Binding with a Conditional Expression in Java

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Binding with a Conditional Expression
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The ability to bind to an expression includes binding to the result of an if expression (or to an expression that includes one or more if expressions)As an example of why this is useful, suppose you are implementing a user interface that shows the progress of a long operation, such as a download, and you want to show how much time remains If the time remaining is in a variable called timeLeft, which is updated as the download progresses, you can display a suitable message like this:
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Label { text: bind "{timeLeft} seconds remaining" }
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This works, but there is a grammatical issue with the text that appears 1 second before the operation completes, as shown in Figure 9-4
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Unwanted plural
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Figure 9-4
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An incorrect progress message
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Binding to Variables and Expressions
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What you need to do to fix this is to use a slightly different message when only 1 second remainsThis is easily achieved by binding the Label s text variable to the result of an if expression, like this2:
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Label { text: bind if (timeLeft == 1) "{timeLeft} second remaining" else "{timeLeft} seconds remaining" }
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This gives the result shown in Figure 9-5As you can see, the code is quite readable and conveys the intent clearly
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Figure 9-5
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Using a bind to a conditional expression
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Bidirectional Binding
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The bindings you have seen so far have all been unidirectional Sometimes it is useful to make a binding bidirectionalA bidirectional binding is established through the use of the keywords with inverse:
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var a = 23; var b = bind a with inverse;
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Now, as with a unidirectional binding, changes to the value of a cause b to be updated However, a bidirectional binding also allows the bound variable, in this case b, to be modified and ensures that the value of a stays in step with itAs a consequence, this assignment, which would not be allowed in the case of a unidirectional binding, is legal:
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b = 100;
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This causes the value of a to also be changed to 100 so that the invariant expressed by the binding is maintained Note that at the time of this writing, the only form of bidirectional binding allowed is to a script variable or to instance variable It is not possible to bind to the value of a more general expression, such as this:
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var b = bind a + 23 with inverse; // Not allowed
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You ll find this code in the file javafxbinding/BindLabelToExpression3fx in the JavaFX Book Desktop project
9 Binding
The following is an example of a bidirectional binding to an instance variable, using the ValueHolder class that we created in an earlier example:
1 2 3 4 5 6 7 8 9 10 11 12 class ValueHolder { var value:Integer; } var holder = ValueHolder { value: 1234 }; var boundValue = bind holdervalue with inverse; println("boundValue is {boundValue}"); boundValue = 4567; println("Value is holder is now {holdervalue}"); holdervalue = 7890; println("boundValue changed to {boundValue}");
The code on line 6 creates a bidirectional binding between the variable boundValue and the value variable of an instance of the ValueHolder classThe output from this script illustrates how the bidirectional binding works:
boundValue is 1234 Value is holder is now 4567 boundValue changed to 7890
Bidirectional bindings are often needed when building user interfaces It is common to use a GUI component to map an instance variable of a model class that represents part of the state of an application If you use a unidirectional binding, you can arrange for the content of the model to be reflected in the state of the GUI component, or vice versa, depending on which way you define the binding However, if you want to allow both the user and the application to change the state of the model and to keep those states synchronized, you need to bind them bidirectionally We illustrate this by using a variation on the example shown in Figure 9-3, which bound the text variable of a Label to the rawText variable of a TextBox First, we move the bound text to a script variable, and then we bind the text variables of the Label and the rawText variable of the TextBox to that script variable, instead of creating a binding between the two GUI components3:
TextBox { text: bind modelText selectOnFocus: true } Label { text: bind "You typed: {modelText}"
The code for this example is in the file javafxbinding/BidirectionalBindingfx in the JavaFX Book GUI project