谁在我的类中调用paintComponent（）方法？ [英] Who is calling the paintComponent() method in my class?

问题描述

我有一个简单的类，用于在JPanel中绘制图形。这是我的班级：

  import java.awt.Color; 
 import java.awt.Dimension; 
 import java.awt.Graphics; 
 import javax.swing.JPanel; 
 
 class Drawing_panel扩展JPanel {
 public void paintComponent（Graphics g）{
 super.paintComponent（g）; 
 this.setBackground（Color.white）; 
 g.setColor（Color.red）; 
 g.fillRect（150,80,20,20）; 
} 
 
 public Dimension getPreferredSize（）{
 return new Dimension（500,500）; 
} 
  

}

我有另一个实例化这个的类：

  Drawing_panel dp = new Drawing_panel（）; 
  

Drawing_panel 类中没有构造函数和/或没有显式调用 paintComponent（）或 getPreferredSize（）方法。我假设该方法是在父 JPanel 构造函数中调用的，但是我没有看到那里的调用。

解决方案

从几个不同的地方调用 paintComponent 。来自 JComponent.paint 的调用可能是您正在寻找的。

请注意 paintComponent 未从任何构造函数调用。 paintComponent 被称为按需，即当系统决定需要重新绘制该组件时。 （例如，可以在调整组件大小时，或者从最小化状态恢复窗口时。）要清楚：组件不是涂漆，然后使用，而是使用，然后在需要时涂漆。 / p>

这整个绘画调用链都不是你应该打扰的，因为它完全由Swing和所谓的事件调度线程完成。

I have a simple class that paints a graphic in a JPanel. This is my class:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import javax.swing.JPanel;

class Drawing_panel extends JPanel {
    public void paintComponent(Graphics g) {
    super.paintComponent(g);       
    this.setBackground(Color.white);
    g.setColor(Color.red);
    g.fillRect(150, 80, 20, 20);
}  

public Dimension getPreferredSize(){
    return new Dimension(500,500);
}

}

I have another class that instantiates this one:

Drawing_panel dp = new Drawing_panel();

There is no constructor in the Drawing_panel class and/or no explicit call to either the paintComponent() or getPreferredSize() methods. I assume the method is being called in the parent JPanel constructor, but I did not see the calls there either.

解决方案

The paintComponent is called from a few different places. The call from JComponent.paint is probably the one you're looking for.

Note that paintComponent is not called from any constructor. The paintComponent is called "on-demand" i.e. when the system decides that the component needs to be redrawn. (Could for instance be when the component is resized, or when the window is restored from a minimized state.) To be clear: The component is not "painted, then used", it is "used, then painted when needed".

This whole chain of painting-calls is nothing you should bother about, as it is taken care of entirely by the Swing and the so called Event Dispatch Thread.

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