java - 这个程序什么时候调用paint？另外，为什么它扩展 Canvas ？

Question

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

public class GraphicsDemo1 extends Canvas
{
    public void paint( Graphics g )
    {
        g.setColor(Color.green);
        g.drawRect(50,20,100,200);  // draw a rectangle
        g.fillOval(160,20,100,200); // draw a filled-in oval
        g.setColor(Color.blue);
        g.fillRect(200,400,200,20); // a filled-in rectangle
        g.drawOval(200,430,200,100);

        g.setColor(Color.black);
        g.drawString("Graphics are pretty neat.", 500, 100);
        int x = getWidth() / 2;
        int y = getHeight() / 2;
        g.drawString("The first letter of this string is at (" + x + "," + y + ")", x, y);
    }

    public static void main( String[] args )
    {
        // You can change the title or size here if you want.
        JFrame win = new JFrame("GraphicsDemo1");
        win.setSize(800,600);
        win.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        GraphicsDemo1 canvas = new GraphicsDemo1();
        win.add( canvas );
        win.setVisible(true);
    }
}

谢谢。 awt 和 swing 让我很困惑。

Answer 1

why does it extend canvas?

因为写它的人选择这样做。只有那些从 Component 扩展的类才能真正被绘制到屏幕上，并且只有当它们附加到有效的可见窗口时才可以绘制

When is paint called in this program?

绘画是 RepaintManager 的职责。它将决定何时需要重绘组件并在事件调度线程上安排重绘事件。这反过来会代表您调用(在您的情况下 update 调用)paint。

您可能希望阅读 Painting in AWT and Swing有关该主题的更多详细信息