move automatically Whenever the user deletes a box all the arrows that have ends on it will be deleted automatically Figure 9 A box and arrow editor The tool panel contains four user built buttons laid out in a column A table constraint is used to specify the layout To ensure that the arrow and the delete buttons are sufficiently apart from each o
13. id constraints are sufficient for restricting the positions of the stars The order in which components are shown is very important When two components overlap the component shown later will overwrite the one shown earlier So for the Antigua flag the red rectangle must be shown first and the white triangle must be shown last Aruba Antigua China Japan USA Singapore Figure 3 Drawing flags 3 2 Drawing Trees The constraint cgTree can be used to draw many different kinds of trees Figure 4 shows a complete binary tree The constraint cgTree Tree top_down 0 10 centered is used to determine the layout where Tree is a term that represents the nodes and the parent children relation top_down means that each root is located above its children 0 means that the x distance between any two leaves is at least 0 pixel 10 means that the y distance between a node and its children is 10 pixels and the last argument centered means that the root of any sub tree is centered with respect to its children Figure is a presentation of the Java AWT s class hierarchy Each node is a text box The nodes run from left to right and are connected by horizontal and vertical lines The positions for the lines depend on those of the nodes So it is important to pack the nodes before the lines CREAREA ae tase Figure 4 A complete binary tree FileDialog CheckboxMenuttem Figure 5 The hierarchy of Java AWT s cl
14. ing Graphical user interface design Agents Action rules Prolog 1 INTRODUCTION The widespread use of window systems has made a graphics package indispensable for any programming languages 9 Prolog and CLP Constraint Logic Programming languages in general are not an exception Some efforts have been made to introduce graphics programming into CLP One widely used approach is to This research was supported by grants from PSC CUNY CUNY Software Institute and ONR N00014 96 1 1057 adopt an external interface with a language that supports graphics programming For example JIPL is an interface developed by Kino 8 that bridges Prolog and Java bi directionally Any Prolog system that adopts the interface will be able to access the graphics library of Java This approach is not satisfactory for the following two reasons First the programmer has to write code in two languages This is especially daunting when there are interactions involved Second the graphics library in the adopted language whether it is C C Java VB or Tcl Tk is at such a low level that it does not match well with a high level language like Prolog CLP languages deserve a high level graphics library that can be used to specify easily the states layouts and behaviors of graphical objects Motivated by the observations we designed and implemented a high level and constraint based graphics library called CGLIB for B Prolog The library provides primi
15. ing components is very simple If Selected is a component a box or an arrow when the delete button or the DEL key is pressed then the component will be deleted If the component is a box then all the arrows attached to it will be deleted as well The rule for moving boxes works as follows When a mouse drag event occurs while Selected is a box the editor updates the position of the box and also updates the arrows attached to the box 4 IMPLEMENTATION AND EXPERIENCE CGLIB works with B Prolog Version 6 0 13 14 15 CGLIB was implemented in three languages 4500 lines of code in B Prolog 3000 lines of code in Java and several hundreds of lines of code in C The B Prolog part is responsible for creating objects and solving constraints The Java part handles drawing requests from the B Prolog part and manages graphical objects The C part is responsible for delegating Java events to B Prolog Efforts were made to keep the modifications on the B Prolog system as minor as possible For example existing data structures of B Prolog are used to represent graphical objects Because of this the garbage collector is kept untouched Action rules were designed originally for programming constraint propagators 14 Because of the lightness of constraint propagators B Prolog has successfully sustained applications that have millions of propagators running co routinely Our experience tells that action rules serve very well as a language for pro
16. le T i Rectangle Polygon ie Square Star i Textarea TextField Text F TextBox Triangle Da Figure 1 Base objects Each object has a certain number of attributes and each attribute has a name and a domain The attribute attr of an object O is referred to by the notation O attr The more commonly used notation O att r cannot be used here since the has been used as the constructor for lists in Prolog Each object has the following attributes e window The containing window of the object e x The x coordinate of the top left corner of the object e y The y coordinate of top left corner of the object e width The width of the object e height The height of the object e color The color of the object These attributes are stored explicitly and are called base attributes In addition to the base attributes each object also has some derivative attributes that are not stored explicitly but are derived from the base attributes For example there is a derivative attribute called centerX in every object When centerX of an object O is accessed the system replaces O centerX with a variable say V and generates a constraint vV O x O0 width 2 which forces V to be the central x coordinate of O Each object type may also have some specific attributes in addition to the base and their derivative attributes For example a line has the following attributes e xl yl1 x2 y2 coordinates of the two ends e thickness
17. les for an agent All conditions on the left hand side must be in line and cannot bind variables in Agent Example conditions include X Y X gt Y arg N T A and functor T F N There are a set of built in events for programming constraint propagators ZHOU98 and interactive user interfaces For example ins X is an event that is posted when the variable X is instantiated actionPerformed O is posted when O is clicked if O is a button or when O is selected if O is a menu item Events are posted in most cases by built ins but a user s program can also post events by using the primitive post Event Recall the action rule in our illustrative example handleButtonClick B actionPerformed B gt halt There is no condition specified on the agent handleButtonClick B When an event actionPerformed B is posted the action halt will be executed which terminates the execution For some events the event source does not tell all the information about the event For example when a mouse is clicked on a window or a key is typed while a window has the focus the window cannot know where the mouse click occurred or which key was typed An event can have a second argument that carries the information For example handleMouseEvent Win mousePressed Win E gt E x X y Y write mouse pressed at write X Y AE The term E is called an event object Each type of event objects has certain
18. nology 119 128 1996 11 Ivan Sutherland Sketchpad A Man Machine Graphical Communication System JFIPS Proc of the Spring Joint Computer Conference 1963 12 B V Zanden and B A Myers Demonstrational and Constraint Based Techniques for Pictorially Specifying Application Objects and Behaviors ACM Transactions on Computer Human Interaction 2 4 308 356 1995 13 Neng Fa Zhou Parameter Passing and Control Stack Management in Prolog Implementation Revisited ACM Transactions on Programming Languages and Systems 18 6 752 779 1996 14 Neng Fa Zhou Programming Constraint Propagators in Action Rules submitted preliminary version appears in Proc Joint International Conference and Symposium on Logic Programming JICSLP 70 84 1998 15 Neng Fa Zhou B Prolog User s Manual CUNY Brooklyn amp Kyushu Institute of Technology 1994 2001 http www probp com
19. number of attributes A mouse event for example has two attributes named x and y that tell where the mouse event occured There is no primitive for killing agents For an agent if the condition of an action rule in its definition is not satisfied the next alternative rule will be tried If that rule does not contain an event then the agent will be killed automatically after the action is executed For example handleButtonClick B Flag var Flag actionPerformed B gt Flag 1 handleButtonClick B Flag gt true When the button B is clicked the first time Flag is bound to 1 On the second time the first rule becomes inapplicable because the condition var Flag fails The agent haddleButtonClick B Flag disappears after the second tule is applied to it Notice that the second rule is necessary here Without it the agent would fail when the button B is clicked the second time An agent can be defined by multiple action rules but the second tule will be tried only after the condition of the first one fails Therefore in the following definition handleButtonClick B actionPerformed B gt p B handleButtonClick B actionPerformed B gt q B the second rule will never be tried since the condition for the first tule always succeeds To let both of the actions p B and q B be executed when B is clicked one has to either put them into one rule or create two agents one executing p B and the
20. ousePress BA X Y The action of the handler depends on the current status of the editor For example if a mouse press occurs in the drawing area while Selected is the label button then the editor is ready to receive a new text box This action is described as follows handleMousePress BA X Y BA ba Win Area L A D Q _X _Y L Bs As L selected inside X Y Area setPosistion BA X Y setSelected BA newLabel _ Where setPosition updates the X and Y arguments and setSelected updates the Selected argument of BA The handler for keystrokes which is defined below is ready to receive the input characters after this action i e after newLabel Lab becomes selected handleKeyStroke BA BA ba Win Area L A D Q _X _Y Selected Bs As keyTyped Win E gt E char Char char_code Char Code Selected newLabel Lab gt attach Code Lab true The handler adds the code of the typed character to the end of the list Lab If the mouse is pressed while Selected is newLabel Lab then the editor creates a box with Lab as its text and shows it Rules for adding arrows work as follows When a box Box1 is clicked while Selected is the arrow button then arrowStart Box1 becomes the newly selected component If another box Box2 is clicked after this the editor will create an arrow that goes from Box1 to Box2 It also records the arrow and the two connected boxes The rule for delet
21. pecial operator is provided to update the attributes of objects Let O be an object Att r be an attribute the following call resets the attribute value to NewValue O Attr NewValue The update is destructive and is not undone upon backtracking The library also provides primitives for scaling resizing and moving objects The primitive cgScale O F scales the object or list of objects O by the factor F The primitive cgResize O Width Height resizes the object or list of objects O such that the object or list of objects can be covered by a rectangle of the size Width by Height The primitive cgMove O X Y moves the object or list of objects O to the position lt X Y gt which will become the new top left corner of O These primitives are implemented by using the update operator and the updates are destructive 2 8 Animation To construct animations we not only need to construct the static images but also need to decide the order in which the images are rendered and the rate at which changes of images take place It is possible to suspend the system for a certain period of time The cgSleep Time is introduced for this purpose While the system is be suspended no events will be handled The following example displays a circle in an animated fashion with the color changing from red to black animateCircle Colors red pink yellow blue black cgCircle C C width 100 member Color Colors C color Color cgSho
22. rmance should be improved further for not only constraint solving but also graphics rendering 7 REFERENCES 1 Alan Borning The Programming Language Aspects of ThingLab a Constraint Oriented Simulation Laboratory ACM Transactions on Programming Languages and Systems Vol 3 353 387 1981 2 A Borning R Lin and K Marriott Constraint Based Document Layout for the Web ACM Multimedia Systems Journal 2000 3 J Cohen Constraint logic programming languages CACM No 7 pp 52 68 1990 4 I Cruz K Marroitt and P V Hentenryck Special Issue on Constraints Graphics and Visualization Eds Constraints An International Journal Vol 3 No 1 1998 5 C Elliott and P Hudak Functional Reactive Animation Proc of ACM SIGPLAN International Conference on Functional Programming 1997 6 Paul Hudak The Haskell School of Expression Learning Functional Programming Through Multimedia Cambridge University Press 2000 7 S Hudson and I Smith SubArctic UI Toolkit User s Manual Georgia Institute of Technology 1996 http www cc gatech edu gvu ui sub_arctic 8 JIPL A Java http www kprolog com jipl html Interface for Prolog 9 Brad A Myers ed Languages for Developing User Interfaces Janes and Bartlett Publishers 1992 10 B A Myers R C Miller R McDaniel and A Ferrency Easily Adding Animations to Interfaces Using Constraints Proc ACM User Interface and Software Tech
23. tem developed by Sutherland 11 Currently all popular OOP languages such as C Java Smalltalk and CLOS come with some graphics packages Although OOP provides a module scheme for graphical objects the control part of objects has to be described procedurally The programmers have to maintain the relationship between the internal data structures of objects and their visual representation Elliott and Hudak 5 6 have developed a graphics library for the functional language Haskell The library consists of new data types and functions for representing time objects events and behaviors The library facilitates the composition of new objects from existing ones In terms of the specification of layouts however the functional programming approach is not as powerful as the constraint approach since computation is deterministic and information flow is one directional Constraints have been used in building graphics for many years In the Sketchpad system 11 constraints were used to represent the relationships of objects and a relaxation algorithm was used to maintain constraints dynamically Many researchers have explored this declarative approach to graphics development 1 4 10 Systems such as ThingLab 1 and Amulet 12 are drawing editors that allow the user to draw graphical objects and impose constraints on their layouts To help the constraint solvers maintain the consistency of the constraints the user is usually required to spe
24. ther we use a rectangle as a pad between these two buttons Let L A D and Q be the four buttons and Pad be the pad then the constraint for the buttons can be written as follows cgTable L A Pad D Q 0 5 There are five rows in the tabular form and there is a 5 pixel space between each two rows The drawing area is a rectangle located to the right of the tool panel A Prolog object of the following form is used to keep track of the status of the editor ba Win Area L A D Q X Y Selected Bs As where e Win is the enclosing window e Area is the drawing area e L A D and Q are the four buttons X and Y indicate the position of the last mouse click e Selected is the selected component It is nil in the beginning indicating that nothing has been selected e Bs is the list of boxes in the drawing area e As is the list of arrows that have been created For each arrow in addition to the arrow itself the two boxes connected by the arrow are also memorized The editor behaves in an event driven fashion It creates the initial interface and creates several event handlers all of which take this object There are handlers for handling mouse presses keystrokes mouse drags window resizing etc The following is the action rule that defines the handler for mouse presses handleMousePress BA BA ba Win Area L A D Q _X _Y Selected Bs As mousePressed Win E gt E x X E y Y handleM
25. thickness of the line e arrowl has an arrow at the first end if 1 e arrow2 has an arrow at the second end if 1 In addition to objects listed in Figure 1 there are also primitives for creating menus dialogs and non visible objects such as colors fonts points and dimensions Non visible objects serve as attribute values of visible objects 2 3 Constraints A constraint is a relation among objects For instance the constraint O1 x Ol width 2 O2 x ensures that the x coordinate of O2 is equal to the central x coordinate of O1 Here the operator denotes division on integers and the operator denotes the equality constraint A simpler way to write the constraint is O1 centerX 02 x In addition to equality disequality and inequality gt gt lt lt constraints that are already available in B Prolog the constraint library also contains many symbolic constraints that restrict the layout of objects without accessing their attributes explicitly These constraints include relative positioning same property not overlap grid table and tree constraints A positioning constraint specifies the relative positions of objects For instance the constraint cgInside 01 02 ensures that O1 is located inside 02 A same property constraint forces a list of objects to have the same value for an attribute For instance the constraint cgSame 01 02 03 size ensures that the three objects O1 O2 and O3
26. tives on graphical objects and provides various kinds of constraints that facilitate the specification of the layouts of graphical objects The constraint solver of B Prolog serves as a general purpose layout manager that is significantly more flexible than the special purpose layout managers used in Java and Tcl Tk The library adopts a construct called action rules available in B Prolog for programming interactions An action rule consists of a pattern for agents a pattern for events that can activate the agents and an action the agents carry out when activated Agents can communicate with each other through logical or global variables Agents can behave concurrently and sequentially as well CGLIB is implemented in B Prolog Java and C CGLIB enables the user to use the Java graphics package without the need to write any code in Java CGLIB however is not just another syntax sugar for Java s graphics package It has a significantly higher abstraction level and is much easier to learn and use than Java s graphics package One of the distinguishable features of CGLIB is the use of constraints in the specification of layouts In graphics languages such as PostScript and Tex the programmer has to specify every single detail about the graphical objects including their sizes and positions In languages such as Java and Tcl Tk the layout managers help determine the layouts of objects for certain applications but the layout algorithms lack fle
27. to update a structure without making a copy of it This predicate is not in the ISO standard but many Prolog systems support it Size a Rate Reset Start 200 Resume Figure 7 Animating the Towers of Hanoi The interface of the animator is made of two parts The first part is a control panel that consists of several buttons and two text fields and the second part is a panel that shows a table three poles and several disks The number of disks and the animation rate can be input through the text fields in the control panel We use a structure of the following form to represent the control panel cp InputSize InputRate Reset Start Pause Resume Status EventVar where Input Size and InputRate are two text fields the four arguments following the text fields are control buttons and the argument Status denotes the status of the animator which can be one of the following run meaning the disks are moving wait meaning that the animator is waiting for the user to press the Resume button dead meaning that the animator has been killed The last argument Event Var is an event variable used to notify the animator when the status changes The predicate for updating the status is as follows updateStatus CP NewStatus getEventVar CP EventVar setarg 7 CP NewStatus post event EventVar The call getEventVar CP EventVar gets the event variable from the control panel and the call post posts
28. tor and the constraint on the layout Prolog is not an object oriented language but this does not prevent us from writing programs in the object oriented style A graphical user interface usually generates many handlers each of which carrries the objects it manipulates when activated In the calculator there is an event handler defined as follows for each key on the keyboard keyInput Key Cal actionPerformed Key gt Key text KeyLab process KeyLab Cal The handler takes the object Cal and updates it when activated The predicate process KeyLab Cal processes a click of the button labeled KeyLab Different actions are taken according to the types of keys For example when a digit key is clicked if the number on display is a result then the calculator cleans the display and shows the digit as part of the next operand if the number on display is an operand then the calculator appends the digit to the end of the operand 3 4 Animating the Towers of Hanoi An animation is a sequence of frames of objects that change along a time line Animation is better than static graphics for presenting many kinds of data because of its dynamic nature We consider how to develop an animator for the Towers of Hanoi Figure 7 shows the interface B Prolog provides a built in called setarg for updating structures The built in setarg I T NewA sets the Ith argument of the structure T to NewA This predicate makes it possible
29. ts and maintain the constraints Let Q be the outer and P be the inner quadrilaterals The agent handleMouseDown is triggered when the mouse is pressed It detects whether a point of Q is selected The agent handleMouseUp is triggered when the mouse is released It deselects the selected point if any The agent handleMouseDrag is activated when a point is dragged If the mouse drag occurs while a point is selected the agent moves the point and re computes Q s points The three agents are defined in the following handleMouseDown Win O mousePressed Win E gt E x X E y Y selectPoint X Y 0O PrRPrRPRRRrR oO yoo BUNE OPBWNRrRO handleMouseUp Win O mouseReleased Win E deselectPoint 0 ll Vv handleMouseDrag Win O mouseDragged Win E gt selectedPoint O I gt E x X E y Y updatePoint O I X Y recomputeP O true All of the three agents share the window Win and an object O that is a Prolog term of the following structure quadrilaterals Q P Selected where Q and P are two polygons and Selected is the index of the selected point Selected is 1 when no point of Q is selected The predicate selectPoint X Y O in line 4 sets Selected to I if the Ith point of Q is close enough to lt X Y gt The predicate deselectPoint O in line 7 sets Selected to 1 The predicate selectedPoint O I in line 7 succeeds if I is the selected point T
30. w C cgSleep 100 fail 2 9 Generating Java Applets There are demands for images and animations that can be embedded into Web documents In CGLIB it is possible to record images into a Java applet The call cgStartRecord File starts and the primitive cgEndRecord ends recording For example the following program generates a Java applet named circle java that animates the color changing circle go cgStartRecord circle animateCircle r cgStopRecord Generated Java programs are standalone Therefore they can show static and dynamic images but cannot handle interactions 3 SAMPLE APPLICATIONS We present six sample applications in this section Due to the limit on space only part of the code will be given The full sources are available from www probp com examples htm The examples to be described here are mostly illustrative The reader can find more examples on the web site 3 1 Drawing Flags Figure 3 shows six flags drawn by using the CGLIB The six flags and the labels are laid out in a tabular form Each flag is composed of several components whose sizes are in certain proportions and whose relative positions are constrained properly For example the Antigua flag is composed of a red rectangle a black triangle a yellow 16 pointed star a blue triangle and a white triangle The USA flag is composed of 50 white 5 pointed stars laid out on a blue background and seven red strips on a white background Two gr
31. xibility and are difficult to learn and use The use of constraints significantly relieves the burden of the programmers to describe many tedious details about layouts Another distinguishable feature of CGLIB is its support of action rules for specifying interactions Action rules can be used to describe and synchronize sophisticated and dynamic behaviors of graphical objects in a simple and straightforward manner CGLIB can be used in many areas such as drawing editors interactive user interfaces document authoring animation information visualization intelligent agents and games In this paper we describe the library and present several sample applications Familiarity with logic and constraint programming concepts is assumed but most of the material should be readable for readers who don t have any programming experience in Prolog or CLP The reader is referred to 3 for the basics 2 THE CGLIB 2 1 An Illustrative Example Let us start with the following program go m Dm WN WH cgButton B B text Hello World handleButtonClick B cgShow B handleButtonClick B actionPerformed B gt 5 6 7 halt 8 The call cgButton B in line 1 creates a button B The constraint in line 2 binds the text attribute of B to Hello World Line 3 creates an event handler which will be activated when the button B is clicked The call cgShow B in line 4 packs the button i
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