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java - JavaFx 的 setOnAction 方法在底层是如何工作的？

问题描述

在我的面向对象编程课程中，我们最近的主题是通过 JavaFX 进行事件驱动编程。这些概念相对有趣，尽管有时让我很困惑。我一直在努力理解的一件事是该setOnAction()方法的工作原理。

正如我的导师所说，这个方法接受一个具有句柄方法的类的对象，并在触发事件时调用该句柄方法。

例如在我们的start方法中：

class MyHandler implements EventHandler<ActionEvent> {
        @Override
        public void handle(ActionEvent actionEvent) {
            System.out.println("Handled");
        }
    }

Button button = new Button();
button.setOnAction(new MyHandler());

但是，我无法理解这是如何在幕后工作的。方法究竟是如何setOnAction“知道”调用对象的句柄方法的MyHandler？是否可以实现我自己的 a 版本setOnAction，在其中我调用任何传入对象的特定命名方法？

如果我们根本不实现句柄方法，而是说一个fakeHandle方法，就像这样。

class MyHandler {
        public void fakeHandle(ActionEvent actionEvent) {
            System.out.println("Handled");
        }
    }

Button button = new Button();
button.setOnAction(new MyHandler());

我们的代码还能工作吗？

标签： javajavafxevent-handling

解决方案

正如我的导师所说，这个方法接受一个具有句柄方法的类的对象，并在触发事件时调用该句柄方法。

I suspect there's a slight misunderstanding or a poor choice of words happening here. You cannot pass any arbitrary object whose class defines a handle(ActionEvent) method to the setOnAction method. The setOnAction method is defined to accept an EventHandler<ActionEvent> implementation, and it's that interface that defines the handle method.

This is exactly what interfaces are for. They define a contract that all implementations must follow. It doesn't matter what object you pass to setOnAction so long as its class implements EventHandler<ActionEvent>. And that's how JavaFX knows about the handle method, because it knows about the EventHandler interface.

Your second code example won't compile because myHandler doesn't implement EventHandler<ActionEvent>. But even if it did your fakeHandle method would not be invoked (by JavaFX code) because JavaFX is not aware of it.

Explaining exactly how the entire event-handling process works under-the-hood in JavaFX is an in-depth process. If you really want to understand this then I suggest reading the code—<a href="https://github.com/openjdk/jfx" rel="nofollow noreferrer">JavaFX is open source. However, if you only want to understand event-handling from a user's perspective (i.e. an API level) then this tutorial should help.

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