CODEWITHSUNDEEP
javafx-8
Raphael Roth
Raphael Roth

Reputation: 27373

disable buttons according to Predicate in JavaFX8

I'm working on a PDF viewer using JavaFX8 and have "page up" and "page down" button. I want the "page up" button to be disabled if the currently displayed page is the first page.

First, I haved added a isFirstPage BooleanProperty instance variable to my controller (which I set to true or false depending on the current page number), this works fine, but it seems overkill to add an instance variable for every state of the application (isFirstPage, isLastPage, isDocumentLoaded etc).

What I want to achieve is to bind the disableProperty to a Predicate (whichs is evaluated every time the selectedPageNo changes), but apparently I'm doing somethign wrong as the code is not working (the buttons is never disabled):

nextPageButton.disableProperty().bind(Bindings.createBooleanBinding(() ->
                 selectedPageNo.intValue() >= currentDocument.getNumberOfPages()));

Here selectedPageNo is a IntegerProperty and currentDocument a org.apache.pdfbox.pdmodel.PDDocument

Upvotes: 1

Views: 45

Answers (1)

James_D
James_D

Reputation: 209684

Three different ways:

IntegerProperty API

You can just do

nextPageButton.disableProperty().bind(
    selectedPageNo.greaterThanOrEqualTo(currentDocument.getNumberOfPages()));

If the number of pages might change and is also represented by an IntegerProperty, then 

nextPageButton.disableProperty().bind(
    selectedPageNo.greaterThanOrEqualTo(currentDocument.numberOfPagesProperty()));

will observe both properties and update as required.

Bindings API

For more general calculations that aren't supported by the methods IntegerProperty inherits from NumberExpressionBase, you can use the Bindings API almost as you used it. The thing you omitted was to provide the properties that the binding needs to observe:

nextPageButton.disableProperty().bind(Bindings.createBooleanBinding(() -> 
    selectedPageNo.intValue() >= currentDocument.getNumberOfPages(),
    selectedPageNo));

Subclass BooleanBinding

An alternative is to subclass BooleanBinding directly:

nextPageButton.disableProperty().bind(new BooleanBinding() {
    {
        bind(selectedPageNo);
    }

    @Override
    protected boolean computeValue() {
        return selectedPageNo.intValue() >= currentDocument.getNumberOfPages();
    }
});

Upvotes: 1

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