toFront()抛出UnsupportedOperationException [英] toFront() throwing UnsupportedOperationException
问题描述
我想创建一个StackPane，无论添加什么内容，我选择的节点始终位于最前面(而不是要注意添加顺序，也不必记住在toFront()上调用该特定节点，只要添加了任何内容.)
I want to create a StackPane where, whatever is added, a node of my choice is always at the front (rather than having to be careful about the order I add things in, or remembering to call toFront() on that particular node whenever anything is added.)
为此，我只是将一个侦听器放在相关的StackPane对象的子列表上，这样，只要发生任何更改，它都会在相关的节点上调用toFront()，例如:
In order to do this, I simply place a listener on the child list of the relevant StackPane object, so that whenever anything changes, it calls toFront() on the relevant node, for example:
public class Test extends Application {
@Override
public void start(Stage stage) {
StackPane root = new StackPane();
final Rectangle r1 = new Rectangle(50, 50);
root.getChildren().add(r1);
root.getChildren().addListener(new ListChangeListener<Node>() {
@Override
public void onChanged(ListChangeListener.Change<? extends Node> change) {
try {
while(change.next()) {
if(change.wasAdded()) {
r1.toFront();
}
}
}
catch(Exception ex) {
ex.printStackTrace();
}
}
});
root.getChildren().add(new Rectangle(50, 50));
stage.setScene(new Scene(root));
stage.show();
}
}
在Java 7中，这很好用.但是，在JFX8(最新下载的最新版本)中，它失败并显示以下内容:
In Java 7, this works just fine. However, in JFX8 (latest build downloaded just now), it fails with the following:
java.lang.UnsupportedOperationException
at java.util.Collections$UnmodifiableList.add(Collections.java:1374)
at javafx.collections.ListChangeBuilder.nextRemove(ListChangeBuilder.java:208)
at javafx.collections.ObservableListBase.nextRemove(ObservableListBase.java:150)
at javafx.collections.ModifiableObservableListBase.remove(ModifiableObservableListBase.java:181)
at com.sun.javafx.collections.VetoableListDecorator.remove(VetoableListDecorator.java:284)
at com.sun.javafx.collections.VetoableListDecorator.remove(VetoableListDecorator.java:209)
at javafx.scene.Parent.impl_toFront(Parent.java:624)
at javafx.scene.Node.toFront(Node.java:1713)
at test.Test$1.onChanged(Test.java:34)
at com.sun.javafx.collections.ListListenerHelper$Generic.fireValueChangedEvent(ListListenerHelper.java:315)
at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:72)
at com.sun.javafx.collections.VetoableListDecorator$1.onChanged(VetoableListDecorator.java:77)
at com.sun.javafx.collections.ListListenerHelper$Generic.fireValueChangedEvent(ListListenerHelper.java:315)
at com.sun.javafx.collections.ListListenerHelper.fireValueChangedEvent(ListListenerHelper.java:72)
at javafx.collections.ObservableListBase.fireChange(ObservableListBase.java:233)
at javafx.collections.ListChangeBuilder.commit(ListChangeBuilder.java:482)
at javafx.collections.ListChangeBuilder.endChange(ListChangeBuilder.java:541)
at javafx.collections.ObservableListBase.endChange(ObservableListBase.java:205)
at javafx.collections.ModifiableObservableListBase.add(ModifiableObservableListBase.java:155)
at java.util.AbstractList.add(AbstractList.java:108)
at com.sun.javafx.collections.VetoableListDecorator.add(VetoableListDecorator.java:200)
at test.Test.start(Test.java:41)
at com.sun.javafx.application.LauncherImpl$8.run(LauncherImpl.java:837)
at com.sun.javafx.application.PlatformImpl$7.run(PlatformImpl.java:331)
at com.sun.javafx.application.PlatformImpl$6$1.run(PlatformImpl.java:297)
at com.sun.javafx.application.PlatformImpl$6$1.run(PlatformImpl.java:294)
at java.security.AccessController.doPrivileged(Native Method)
at com.sun.javafx.application.PlatformImpl$6.run(PlatformImpl.java:294)
at com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:95)
at com.sun.glass.ui.win.WinApplication._runLoop(Native Method)
at com.sun.glass.ui.win.WinApplication.access$300(WinApplication.java:39)
at com.sun.glass.ui.win.WinApplication$4$1.run(WinApplication.java:112)
at java.lang.Thread.run(Thread.java:744)
是的，test.Test$1.onChanged(Test.java:34)确实是指r1.toFront();.
这被认为是错误，还是我试图通过这种方式实现目标而违反了我不知道的某些规则?我确实想知道在执行onChanged()方法时列表是否仍在更改，并且toFront()还会更改列表内容，因此是例外-但是onChanged()的Javadoc明确指出:
Is this to be considered a bug, or am I breaking some rule I'm unaware of by trying to achieve things this way? I did wonder whether the list was still being changed while the onChanged() method was executing, and toFront() would also change the list contents, hence the exception - but the Javadoc to onChanged() clearly says:
在之后被称为对ObservableList的更改.
Called after a change has been made to an ObservableList.
(粗体是我的.)
At this point I'm more certain that it's a bug, so the related bug report is here.
推荐答案
似乎不允许您在事件处理程序内修改(JavaFX)列表，该事件处理程序当前正在处理同一列表的另一个(先前)修改事件.尽管这似乎是合理的，但这不是不言而喻的，因此在这种情况下应该有一个更明显的例外.
It seems that you are not allowed to modify a (JavaFX) list inside an event handler that is currently handling another (previous) modification event of the same list. Although this seems reasonable it is not self-evident, so there should be a more obvious exception in that case.
不幸的是，非偷看异常在JavaFX中非常常见.
Unfortunately non-speeking exceptions are very common in JavaFX.
幸运的是，解决方案/解决方案非常简单:通过Platform.runLater()调用您的修改代码(此处为r1.toFront)，这将延迟您的修改在发生原始事件之后发生:
Fortunately the solution/workaround is pretty easy: Call your modifying code (here: r1.toFront) by Platform.runLater(), it will delay your modification to happen after the originating event:
root.getChildren().addListener(new ListChangeListener<Node>() {
@Override
public void onChanged(ListChangeListener.Change<? extends Node> change) {
Platform.runLater(new Runnable() {
public void run() { r1.toFront(); }
});
}
});
侧面注释:toFront不执行任何操作(如果组件已经在前面).这样可以防止无限循环.但是，由于在文档中没有明确提及，因此您可能不依赖于此.
Sidenote: toFront does nothing, if the component is already at front. This prevents infinite loops. Nevertheless, as this is not explicitely mentioned in the documentation, you might not rely on that.
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