paintComponent是如何工作的？

Question

这可能是一个非常无聊的问题.我刚刚开始学习Java

我不明白paintComponent方法的操作.我知道如果我想画一些东西,我必须覆盖paintComponent方法.

public void paintComponent(Graphics g)
{
   ...
}

但它何时被称为？我从来没有看到像"object.paintComponent(g)"这样的东西,但它仍然是在程序运行时绘制的.

什么是Graphics参数？这个从哪里来？调用方法时必须提供参数.但正如我之前所说,似乎永远不会明确地调用此方法.那么谁提供这个参数呢？为什么我们必须将其转换为Graphics2D？

public void paintComponent(Graphics g)
{
    ...
    Graphics2D g2= (Graphics2D) g;
    ...
}

Answer 1

对你的问题的(非常)简短回答paintComponent是"当它需要时".有时将Java Swing GUI系统视为"黑盒子"更容易,其中许多内部构件在没有太多可见性的情况下处理.

有许多因素决定了何时需要重新绘制一个组件,包括移动,重新调整大小,改变焦点,被其他框架隐藏,等等.其中许多事件都是自动神奇地检测到的,并且paintComponent在确定需要进行该操作时在内部调用.

我和Swing一起工作了很多年,我认为我没有paintComponent直接打过电话,甚至没有直接打过电话.我最接近的是使用repaint()方法以编程方式触发某些组件的重绘(我假设调用paintComponent下游的正确方法).

根据我的经验,paintComponent很少直接覆盖.我承认有些自定义渲染任务需要这样的粒度,但Java Swing确实提供了一套(相当)强大的JComponents和Layouts,可用于完成大部分繁重工作而无需直接覆盖paintComponent.我想我的观点是确保在您尝试滚动自己的自定义渲染组件之前,您无法对本机JComponents和Layouts执行某些操作.

Answer 2

你可以在这做两件事:

1. 阅读AWT和Swing中的绘画
2. 使用调试器并在paintComponent方法中放置断点.然后向上移动堆栈跟踪,看看如何提供Graphics参数.

仅供参考,以下是我在最后发布的代码示例中得到的堆栈跟踪:

Thread [AWT-EventQueue-0] (Suspended (breakpoint at line 15 in TestPaint))  
    TestPaint.paintComponent(Graphics) line: 15 
    TestPaint(JComponent).paint(Graphics) line: 1054    
    JPanel(JComponent).paintChildren(Graphics) line: 887    
    JPanel(JComponent).paint(Graphics) line: 1063   
    JLayeredPane(JComponent).paintChildren(Graphics) line: 887  
    JLayeredPane(JComponent).paint(Graphics) line: 1063 
    JLayeredPane.paint(Graphics) line: 585  
    JRootPane(JComponent).paintChildren(Graphics) line: 887 
    JRootPane(JComponent).paintToOffscreen(Graphics, int, int, int, int, int, int) line: 5228   
    RepaintManager$PaintManager.paintDoubleBuffered(JComponent, Image, Graphics, int, int, int, int) line: 1482 
    RepaintManager$PaintManager.paint(JComponent, JComponent, Graphics, int, int, int, int) line: 1413  
    RepaintManager.paint(JComponent, JComponent, Graphics, int, int, int, int) line: 1206   
    JRootPane(JComponent).paint(Graphics) line: 1040    
    GraphicsCallback$PaintCallback.run(Component, Graphics) line: 39    
    GraphicsCallback$PaintCallback(SunGraphicsCallback).runOneComponent(Component, Rectangle, Graphics, Shape, int) line: 78    
    GraphicsCallback$PaintCallback(SunGraphicsCallback).runComponents(Component[], Graphics, int) line: 115 
    JFrame(Container).paint(Graphics) line: 1967    
    JFrame(Window).paint(Graphics) line: 3867   
    RepaintManager.paintDirtyRegions(Map<Component,Rectangle>) line: 781    
    RepaintManager.paintDirtyRegions() line: 728    
    RepaintManager.prePaintDirtyRegions() line: 677 
    RepaintManager.access$700(RepaintManager) line: 59  
    RepaintManager$ProcessingRunnable.run() line: 1621  
    InvocationEvent.dispatch() line: 251    
    EventQueue.dispatchEventImpl(AWTEvent, Object) line: 705    
    EventQueue.access$000(EventQueue, AWTEvent, Object) line: 101   
    EventQueue$3.run() line: 666    
    EventQueue$3.run() line: 664    
    AccessController.doPrivileged(PrivilegedAction<T>, AccessControlContext) line: not available [native method]    
    ProtectionDomain$1.doIntersectionPrivilege(PrivilegedAction<T>, AccessControlContext, AccessControlContext) line: 76    
    EventQueue.dispatchEvent(AWTEvent) line: 675    
    EventDispatchThread.pumpOneEventForFilters(int) line: 211   
    EventDispatchThread.pumpEventsForFilter(int, Conditional, EventFilter) line: 128    
    EventDispatchThread.pumpEventsForHierarchy(int, Conditional, Component) line: 117   
    EventDispatchThread.pumpEvents(int, Conditional) line: 113  
    EventDispatchThread.pumpEvents(Conditional) line: 105   
    EventDispatchThread.run() line: 90  

Graphics参数来自这里:

RepaintManager.paintDirtyRegions(Map) line: 781 

涉及的片段如下:

Graphics g = JComponent.safelyGetGraphics(
                        dirtyComponent, dirtyComponent);
                // If the Graphics goes away, it means someone disposed of
                // the window, don't do anything.
                if (g != null) {
                    g.setClip(rect.x, rect.y, rect.width, rect.height);
                    try {
                        dirtyComponent.paint(g); // This will eventually call paintComponent()
                    } finally {
                        g.dispose();
                    }
                }

如果你看一下它,你会发现它从JComponent本身(间接地javax.swing.JComponent.safelyGetGraphics(Component, Component))检索图形,它最终从它的第一个"重量级父级"(剪切到组件边界)中获取它,它自己从它获取它相应的原生资源.

关于你必须转换Graphics为a 的事实,Graphics2D只是在使用Window Toolkit时,Graphics实际上是扩展了Graphics2D,但你可以使用其他Graphics"不必"扩展Graphics2D(它不经常发生但是AWT/Swing允许你这样做).

import java.awt.Color;
import java.awt.Graphics;

import javax.swing.JFrame;
import javax.swing.JPanel;

class TestPaint extends JPanel {

    public TestPaint() {
        setBackground(Color.WHITE);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawOval(0, 0, getWidth(), getHeight());
    }

    public static void main(String[] args) {
        JFrame jFrame = new JFrame();
        jFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jFrame.setSize(300, 300);
        jFrame.add(new TestPaint());
        jFrame.setVisible(true);
    }
}