Triggers and Instance Variables in Java

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Triggers and Instance Variables
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This code sets the value of the visible variable to true and adds a trigger that is called whenever the variable s value changesThe override keyword indicates that we are referring to the existing variable called visible in the class definition if you omit it, it will look like you are trying to declare a local variable of that name in the object literal Note that it is not possible to add a trigger to a variable in an object that has already been instantiatedYou cannot, therefore, directly attach a triggers to an instance variable of an object that your code does not create If you want to add a trigger to a variable in an arbitrary class, or one to which you do not have write access, you have to do so indirectly by creating a new variable that is bound to the variable s value and then attaching the trigger to your new variableThis works because the binding will keep your variable synchronized with the original, so changes to the original will fire the trigger on your variable For example, the Node class in the GUI libraries, which represents a user interface element, has a variable called hover that is true when the cursor is over the screen space occupied by the nodeThis variable is read-only, so you cannot override it in an object literal to attach a trigger to itThe following code, which you ll find in the file javafxtriggers/InstanceVariableTriggerfx, shows how you can track the state of this variable:
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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Stage { title: "Instance Trigger" scene: Scene { var rect: Rectangle; width: 200 height: 200 content: [ rect = Rectangle { x: 10 y: 10 width: 100 height: 100 fill: ColorYELLOW def hover = bind recthover on replace { println("Hover is {hover}"); } } ] } }
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Line 4 declares a variable that we use to refer to the node whose hover variable is to be tracked, which in this case is a rectangle Lines 10 to 12 declare a local variable that is bound to the hover variable of the rectangle and attach a trigger to it Here, the trigger just prints the value of the variable whenever it changes, which is when the mouse enters and leaves the bounds of the rectangle Adding a trigger to a variable in an object literal does not replace any trigger that may have been added to it by the author of that class both triggers are registered and both fire when the variable is initialized or changedThe JavaFX runtime does not guarantee the order in which these triggers will be executed
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JavaFX Script Classes
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In Java, classes are everywhere, buthaving toiscreate a single class of your own Most ofcan JavaFX a much more informal language you write whole applications without the
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examples in the first half of this book do not include a class declaration1 However, if you are going to be creating reusable functionality, such as new controls to be used in your own or your company s applications, you need to have a good understanding of JavaFX classes, which are the subject of this chapter As in Java, a JavaFX class may have state, represented by its instance variables, and operations that use/modify that state, in the form of instance functionsAlthough Java and JavaFX classes have much in common, there are various differences:
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JavaFX classes do not have constructors Instead, initialization is performed using an object literalA JavaFX class can, however, contain code that performs a similar function to a constructor, which will be executed while it is being initialized See the Class Initialization section, later in this chapter JavaFX classes allow function overloading and overriding In JavaFX, function overriding must be explicitly indicated using the override keyword See the Function Overloading and Function Overriding section, later in this chapter JavaFX does not have a static keyword to declare class members and functions, but it does have script variables and script functions See the Script Functions and Variables section, later in this chapter There is no concept of a JavaFX interface, but a JavaFX class can implement a Java interface Both Java and JavaFX have abstract classesA JavaFX class in which all functions are declared to be abstract is a good approximation to a Java interface
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This is not the same as saying that there aren t any classes As you ll see in this chapter, all JavaFX scripts are compiled to at least one Java class even if there is no explicit class declaration
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