Binding and Functions in Java

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Binding and Functions
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You can create a binding to the return value of a function by simply invoking the function (alone or as part of an expression) on the right side of a binding assignment JavaFX has two types of function, called unbound and bound functionsThe functions that you have seen so far in this book have all been unbound, and the example in this section also uses an unbound functionYou ll be introduced to bound functions in the section Binding and Bound Functions, later in this chapter
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Binding and Unbound Functions
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Suppose we want to write a function that calculates the area of a circle, rounded to the nearest integer, given its radiusThat s simple enough it takes only three lines of code:
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function computeArea(radius:Integer):Integer { Mathround(MathPI * radius * radius) as Integer }
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Now suppose we want to write an application that displays a circle and a slider that controls the circle s radius, as shown in Figure 9-84 As the user drags the slider, the circle s radius should change to match the slider s value, and both the radius and the area should be updated
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Figure 9-8
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Binding to a function that calculates the area of a circle of varying radius
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The source code for this example is in the file javafxbinding/FunctionBindingfx in the JavaFX Book GUI project
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9 Binding
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You should by now recognize that there is an obvious case here for binding the variable should be bound to the value of the slider, and the strings that display the radius and the area should be bound to the radius variable and to the result of the function that calculates the area, respectivelyWe haven t yet covered the GUI classes that you need to create this application, so we ll skip those details and show you only those parts of the code that involve bindings First, the radius is defined and set to a reasonable initial value:
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radius var radius = 100;
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The linkage from the slider value to the radius variable is a binding in the object literal that initializes the slider when it is created:
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Slider { min: 1 max: 50 vertical: false value: bind radius with inverse translateX: 20 translateY: 120 }
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The slider value is allowed to range from 1 up to 50 and, because of the binding, it starts with the initial value of the variable radius, which is 10As the user drags the slider, its value variable changes, and this is propagated to the radius variable because of the bindingWe set up a bidirectional binding because we need to copy the value of the slider to the radius variable, not the other way aroundAnother way to do this is to create a unidirectional binding from the radius variable to the value variable of the slider:
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var slider = Slider { min: 1 max: 50 vertical: false value: 10 translateX: 20 translateY: 120 } var radius = bind slidervalue;
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This works, but it is not good practice to have application state, which is what the variable represents, depend directly on a GUI component in this wayAs a rule, dependencies should be from the GUI components to values in the application model This makes it possible to change the components that are used in the user interface without requiring any modification of the application logic The text that represents the radius is displayed using a Text object:
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radius Text { content: bind "Radius: {radius}" x: 120
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Binding and Functions
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y: 50 }
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The content variable is bound to a string containing the value of the radius variable As a result, it will be updated whenever the radius changes, and the Text object will display the new value on the screen The content variable of the second Text object is bound to an expression that includes the result of invoking the computeArea() function with the current radius as its parameter:
Text { content: bind "Area: {computeArea(radius)}" x: 120 y: 80 }
The highlighted code is an example of a binding to a functionAlthough in this case only the function result is required, it is possible to create a bound expression that involves more than one function invocation, as follows:
var v = bind fn1(arg1) + fn2(arg2);
In this example, fn1() and fn2() are assumed to be functions that each require a single argument When a binding involves a function invocation, it is necessary to assign a new value to the target of the binding whenever the result of the function would change In the case of an unbound function, the function is invoked automatically, and its value substituted in its surrounding expression whenever the value of any of its arguments changes In the case of the computeArea() function, a change in the value of the radius variable, which is the function s sole argument, will cause the function to be invoked again and its new value substituted into the bind expression, which is then assigned to the text variable of the Text node Hence, as the user moves the slider on the screen, the corresponding change in the radius variable causes the displayed area to be updated