在已经可见的框架上调用 setVisible(true)

Question

问题：

在已经可见的 JFrame 上调用 setVisible(true) 有什么作用？我正在研究JFrame 的源代码，最终，它归结为this function in Component，如果它已经可见，它对框架没有任何作用。为什么它的行为类似于revalidate(); repaint();？ （见下面的 SSCCE）

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动机：

我正在开发一个 java 应用程序，为此我编写了一个类 JImagePanel，它扩展了 JPanel 并允许用户将图像设置为背景（参见 SSCCE）。我发现在编辑面板的背景后，我在将背景重新绘制为正确大小时遇到​​了问题。在网上搜索后，我发现以下工作：

if(frame.isVisible()) frame.setVisible(true);

最终，我使用

解决了这个问题

panel.revalidate();
panel.repaint();

，我认为这是更好的解决方案，但它让我思考setVisible(true) 在已经可见的框架上实际上做了什么。从我的角度来看，它不应该起作用 - 但事实上它确实起作用。

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SSCCE

这是一个说明我的问题的示例。如果不出意外，希望你会发现这个类在未来非常有用。

注意：此文件的更新源可以在项目主页GitHub 上找到。

享受吧！

package com.dberm22.utils;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.RenderingHints;
import java.awt.Toolkit;
import java.awt.image.BufferedImage;

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

public class JImagePanel extends JPanel {

    private static final long serialVersionUID = 6841876236948317038L;
    private Image img = null;
    private Position position = Position.CENTER;

  public enum Position{
      STRETCH,
      CENTER,
      FIT,
      FILL,
      NONE;
  }

  public JImagePanel() {
      }

  public JImagePanel(String img) {
    this(new ImageIcon(img).getImage());
  }

  public JImagePanel(Image img) {

      setBackgroundImage(img);
  }

  @Override
  public void paintComponent(Graphics g)
  {
    super.paintComponent(g);

    Graphics2D g2 = (Graphics2D) g;

    g2.setColor(getBackground());
    g2.fillRect(0, 0, getWidth(), getHeight());

    if (this.position.equals(Position.STRETCH))
    { 
        if(this.img != null) g2.drawImage(img, 0, 0, getWidth(), getHeight(), null); 
    }
    else if (this.position.equals(Position.FILL) || this.position.equals(Position.FIT))
    { 
        if(this.img != null)
        {

             double scaleFactor = getScaleFactor(new Dimension(img.getWidth(null), img.getHeight(null)), getSize());
             int scaleWidth = (int) Math.round(img.getWidth(null) * scaleFactor);
             int scaleHeight = (int) Math.round(img.getHeight(null) * scaleFactor);

             //Image img_scaled = img.getScaledInstance(scaleWidth, scaleHeight, Image.SCALE_SMOOTH);
            g2.drawImage(scaleImage(img, scaleWidth, scaleHeight, getBackground()), (getWidth() - scaleWidth)/2, (getHeight() - scaleHeight)/2, scaleWidth, scaleHeight, null); 
        }
    }
    else if (this.position.equals(Position.CENTER)) { if(this.img != null) g2.drawImage(img, (getWidth() - img.getWidth(null))/2, (getHeight() - img.getHeight(null))/2, null); }
  }

  public void setBackgroundImage(String img) 
  {
      setBackgroundImage(new ImageIcon(img).getImage());
  }

  public void setBackgroundImage(Image img)
  {
        this.img = img;
        Dimension size = new Dimension(img.getWidth(null), img.getHeight(null));
        setPreferredSize(size);
        setMinimumSize(size);
        setMaximumSize(size);
        setSize(size);

        repaint();
  }

  public static double getScaleFactor(int iMasterSize, int iTargetSize) {

        double dScale = 1;
        if (iMasterSize > iTargetSize) {

            dScale = (double) iTargetSize / (double) iMasterSize;

        } else {

            dScale = (double) iTargetSize / (double) iMasterSize;

        }

        return dScale;

    }

    public double getScaleFactor(Dimension original, Dimension targetSize) {

        double dScale = 1d;

        if (original != null && targetSize != null) {

            double dScaleWidth = getScaleFactor(original.width, targetSize.width);
            double dScaleHeight = getScaleFactor(original.height, targetSize.height);

            if (this.position.equals(Position.FIT)) dScale = Math.min(dScaleHeight, dScaleWidth);
            else if(this.position.equals(Position.FILL)) dScale = Math.max(dScaleHeight, dScaleWidth);

        }

        return dScale;

    }

    public BufferedImage scaleImage(Image img, int width, int height, Color background) {

        BufferedImage newImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
        Graphics2D g = newImage.createGraphics();
        try {
            g.setRenderingHint(RenderingHints.KEY_INTERPOLATION,
                    RenderingHints.VALUE_INTERPOLATION_BICUBIC);
            g.setBackground(background);
            g.clearRect(0, 0, width, height);
            g.drawImage(img, 0, 0, width, height, null);
        } finally {
            g.dispose();
        }
        return newImage;
    }

  public void setBackgroundImagePosition(String pos)
  {
      if("Stretch".equals(pos)) setBackgroundImagePosition(Position.STRETCH);
      else if("Center".equals(pos))  setBackgroundImagePosition(Position.CENTER);
      else if("Fit".equals(pos)) setBackgroundImagePosition(Position.FIT);
      else if("Fill".equals(pos))  setBackgroundImagePosition(Position.FILL);
      else if("None".equals(pos)) setBackgroundImagePosition(Position.NONE);
  }
  public void setBackgroundImagePosition(Position pos)
  {
      this.position = pos;
      repaint();
  }

  public static void main(String[] args)
  {

      JFrame frame = new JFrame("JImagePanel Test");
      frame.setSize( Toolkit.getDefaultToolkit().getScreenSize());
      frame.setPreferredSize( Toolkit.getDefaultToolkit().getScreenSize());
      frame.setExtendedState(JFrame.MAXIMIZED_BOTH); //sets appropriate size for frame

      JImagePanel panel = new JImagePanel();
      frame.add(panel);

      frame.setVisible(true);

      try {Thread.sleep(2000);} catch (InterruptedException e) {}

      panel.setBackgroundImage("C:\\Users\\David\\Pictures\\Wood.jpg");
      panel.setBackgroundImagePosition(JImagePanel.Position.STRETCH);

      panel.revalidate(); // need to revalidate()
      panel.repaint(); //doesnt work by itself

      try {Thread.sleep(2000);} catch (InterruptedException e) {}

      panel.setBackgroundImage("C:\\Users\\David\\Pictures\\Wood.jpg");
      panel.setBackgroundImagePosition(JImagePanel.Position.FIT);

      frame.setVisible(true); //also works --why?

  }

}

Answer 1

在已经可见的JFrame 上调用setVisible(true) 对您有效，因为这最终会在内部调用validate()，进而重新验证框架中的所有子组件。

要了解原因，请参考Component.setVisible(boolean b)的实现：

public void setVisible(boolean b) {
    show(b);
}

public void show(boolean b) {
    if (b) {
        show();
    } else {
        hide();
    }
}

但是show() 方法在Window 中被覆盖（其中JFrame 是一个子类）。所以这最终会调用Window.show()：

public void show() {
    if (peer == null) {
        addNotify();
    }
    validate();
    [...]

希望这能解释您所看到的行为。

Answer 2

假设您已经完成了 imagePanel 的中途清理：

• 让面板根据需要自行重新验证/绘制（与应用程序代码相比）
• do not call setXXSize，从来没有，甚至不在面板内部

这就是更改您的设置器并覆盖 getXXSize。请注意，仅基于图像大小的尺寸提示不是您想要在现实世界代码中执行的操作，您可能还需要考虑 super 的提示（如果面板有孩子，则为 fi）

@Override
public Dimension getPreferredSize() {
    if (img != null) {
        return new Dimension(img.getWidth(this), img.getHeight(this));
    }
    return super.getPreferredSize();
}

public void setBackgroundImage(Image img) {
    this.img = img;
    // need to revalidate as our sizing hints might have changed
    revalidate();
    repaint();
}

public void setBackgroundImagePosition(Position pos) {
    this.position = pos;
    repaint();
}

使用它的应用程序代码现在只调用设置器，没有别的：

    JFrame frame = new JFrame("JImagePanel Test");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    // frame.setLayout(new FlowLayout()); // just to see the effect of a pref-respecting layout
    final JImagePanel panel = new JImagePanel();
    frame.add(panel);

    final Image[] images = new Image[]{
            XTestUtils.loadDefaultImage(), XTestUtils.loadDefaultImage("500by500.png"), null};
    Action toggleImage = new AbstractAction("toggle image") {
        int index = 0;
        @Override
        public void actionPerformed(ActionEvent e) {
            panel.setBackgroundImage(images[index]);
            index = (index +1) % images.length;
        }
    };
    Action togglePosition = new AbstractAction("toggle position") {
        int index = 0;
        @Override
        public void actionPerformed(ActionEvent e) {
            panel.setBackgroundImagePosition(Position.values()[index]);
            index = (index +1) % Position.values().length;
        }
    };
    frame.add(new JButton(toggleImage), BorderLayout.NORTH);
    frame.add(new JButton(togglePosition), BorderLayout.SOUTH);
    // size for frame
    //frame.setSize(800, 800);
    frame.setExtendedState(JFrame.MAXIMIZED_BOTH); //sets appropriate
    frame.setVisible(true);