问题描述
我正在尝试将拖放操作放到我的程序中；我发现以下示例说明了我正在尝试做的很多事情：
package sandBox;
import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Component;
import java.awt.Container;
import java.awt.Cursor;
import java.awt.Dimension;
import java.awt.GridLayout;
import java.awt.LayoutManager;
import java.awt.Point;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JlayeredPane;
import javax.swing.JPanel;
/**
* Example showing the use of a JlayeredPane to implement dragging an object
* across a JPanel containing other objects.
* <P>
* Basic idea: Create a JlayeredPane as a container,then put the JPanel containing
* the application's components or whatever in the JlayeredPane.DEFAULT_LAYER layer of that layered pane.
* The code is going to drag a JComponent object by calling JComponent.setPosition(x,y)
* on the component. When a mouse is clicked on the panel to start the dragging,put the
* component on the drag layer of the layered pane; as it is dragged,continue to call
* setPosition to move it. When the mouse is released,use the x.y position of the release
* to decide what to do with it next.
*
*/
public class ChessBoard extends JFrame implements MouseListener,MouseMotionListener
{
private static final long serialVersionUID = 1L;
JlayeredPane layeredPane;
JPanel chessBoard;
JLabel chesspiece;
int xAdjustment;
int yAdjustment;
public ChessBoard()
{
Dimension boardSize = new Dimension(600,600);
// Use a layered Pane for this application
layeredPane = new JlayeredPane();
layeredPane.setPreferredSize( boardSize );
layeredPane.addMouseListener( this );
layeredPane.addMouseMotionListener( this );
getContentPane().add(layeredPane);
//debug
LayoutManager lm = layeredPane.getLayout();
System.out.println("layered pane layout name is " + (lm == null? "<null>" : lm.getClass().getName()));
// Add a chess board to the layered Pane on the DEFAULT layer
chessBoard = new JPanel();
chessBoard.setLayout( new GridLayout(8,8) );
chessBoard.setPreferredSize( boardSize );
chessBoard.setBounds(0,boardSize.width,boardSize.height);
layeredPane.add(chessBoard,JlayeredPane.DEFAULT_LAYER);
// Build the Chess Board squares
// We use an 8x8 grid,and put a JPanel with BorderLayout on each square.
for (int i = 0; i < 8; i++)
{
for (int j = 0; j < 8; j++)
{
JPanel square = new JPanel( new BorderLayout() );
square.setBackground( (i + j) % 2 == 0 ? Color.gray : Color.white );
chessBoard.add( square );
}
}
// Add a few pieces to the board
// we do this with an ImageIcon that gets added to the square's panel.
ImageIcon duke = new ImageIcon("granary.gif"); // this is the image to add to each space.
addDuke(duke,0);
addDuke(duke,6);
addDuke(duke,15);
addDuke(duke,20);
}
private void addDuke(ImageIcon duke,int boardPosition)
{
JLabel pieceLabel = new JLabel(duke);
JPanel piecePanel = (JPanel)chessBoard.getComponent(boardPosition);
piecePanel.add(pieceLabel);
}
/*
** Add the selected chess piece to the dragging layer so it can be moved
*/
public void mousepressed(MouseEvent e)
{
// get the component where the user pressed; iff that's not a panel,// we'll put it on the dragging layer.
chesspiece = null; // change1 swap the change1 lines
// chesspiece = new JLabel(new ImageIcon("house1x1.gif")); // change1
Component c = chessBoard.findComponentAt(e.getX(),e.getY());
if (c instanceof JPanel) return;
// get the location of the panel containing the image panel,i.e.,// the square's panel. we adjust the location to which we move the
// piece by this amount so the piece doesn't 'snap to' the cursor
// location.
Point parentLocation = c.getParent().getLocation();
xAdjustment = parentLocation.x - e.getX();
yAdjustment = parentLocation.y - e.getY();
chesspiece = (JLabel)c; // change2 - comment out
chesspiece.setLocation(e.getX() + xAdjustment,e.getY() + yAdjustment);
layeredPane.add(chesspiece,JlayeredPane.DRAG_LAYER); // evidently this removes it from the default layer also.
layeredPane.setCursor(Cursor.getPredefinedCursor(Cursor.MOVE_CURSOR));
}
/*
** Move the chess piece around
*/
public void mouseDragged(MouseEvent me)
{
if (chesspiece == null) return;
// The drag location should be within the bounds of the chess board
int x = me.getX() + xAdjustment;
int xMax = layeredPane.getWidth() - chesspiece.getWidth();
x = Math.min(x,xMax);
x = Math.max(x,0);
int y = me.getY() + yAdjustment;
int yMax = layeredPane.getHeight() - chesspiece.getHeight();
y = Math.min(y,yMax);
y = Math.max(y,0);
chesspiece.setLocation(x,y); // evidently this works for whatever layer contains the piece.
// also,the layout manager of its new home is evidently not the same as lower layers.
}
/*
** Drop the chess piece back onto the chess board
*/
public void mouseReleased(MouseEvent e)
{
layeredPane.setCursor(null);
if (chesspiece == null) return;
// Make sure the chess piece is no longer painted on the layered pane
chesspiece.setVisible(false);
layeredPane.remove(chesspiece);
chesspiece.setVisible(true);
// The drop location should be within the bounds of the chess board
int xMax = layeredPane.getWidth() - chesspiece.getWidth();
int x = Math.min(e.getX(),0);
int yMax = layeredPane.getHeight() - chesspiece.getHeight();
int y = Math.min(e.getY(),0);
Component c = chessBoard.findComponentAt(x,y);
Container parent = null;
if (c instanceof JLabel)
{
parent = c.getParent(); // there's a piece on the square already; remove it from the panel.
parent.remove(0);
}
else
{
parent = (Container)c;
}
parent.add( chesspiece ); // this adds the piece back to the default layer
parent.validate();
}
public void mouseClicked(MouseEvent e) {}
public void mouseMoved(MouseEvent e) {}
public void mouseEntered(MouseEvent e) {}
public void mouseExited(MouseEvent e) {}
public static void main(String[] args)
{
JFrame frame = new ChessBoard();
frame.setDefaultCloSEOperation( disPOSE_ON_CLOSE );
frame.setResizable( false );
frame.pack();
frame.setLocationRelativeto( null );
frame.setVisible(true);
}
}
这对于棋盘来说是可行的，即允许用户将棋盘上的任何棋子拖到另一个正方形上。
在我正在编写的应用程序中，要被拖动的项目不存在，直到用户单击启动拖动操作的内容为止。我在弄清楚如何进行创建并将其显示时遇到麻烦。
我当前的尝试是在标有“ change1”和“ change2”的行上进行；您将两行替换为“ change1”，并用“ change2”注释掉其中的一行。换句话说，在按下鼠标时创建JLabel，并（希望）拖动它。但是当我执行该操作时，图像在按下时或拖动过程中不会显示，但是确实会在拖动结束时显示在正方形上。
我在这里错过了什么？我对JlayeredPane有点困惑，javadoc说它将遵循布局规则，但是布局规则是否适用于所有层上的所有组件，还是仅适用于底层，还是适用于所有层，但又分别适用于什么呢？我不认为这是布局问题，但我不知道出什么问题了。我需要在某处进行某种UI更新吗？我以为添加组件会使面板失效。
解决方法
原始代码是在您单击棋子的情况下编写的。
现在，您要单击一个空白单元格，这将需要进行以下更改。
- 国际象棋棋盘在每个单元格中均包含JPanel。一些单元格将包含代表棋子的JLabel。 mousePressed事件中的当前逻辑期望您单击JLabel，否则将跳过某些处理。
您需要删除：
//if (c instanceof JPanel) return;
- 默认情况下，Swing组件在创建时大小为0。
您需要给它一个尺寸：
chessPiece.setSize( chessPiece.getPreferredSize() );
- 标签的定位逻辑基于找到被单击的组件相对于父组件的位置。由于没有标签，因此此逻辑现在基于相对于分层窗格的面板。
您需要调整此逻辑以使其再次相对于父面板：
//Point parentLocation = c.getParent().getLocation();
Point parentLocation = c.getLocation();
我更新的mousePressed方法如下：
public void mousePressed(MouseEvent e)
{
// get the component where the user pressed; iff that's not a panel,// we'll put it on the dragging layer.
//chessPiece = null; // change1 swap the change1 lines
chessPiece = new JLabel(new ImageIcon("dukewavered.gif")); // change1
chessPiece.setSize( chessPiece.getPreferredSize() );
Component c = chessBoard.findComponentAt(e.getX(),e.getY());
//if (c instanceof JPanel) return;
// get the location of the panel containing the image panel,i.e.,// the square's panel. we adjust the location to which we move the
// piece by this amount so the piece doesn't 'snap to' the cursor
// location.
//Point parentLocation = c.getParent().getLocation();
Point parentLocation = c.getLocation();
xAdjustment = parentLocation.x - e.getX();
yAdjustment = parentLocation.y - e.getY();
//chessPiece = (JLabel)c; // change2 - comment out
chessPiece.setLocation(e.getX() + xAdjustment,e.getY() + yAdjustment);
layeredPane.add(chessPiece,JLayeredPane.DRAG_LAYER); // evidently this removes it from the default layer also.
layeredPane.setCursor(Cursor.getPredefinedCursor(Cursor.MOVE_CURSOR));
}
请注意，以上更改将破坏能够拖动现有标签的旧功能。如果您需要同时使用这两种功能，那么将根据是单击JLabel（在这种情况下使用旧的逻辑）还是单击JPanel（在这种情况下使用新的逻辑）确定逻辑。