Java 图形用户界面 : paintComponent method in the JComponent class

Question

因此，首先我创建了一个 JFrame 对象，然后创建了一个使用 PaintComponent 方法的适当重写版本来扩展 JComponent 的类。

最后，我将此类的一个对象添加到 JFrame 对象中。

引用代码如下:

import java.awt.*;
import javax.swing.*;

public class MyFrame {

    public static void main(String[] args) {
        JFrame frame = new JFrame("My Frame");
        Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize();
        frame.setSize(screenSize);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        MyComponent comp = new MyComponent();
        frame.add(comp);
    }

}

class MyComponent extends JComponent {

    public void paintComponent(Graphics g) {
        Graphics2D g2 = (Graphics2D) g;
        Rectangle r = new Rectangle(0,0,100,100);
        g2.draw(r);
    }
}

我的问题是...paintComponent 方法采用 Graphics2D 对象作为参数，但是谁传递了这个对象，因为我不是。

Answer 1

My question is...the paintComponent method takes a Graphics2D object as it's argument, but who is passing this coz I'm not.

当在 EDT 上处理绘制事件时，绘制系统会将其传入。 .

在许多方面，paintComponent 与其他事件处理程序(actionPerformed、itemStateChanged 等)类似，因为您无需调用它明确地。系统准备传递给您的对象(在本例中为Graphics)并在需要时调用它。

摘自Painting in AWT and Swing :

[If] the paint request originates on the first heavyweight ancestor (usually JFrame, JDialog, JWindow, or JApplet):

• the event dispatching thread invokes paint() on that ancestor

• The default implementation of Container.paint() recursively calls paint() on any lightweight descendents

[If] the paint request originates from a call to repaint() on an extension of javax.swing.JComponent:

• JComponent.repaint() registers an asynchronous repaint request to the component's RepaintManager, which uses invokeLater() to queue a Runnable to later process the request on the event dispatching thread.

• The runnable executes on the event dispatching thread and causes the component's RepaintManager to invoke paintImmediately() on the component, which [...] invokes paint() on the root component.

(并且 paintComponent 由 paint 调用。)