拖放图像

Question

我正在制作一个用 Java 编辑图像的程序。我想知道是否可以将图像从用户桌面拖到JPanel 上，并将被拖动的图像设置为JPanel 的背景。这在Java中可能吗？如果是这样，我该怎么做？

Answer 1

简短的回答是：是的，有可能。

有几种方法可以做到这一点，我将为您提供最简单的代码：将图像文件从某处拖动到您的 Swing 应用程序。但是，您也可以将图像从图像编辑器复制到剪贴板并将其粘贴到那里，但我认为您不需要这个。要实现第二个用例，您将不得不使用DataFlavor.imageFlavor 而不是我的示例显示的DataFlavor.javaFileListFlavor。因此，要测试下面的代码，只需将MainPanel 的实例添加到容器中，然后在应用运行时将图像文件拖到面板中。

这是TransferHandler的实现：

import java.awt.Cursor;
import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.io.File;
import java.io.IOException;
import java.util.List;

import javax.swing.JComponent;
import javax.swing.TransferHandler;

public class ImageTransferHandler extends TransferHandler {
    private static final DataFlavor FILE_FLAVOR = DataFlavor.javaFileListFlavor;

    private MainPanel mainPanel;

    public ImageTransferHandler(MainPanel panel) {
        mainPanel = panel;
    }

    /**
     * The method to handle the transfer between the source of data and the
     * destination, which is in our case the main panel.
     */
    public boolean importData(JComponent c, Transferable t) {
        if (canImport(c, t.getTransferDataFlavors())) {
            if (transferFlavor(t.getTransferDataFlavors(), FILE_FLAVOR)) {
                try {
                    List<File> fileList = (List<File>) t.getTransferData(FILE_FLAVOR);
                    if (fileList != null && fileList.toArray() instanceof File[]) {
                        File[] files = (File[]) fileList.toArray();
                        mainPanel.addFiles(files);
                    }
                    return true;
                } catch (IOException e) {
                    e.printStackTrace();
                } catch (UnsupportedFlavorException ex) {
                    ex.printStackTrace();
                }
            }
        }
        return false;
    }

    /**
     * Returns the type of transfer actions to be supported.
     */
    public int getSourceActions(JComponent c) {
        return COPY_OR_MOVE;
    }

    /**
     * Specifies the actions to be performed after the data has been exported.
     */
    protected void exportDone(JComponent c, Transferable data, int action) {
        c.setCursor(Cursor.getPredefinedCursor(Cursor.DEFAULT_CURSOR));
    }

    /**
     * Returns true if the specified flavor is contained in the flavors array,
     * false otherwise.
     */
    private boolean transferFlavor(DataFlavor[] flavors, DataFlavor flavor) {
        boolean found = false;
        for (int i = 0; i < flavors.length && !found; i++) {
            found = flavors[i].equals(flavor);
        }
        return found;
    }

    /**
     * Returns true if the component can import the specified flavours, false
     * otherwise.
     */
    public boolean canImport(JComponent c, DataFlavor[] flavors) {
        for (int i = 0; i < flavors.length; i++) {
            if (FILE_FLAVOR.equals(flavors[i])) {
                return true;
            }
        }
        return false;
    }
}

这是我要添加 TransferHandler 并将图像绘制为背景的面板：

public class MainPanel extends JPanel {
    private Image image;

    MainPanel() {
        setTransferHandler(new ImageTransferHandler(this));
    }

    void addFiles(File[] files) {
        for (File file : files) {
            try {
                image = ImageIO.read(file);
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        if (image != null) {
            repaint();
        }
    }

    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.setColor(Color.BLUE);
        int width = getWidth();
        int height = getHeight();
        drawImage((Graphics2D) g, width, height);
    }

    private void drawImage(Graphics2D g2, int width, int height) {
        if (image != null) {
            g2.drawImage(image, 0, 0, width, height, null);
        } else {
            g2.drawRect(10, 10, width - 20, height - 20);
        }
    }
}