使用JPanel在Java中如何运行addNotify()和requestFocus()?

问题描述

我还不清楚JPanel的super.addNotify()和requestFocus()方法如何正常工作，尤其是在以下代码内如何工作:

I have not got a clear idea how does super.addNotify() and requestFocus() methods of JPanel work in general and within the code below in particular:

public class Panel extends JPanel
implements keyListener, MouseListener, MouseMotionListener {

    public Panel() {
        setPreferredSize(new Dimension(WIDTH, HEIGHT));
        setFocusable(true);
        requestFocus();
}

    public void addNotify() {
        super.addNotify();
        if (thread == null) {
            addKeyListener(this);
            addMouseListener(this);
            addMouseMotionListener(this);
            thread = new Thread(this);
            thread.start();
        }
    }

    // Some unrelated code follows

}

有人可以向我解释吗?

推荐答案

addNotify().因此，可以使用此方法来获取父级信息，而不必具有null父级的风险，而在构造函数中该父级的可能性更大.

addNotify() gets called whenever the Component gets added to a Container. This method can therefore be used to gain parent information without the risk of having a null parent, which in the constructor is more than likely.

requestFocus()发出请求，将给定的Component设置为聚焦状态.此方法要求组件是可显示的，可聚焦的，可见的，并且其所有祖先也都可见.最好调用requestFocusInWindow()，因为该方法不依赖于平台.

requestFocus() makes a request that the given Component gets set to a focused state. This method requires that the component is displayable, focusable, visible and have all it's ancestors be visible too. It is best to call requestFocusInWindow(), as that method is not platform dependent.

在代码示例中，您的JPanel发送一个要关注的请求.这很有用，因为要实现KeyLisener，这将要求面板处于聚焦状态.使用addNotify()，它仅添加侦听器.尽管希望在此代码示例中没有保证，但这将希望仅被调用一次.

In the code example, your JPanel sends a request to be focused. This is useful, since the implementation of a KeyLisener, which would require the panel to be in a focused state. With the addNotify(), it just adds the listeners. This will hopefully only be called once, although no guarantee is made in this code example.

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