本文介绍了调整JPanel大小后，组件消失的处理方法，对大家解决问题具有一定的参考价值，需要的朋友们下面随着小编来一起学习吧！
问题描述
我正在尝试使用鼠标点击后出现的可拖动十字架创建JPanel。一切正常，但是当我调整JPanel的大小时，十字架就会消失。我试图覆盖JPanel中的paintComponent方法，但是所有的十字都在坐标（0,0）处。我该如何解决？
I am trying to create JPanel with draggable crosses which appear after mouse clicking. Everything works fine but when I resize the JPanel the crosses disappear. I tried to override the paintComponent method in my JPanel but then all crosses are at coordinates (0,0). How can I fix it?
import java.awt.*;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionAdapter;
import java.util.ArrayList;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class CrossPanel extends JPanel implements MouseListener {
private int orderOfCross = 0;
private ArrayList<Cross> crosses;
private int defaultSizeOfCrosses = 10;
CrossPanel() {
setOpaque(false);
addMouseListener(this);
crosses = new ArrayList<Cross>();
}
@Override
public void mouseClicked(MouseEvent e) {
int x = e.getX();
int y = e.getY();
Cross cross = new Cross(orderOfCross++, defaultSizeOfCrosses);
crosses.add(cross);
cross.setLocation(x - defaultSizeOfCrosses, y - defaultSizeOfCrosses);
add(cross);
repaint();
}
@Override
public void paintComponent(Graphics g) {
super.paintComponent(g);
// for (int i = 0; i < crosses.size(); i++) {
// crosses.get(i).paint(g);
// }
}
@Override
public void mousePressed(MouseEvent e) {}
@Override
public void mouseReleased(MouseEvent e) {}
@Override
public void mouseEntered(MouseEvent e) {}
@Override
public void mouseExited(MouseEvent e) {}
public static void main(String[] args) {
JFrame f = new JFrame();
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
CrossPanel crossPane = new CrossPanel();
f.getContentPane().add(crossPane);
f.setSize(600, 500);
f.setLocation(200, 200);
f.setVisible(true);
}
}
class Cross extends JComponent {
private int order;
protected Cursor draggingCursor = Cursor.getPredefinedCursor(Cursor.HAND_CURSOR);
private volatile int draggedAtX, draggedAtY;
int size;
public Cross(int order, int size) {
this.order = order;
this.size = size;
this.setBounds(0, 0, 4 * size, 3 * size + 10);
addDragListeners();
setCursor(draggingCursor);
}
@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
g2.setColor(Color.red);
g2.setStroke(new BasicStroke(3));
g2.drawLine(0, size, size + size, size);
g2.drawLine(size, 0, size, size + size);
Font f = new Font("Monospaced", Font.BOLD, size + 10);
g2.setFont(f);
g2.drawString(String.valueOf(order), size - size / 2, 3 * size + 10);
}
private void addDragListeners() {
addMouseListener(new MouseAdapter() {
@Override
public void mousePressed(MouseEvent e) {
draggedAtX = e.getX();
draggedAtY = e.getY();
}
});
addMouseMotionListener(new MouseMotionAdapter() {
@Override
public void mouseDragged(MouseEvent e) {
Point newLocation = new Point(e.getX() - draggedAtX + getLocation().x, e.getY() - draggedAtY + getLocation().y);
setLocation(newLocation);
}
});
}
}
推荐答案
我很少看到用于 null 的布局，对于所有感知的好处，只有很多人退缩。
I very rarely see a use for a null layout, for all the perceived benefits, there are simply to many draw backs.
整个Swing API都是围绕布局管理器的设计而设计的，所以你会疯狂（恕我直言）简单地抛弃所有的工作。
The entire Swing API has been designed around the use of layout managers so you'd be crazy (IMHO) to simply throw all that work away.
如果你发现你自己处于一个可用的布局经理看起来不像你想做的那样的位置，你可能更有价值的时候自己写。
If you find yourself in a position where the available layout managers don't seem to do what you want, it might be more worth while to write you own.
在这里，我是提出了 PropertionalLayoutManager ，其目的是提供布局功能，将基于父组件的宽度/高度的百分比将组件放置在容器上。这意味着，随着父组件的大小调整，子组件将以父级大小的百分比重新定位。
Here, I've presented a PropertionalLayoutManager whose intention is to provide layout capabilities that will place components on a container based an percentage of the width/height of the parent component. This means, as the parent component is resized, the child components will reposition themselves at a percentage of the parent size.
import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Component;
import java.awt.Container;
import java.awt.Cursor;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.FontMetrics;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.LayoutManager2;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionAdapter;
import java.text.NumberFormat;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JPanel;
public class CrossPanel extends JPanel implements MouseListener {
private int orderOfCross = 0;
private ArrayList<Cross> crosses;
private int defaultSizeOfCrosses = 10;
CrossPanel() {
setOpaque(false);
addMouseListener(this);
crosses = new ArrayList<Cross>();
setLayout(new PropertionalLayoutManager());
}
@Override
public void mouseClicked(MouseEvent e) {
int x = e.getX();
int y = e.getY();
Cross cross = new Cross(orderOfCross++, defaultSizeOfCrosses);
float xPos = (float)x / (float)getWidth();
float yPos = (float)y / (float)getHeight();
crosses.add(cross);
add(cross, new PropertionalConstraints(xPos, yPos));
revalidate();
}
public static String format(float value) {
return NumberFormat.getNumberInstance().format(value);
}
@Override
public void mousePressed(MouseEvent e) {
}
@Override
public void mouseReleased(MouseEvent e) {
}
@Override
public void mouseEntered(MouseEvent e) {
}
@Override
public void mouseExited(MouseEvent e) {
}
public static void main(String[] args) {
JFrame f = new JFrame();
f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
CrossPanel crossPane = new CrossPanel();
f.getContentPane().add(crossPane);
f.setSize(600, 500);
f.setLocation(200, 200);
f.setVisible(true);
}
public class Cross extends JComponent {
private int order;
protected Cursor draggingCursor = Cursor.getPredefinedCursor(Cursor.HAND_CURSOR);
private volatile int draggedAtX, draggedAtY;
int size;
public Cross(int order, int size) {
this.order = order;
this.size = size;
// this.setBounds(0, 0, 4 * size, 3 * size + 10);
addDragListeners();
setCursor(draggingCursor);
Font f = new Font("Monospaced", Font.BOLD, size + 10);
setFont(f);
}
@Override
public Dimension getPreferredSize() {
// This is dangrous, you are making assumptions about platforms
// that you have no eviednce to support.
FontMetrics fm = getFontMetrics(getFont());
return new Dimension(Math.max(fm.stringWidth(String.valueOf(order)), size), size + fm.getHeight());
}
@Override
protected void paintComponent(Graphics g) {
super.paintComponent(g);
Graphics2D g2 = (Graphics2D) g;
g2.setColor(Color.red);
g2.setStroke(new BasicStroke(3));
FontMetrics fm = g2.getFontMetrics();
int width = getWidth() - 1;
int height = getHeight() - 1;
int x = (width - fm.stringWidth(String.valueOf(order))) / 2;
int y = fm.getAscent();
g2.drawString(String.valueOf(order), x, y);
int crossSize = Math.min(width, height - fm.getHeight());
x = (width - crossSize) / 2;
y = fm.getHeight();
g2.drawLine(x, y, x + crossSize, y + crossSize);
g2.drawLine(x + crossSize, y, x, y + crossSize);
}
private void addDragListeners() {
addMouseListener(new MouseAdapter() {
@Override
public void mousePressed(MouseEvent e) {
draggedAtX = e.getX();
draggedAtY = e.getY();
}
});
addMouseMotionListener(new MouseMotionAdapter() {
@Override
public void mouseDragged(MouseEvent e) {
Point newLocation = new Point(e.getX() - draggedAtX + getLocation().x, e.getY() - draggedAtY + getLocation().y);
setLocation(newLocation);
}
});
}
}
public class PropertionalConstraints {
private float x;
private float y;
public PropertionalConstraints(float x, float y) {
this.x = x;
this.y = y;
}
public float getX() {
return x;
}
public float getY() {
return y;
}
public void setX(float x) {
if (x > 1f) {
x = 1f;
} else if (x < -0f) {
x = 0f;
}
this.x = x;
}
public void setY(float y) {
if (y > 1f) {
y = 1f;
} else if (y < -0f) {
y = 0f;
}
this.y = y;
}
}
public class PropertionalLayoutManager implements LayoutManager2 {
private Map<Component, PropertionalConstraints> mapConstraints;
public PropertionalLayoutManager() {
mapConstraints = new HashMap<>(25);
}
public PropertionalConstraints getConstraintsFor(Component comp) {
return mapConstraints.get(comp);
}
public void setConstraintsFor(Component comp, PropertionalConstraints pc) {
mapConstraints.put(comp, pc);
}
@Override
public void addLayoutComponent(Component comp, Object constraints) {
if (constraints instanceof PropertionalConstraints) {
mapConstraints.put(comp, (PropertionalConstraints) constraints);
} else {
throw new IllegalArgumentException("Constraints must be PropertionalConstraints");
}
}
@Override
public Dimension maximumLayoutSize(Container target) {
return preferredLayoutSize(target);
}
@Override
public float getLayoutAlignmentX(Container target) {
return 0.5f;
}
@Override
public float getLayoutAlignmentY(Container target) {
return 0.5f;
}
@Override
public void invalidateLayout(Container target) {
}
@Override
public void addLayoutComponent(String name, Component comp) {
}
@Override
public void removeLayoutComponent(Component comp) {
mapConstraints.remove(comp);
}
@Override
public Dimension preferredLayoutSize(Container parent) {
return parent.getSize();
}
@Override
public Dimension minimumLayoutSize(Container parent) {
return preferredLayoutSize(parent);
}
@Override
public void layoutContainer(Container parent) {
int width = parent.getWidth();
int height = parent.getHeight();
for (Component comp : parent.getComponents()) {
PropertionalConstraints con = mapConstraints.get(comp);
if (con != null) {
int x = (int)(width * con.getX());
int y = (int)(height * con.getY());
comp.setSize(comp.getPreferredSize());
comp.setLocation(x, y);
} else {
comp.setBounds(0, 0, 0, 0);
}
}
}
}
}
在某些附注中，您使用魔术数字来确定某些元素的大小和渲染位置。这是一个非常糟糕的主意。您应该，特别是在绘画或打印时，将所有这些值基于经验值。
On some side notes, you are using "magic" numbers to determine the size and rendering position of certain elements. This is a very bad idea. You should, especially when painting or printing, base all these values on empirical values.
在此示例中，我已恢复使用 FontMertrics 提供所需信息，以更准确地计算各种元素的大小和位置。这将允许更好的跨平台支持，因为并非所有平台上的所有字体都呈现相同;）
In this example, I've reverted to using FontMertrics to provide the required information to more accurately calculate the size and positions of various elements. This will allow for better cross platform support, as not all fonts are rendered the same across all platforms ;)
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