我有一个列表,其中包含滚动 Pane 中的许多元素,并且我已经在列表上实现了拖放操作。当我从列表中选择一个项目并将其拖到列表底部时,只要我将鼠标靠近边缘,列表就会自动向下滚动。这在 Windows 上工作正常,但在 Linux 上列表滚动一个元素然后停止。
这是一个揭示这个错误的简短程序:
import java.awt.BorderLayout;
import java.awt.Container;
import java.awt.Dimension;
import java.awt.datatransfer.DataFlavor;
import java.awt.datatransfer.Transferable;
import java.awt.datatransfer.UnsupportedFlavorException;
import java.io.IOException;
import javax.swing.DropMode;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.SwingUtilities;
import javax.swing.TransferHandler;
import javax.swing.WindowConstants;
public class JListAutoscroll {
protected static Container createUI() {
JList<String> jlist = new JList<>(generateData(100));
setDragAndDrop(jlist);
JScrollPane scrollPane = new JScrollPane(jlist);
JPanel panel = new JPanel(new BorderLayout());
panel.add(scrollPane, BorderLayout.CENTER);
return panel;
}
private static void setDragAndDrop(JList<String> jlist) {
jlist.setDragEnabled(true);
jlist.setDropMode(DropMode.INSERT);
jlist.setTransferHandler(new ListTransferHandler());
}
private static String[] generateData(int nRows) {
String rows[] = new String[nRows];
for (int i = 0; i < rows.length; i++) {
rows[i] = "element " + i;
}
return rows;
}
private static class ListTransferHandler extends TransferHandler {
@Override
public int getSourceActions(JComponent component) {
return COPY_OR_MOVE;
}
@Override
protected Transferable createTransferable(JComponent component) {
return new ListItemTransferable((JList)component);
}
@Override
public boolean canImport(TransferHandler.TransferSupport support) {
return true;
}
@Override
public boolean importData(TransferHandler.TransferSupport support) {
return true;
}
}
private static class ListItemTransferable implements Transferable {
private String item;
public ListItemTransferable(JList<String> jlist) {
item = jlist.getSelectedValue();
}
@Override
public DataFlavor[] getTransferDataFlavors() {
return new DataFlavor[] { DataFlavor.stringFlavor };
}
@Override
public boolean isDataFlavorSupported(DataFlavor flavor) {
return flavor.equals(DataFlavor.stringFlavor);
}
@Override
public Object getTransferData(DataFlavor flavor) throws UnsupportedFlavorException, IOException {
if(!isDataFlavorSupported(flavor)) {
throw new UnsupportedFlavorException(flavor);
}
return item;
}
}
public static void main(String args[]) {
SwingUtilities.invokeLater(new Runnable() {
@Override
public void run() {
JFrame frame = new JFrame("JList Autoscroll");
frame.setDefaultCloseOperation(WindowConstants.DISPOSE_ON_CLOSE);
frame.setContentPane(createUI());
frame.setPreferredSize(new Dimension(400, 600));
frame.pack();
frame.setVisible(true);
}
});
}
}
我已经实现了一个简单的 TransferHandler,它在放置时不执行任何操作,但足以在拖动到列表边缘时显示问题。
这似乎是 JDK 中的一个已知错误,在 this report 中对此有最好的描述。 .我看过一些建议的解决方法,例如 this one或 this one ,
但我不清楚如何实现它们。在我看来,我必须创建一个 DropTarget 子类,并且我使用它的组件应该实现 Autoscroll
接口(interface)。但是 JList
并没有实现它!此外,如果我在列表中设置 DropTarget 而不是 TransferHandler,我是否会丢失 TransferHandler 实现的所有默认拖放行为?
那么我该如何修改我的程序来解决这个错误呢?
最佳答案
如 bug description 中所述,有两个处理拖放的类:
DropTargetAutoScroller
, a member class ofjava.awt.dnd.DropTarget
, responsible of supporting components implementing theAutoscroll
interface;DropHandler
, a member class ofjavax.swing.TransferHandler
, that automates d&d autoscrolling on components implementing theScrollable
interface.
所以,确实,解决方法不适合 JList
, 它实现了 Scrollable
而不是 Autoscroll
.但是,如果您查看 DropTarget
的源代码和 TransferHandler
,您会注意到自动滚动代码基本相同,而且在这两种情况下都是错误的。解决方法也与 DropTarget
非常相似代码,只添加了几行。基本上,解决方案是将鼠标光标的位置从组件坐标系转换到屏幕坐标系。这样,在检查鼠标是否移动时,使用绝对坐标。所以我们可以从TransferHandler
复制代码而是添加这几行。
太棒了...但是我们将这段代码放在哪里以及我们如何调用它?
如果我们查看 setTransferHandler()
我们看到它实际上设置了一个 DropTarget
,这是一个名为 SwingDropTarget
的 package-private static 类来自 TransferHandler
类(class)。它将拖放事件委托(delegate)给一个私有(private)静态 DropTargetListener
称为 DropHandler
.这个类完成了拖放过程中发生的所有魔法,当然它使用了来自 TransferHandler
的其他私有(private)方法。 .这意味着我们不能只设置我们自己的 DropTarget
不会丢失 TransferHandler
中已经实现的所有内容.我们可以重写 TransferHandler
(大约 1800 行)我们添加了几行来修复错误,但这不是很现实。
一个更简单的解决方案是写一个 DropTargetListener
,其中我们简单地从 DropHandler
复制与自动滚动相关的代码(它也实现了这个接口(interface)),添加了我们的行。这是类:
import java.awt.Insets;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.Toolkit;
import java.awt.dnd.DropTargetDragEvent;
import java.awt.dnd.DropTargetDropEvent;
import java.awt.dnd.DropTargetEvent;
import java.awt.dnd.DropTargetListener;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.util.TooManyListenersException;
import javax.swing.JComponent;
import javax.swing.Scrollable;
import javax.swing.SwingConstants;
import javax.swing.SwingUtilities;
import javax.swing.Timer;
public class AutoscrollWorkaround implements DropTargetListener, ActionListener {
private JComponent component;
private Point lastPosition;
private Rectangle outer;
private Rectangle inner;
private Timer timer;
private int hysteresis = 10;
private static final int AUTOSCROLL_INSET = 10;
public AutoscrollWorkaround(JComponent component) {
if (!(component instanceof Scrollable)) {
throw new IllegalArgumentException("Component must be Scrollable for autoscroll to work!");
}
this.component = component;
outer = new Rectangle();
inner = new Rectangle();
Toolkit t = Toolkit.getDefaultToolkit();
Integer prop;
prop = (Integer)t.getDesktopProperty("DnD.Autoscroll.interval");
timer = new Timer(prop == null ? 100 : prop.intValue(), this);
prop = (Integer)t.getDesktopProperty("DnD.Autoscroll.initialDelay");
timer.setInitialDelay(prop == null ? 100 : prop.intValue());
prop = (Integer)t.getDesktopProperty("DnD.Autoscroll.cursorHysteresis");
if (prop != null) {
hysteresis = prop.intValue();
}
}
@Override
public void dragEnter(DropTargetDragEvent e) {
lastPosition = e.getLocation();
SwingUtilities.convertPointToScreen(lastPosition, component);
updateRegion();
}
@Override
public void dragOver(DropTargetDragEvent e) {
Point p = e.getLocation();
SwingUtilities.convertPointToScreen(p, component);
if (Math.abs(p.x - lastPosition.x) > hysteresis
|| Math.abs(p.y - lastPosition.y) > hysteresis) {
// no autoscroll
if (timer.isRunning()) timer.stop();
} else {
if (!timer.isRunning()) timer.start();
}
lastPosition = p;
}
@Override
public void dragExit(DropTargetEvent dte) {
cleanup();
}
@Override
public void drop(DropTargetDropEvent dtde) {
cleanup();
}
@Override
public void dropActionChanged(DropTargetDragEvent e) {
}
private void updateRegion() {
// compute the outer
Rectangle visible = component.getVisibleRect();
outer.setBounds(visible.x, visible.y, visible.width, visible.height);
// compute the insets
Insets i = new Insets(0, 0, 0, 0);
if (component instanceof Scrollable) {
int minSize = 2 * AUTOSCROLL_INSET;
if (visible.width >= minSize) {
i.left = i.right = AUTOSCROLL_INSET;
}
if (visible.height >= minSize) {
i.top = i.bottom = AUTOSCROLL_INSET;
}
}
// set the inner from the insets
inner.setBounds(visible.x + i.left,
visible.y + i.top,
visible.width - (i.left + i.right),
visible.height - (i.top + i.bottom));
}
@Override
public void actionPerformed(ActionEvent e) {
updateRegion();
Point componentPosition = new Point(lastPosition);
SwingUtilities.convertPointFromScreen(componentPosition, component);
if (outer.contains(componentPosition) && !inner.contains(componentPosition)) {
autoscroll(componentPosition);
}
}
private void autoscroll(Point position) {
Scrollable s = (Scrollable) component;
if (position.y < inner.y) {
// scroll upward
int dy = s.getScrollableUnitIncrement(outer, SwingConstants.VERTICAL, -1);
Rectangle r = new Rectangle(inner.x, outer.y - dy, inner.width, dy);
component.scrollRectToVisible(r);
} else if (position.y > (inner.y + inner.height)) {
// scroll downard
int dy = s.getScrollableUnitIncrement(outer, SwingConstants.VERTICAL, 1);
Rectangle r = new Rectangle(inner.x, outer.y + outer.height, inner.width, dy);
component.scrollRectToVisible(r);
}
if (position.x < inner.x) {
// scroll left
int dx = s.getScrollableUnitIncrement(outer, SwingConstants.HORIZONTAL, -1);
Rectangle r = new Rectangle(outer.x - dx, inner.y, dx, inner.height);
component.scrollRectToVisible(r);
} else if (position.x > (inner.x + inner.width)) {
// scroll right
int dx = s.getScrollableUnitIncrement(outer, SwingConstants.HORIZONTAL, 1);
Rectangle r = new Rectangle(outer.x + outer.width, inner.y, dx, inner.height);
component.scrollRectToVisible(r);
}
}
private void cleanup() {
timer.stop();
}
}
(您会注意到,基本上只有 SwingUtilities.convertXYZ() 调用是来自 TransferHandler
代码的额外调用)
接下来,我们可以将此监听器添加到 DropTarget
设置 TransferHandler
时安装. (请注意,常规 DropTarget
只接受一个监听器,如果添加另一个监听器将抛出异常。SwingDropTarget
使用 DropHandler
,但幸运的是它也添加了对其他监听器的支持)
所以让我们把这个静态工厂方法添加到AutoscrollWorkaround
类,它为我们做这件事:
public static void applyTo(JComponent component) {
if (component.getTransferHandler() == null) {
throw new IllegalStateException("A TransferHandler must be set before calling this method!");
}
try {
component.getDropTarget().addDropTargetListener(new AutoscrollWorkaround(component));
} catch (TooManyListenersException e) {
throw new IllegalStateException("Something went wrong! DropTarget should have been " +
"SwingDropTarget which accepts multiple listeners", e);
}
}
这提供了一种简单且非常方便的方法,只需调用这一个方法即可将解决方法应用于任何遭受此错误的组件。确保在 setTransferHandler()
之后调用它在组件上。所以,我们只需要在原来的程序中加一行:
private static void setDragAndDrop(JList<String> jlist) {
jlist.setDragEnabled(true);
jlist.setDropMode(DropMode.INSERT);
jlist.setTransferHandler(new ListTransferHandler());
AutoscrollWorkaround.applyTo(jlist); // <--- just this line added
}
自动滚动现在可以在 Windows 和 Linux 上正常运行。 (尽管在 Linux 上,放置位置的行在自动滚动工作之前不会重新绘制,但是哦,好吧。)
此解决方法也适用于 JTable
(我测试过),JTree
以及可能实现 Scrollable
的任何组件.
关于java - 在 Linux 上拖放错误期间如何实现自动滚动的解决方法?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/29126758/