Реализация X EventHandler присоединена к Панели и слушает все MouseEvents. Конечно, X имеет метод handle(), который получает MouseEvents из потока приложений JavaFX.
Панель содержит прямоугольник. Когда панель получает MouseEvent.MOUSE_CLICK на прямоугольник, X делает две вещи:
Удаляет прямоугольник с панели, затем сразу добавляет еще один (это может вызвать дополнительные события.
продолжается некоторая произвольная обработка
Вот вопрос:
Ожидается ли обработка на шаге 2 до до до каких-либо дальнейших событий, передаваемых в X через handle() Потоком приложений JavaFX? Отмечая, что Шаг 1 может вызвать дополнительные события!
Просто ищу ответы да или нет. И аргументация за ваш ответ тоже будет хороша!
Я должен добавить, что больше нигде не задействованы какие-либо другие потоки, включая «произвольную обработку».
Редактировать:
Пример кода
package bareBonesJavaFXBugExample;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.Label;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.Pane;
import javafx.stage.Stage;
/**
* An {@link Application} with one {@link Pane} containing one {@link Label}.
* The {@link Label} has a single {@link javafx.event.EventHandler},
* {@link LabelEventHandler} which processes all {@link MouseEvent}s the {@link Label}
* receives.
*
* To trigger the bug, run the application, then spend a second mouse over the
* little label in the upper left hand corner of the screen. You will see output to
* standard I/O. Then, click the label, which will then disppear. Check the I/O for
* Strings ending in debugCounter is 1.
*
* What that String means and how it proves that the JavaFX Application Thread has
* become reentrant is explained in the javadoc of {@link LabelEventHandler}.
*/
public class JavaFXAnomalyBareBonesApplication extends Application
{
public void start(Stage primaryStage)
{
Pane mainPane = new Pane();
mainPane.setMinHeight(800);
mainPane.setMinWidth(800);
Label label = new Label(" this is quite a bug !!!!");
LabelEventHandler labelEventHandler = new LabelEventHandler(mainPane, label);
label.addEventHandler(MouseEvent.ANY, labelEventHandler);
mainPane.getChildren().add(label);
Scene scene = new Scene(mainPane);
primaryStage.setScene(scene);
primaryStage.show();
}
/**
* The entry point of application.
*
* @param args
* the input arguments
*/
public static void main(String[] args) {
launch(args);
}
}
и вот его единственная зависимость, класс EventListener. Я включил достаточно javadoc, чтобы программа имела смысл. :
package bareBonesJavaFXBugExample;
import javafx.event.Event;
import javafx.event.EventHandler;
import javafx.scene.control.Label;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.Pane;
import java.util.Collection;
import java.util.ConcurrentModificationException;
/**
* An {@link EventHandler} implementation for {@link MouseEvent}s.
* This implementation's {@link EventHandler#handle(Event)} shows the
* relevant debug information to standard output before and after removing
* the member {@link #label} from the {@link #pane}.
*
* <b>discussion</b><br></br>
* <p>
* Users should first satisfy themselves that the value of
* {@link LabelEventHandler#debugCounter} can only be non-zero, in fact 1
* (one) in the method {@link LabelEventHandler#showDebugInformation(String)}
* if the method {@link LabelEventHandler#handle(MouseEvent)} has been
* re-entered recursively, that is, before a previous invocation of
* {@link LabelEventHandler#handle(MouseEvent)} has returned.
* <p>
* Proof:
* 1) <code>debugCounter</code> starts at value 0 (zero).
* 2) <code>debugCounter</code> is only incremented once, by 1 (one), and that
* is after the first call to {@link LabelEventHandler#showDebugInformation(String)}
* has returned.
* 3) <code>debugCounter</code> is only decremented once, by 1 (one) and that
* is before the last call to {@link LabelEventHandler#showDebugInformation(String)}.
* 4) however, because <code>debugCounter</code> is a class variable
* (it's static), if handle() is recurvsively re-entered then it's
* value can be 1 (one) when the re-entrant
*
* Thread executes {@link LabelEventHandler#showDebugInformation(String)}
*
* End proof.
*
* The output of this method to standard I/O is volumnious but searching the
* output for the exact String "debugCounter is 1" will immediately show the
* {@link LabelEventHandler#handle(MouseEvent)} method to have been recursively
* entered.
*
* Some other possibilities other than the JavaFX Application Thread recursing
* into {@code handle()} need to be addressed.
* One is the fact that the compiler is free to reorder statements if it can
* prove that such a reordering would have no effect on the program's correctness.
*
* So somehow the compiler is reordering the increment/decrement of
* {@code debugCounter} and the calls to {@code showDebugInformation}.
* But this would alter the correctness of the program, so this cannot be the case,
* or the compiler is making an error.
*
* Another is the fact that I/O is not instantaneous and can appear to standard
* output later than it actually was executed.
* This is something often seen in debug stack traces, where the output is
* broken up or interleaved by the output of the stack trace even though the
* two sets of statments, i/o and stack trace i/o, were strictly ordered in execution.
* But this can't account for the value of {@code debugCounter}, so it can't
* be the reason "debugCounter is 1" appears in output.
*
* In fact we can make this recursive behaviour more obviously consequential
* to the correctness of the program. If {@code handle() } is being
* recursively re-entered, then we can force a
* {@link ConcurrentModificationException} on a {@link Collection}.
* If we try to invoke {@link Collection#add(Object)} to a {@link Collection}
* while it is being iterated through, then a {@link ConcurrentModificationException}
* will be thrown.
*
* If we re-write this program slightly to first add or remove to or from a
* {@link Collection} then iterate through that {@link Collection} within the
* scope of execution of {@code handle()}, <em>and</em> {@code handle()}
* is being recursively invoked, then we may see a {@link ConcurrentModificationException}.
*
* Two other instances of this same basic program exist at the link provided.
* They are named {@link JavaFXAnomalySimpleVersionApplication} and
* {@link JavaFXAnomalyComplexVersionApplication} which is written to throw a
* {@link ConcurrentModificationException} when the JavaFX Application Thread
* becomes reentrant.
*
* I also have a screen grab (not included here) of the stack trace at a
* specific moment <code>handle()/code> is being invoked, and it can clearly
* be seen that the previous executing line was within the scope of execution
* of the previous invocation of <code>handle()</code>.
*
* In the .zip file at the link there is a readme.txt. In that file.
* I present the two lines of code which need to be added, and where
* they need to be added, so as to generate the same stack trace
* showing the same thing.
*/
public class LabelEventHandler implements EventHandler<MouseEvent> {
/**
* a counter which acts as a recursion detector.
* If {@link #handle(MouseEvent)} is never recursively invoked by
* the JavaFX Application Thread, then it's value will never be other
* than 0 (zero) in {@link #showDebugInformation(String)}.
*/
private static int debugCounter;
/**
* The {@link Label} which will disappear when clicked. This causes
* a MOUSE_EXITED_TARGET event top be fired and that in turn causes
* the JavaFX Event Dispatch Thread to recurse into this class's
* {@link #handle(MouseEvent)}
*/
private Label label;
/**
* The {@link Pane} which contains the {@link Label}. The
* {@link Label} is removed from this {@link Pane}.
*/
private final Pane pane;
/**
* Assign the values to the members {@link Pane} and {@link Label}
*/
public LabelEventHandler(Pane pane, Label label) {
this.pane = pane;
this.label = label;
}
/**
* Causes the member {@link #label} to be removed as a child of the
* member {@link #pane}.
*
* @param mouseEvent the {@link MouseEvent} received from the
* JavaFX Application Thread from the {@link Label} which this
* {@link EventHandler} is listening to.
*/
@Override
public void handle(MouseEvent mouseEvent) {
// debug can only every be 0 (zero) at this point
showDebugInformation("ENTERING");
debugCounter++;
if (mouseEvent.getEventType().equals(MouseEvent.MOUSE_PRESSED)
&& mouseEvent.isPrimaryButtonDown()) {
pane.getChildren().remove(label);
}
debugCounter--;
// debug can only every be 0 (zero) at this point
showDebugInformation("EXITING");
}
/**
* Displays two values to standard output. The first is a
* {@link String} indicating whether the
* {@link LabelEventHandler#handle(MouseEvent)} method is
* being entered or exited and the second is the value of
* {@link LabelEventHandler#debugCounter} at the time this
* method is executed.
*
* @param enterOrExit the string ENTERING or EXITING
* reflecting the point at which this method was invoked
* by {@link LabelEventHandler#handle(MouseEvent)}.
*/
private void showDebugInformation(String enterOrExit) {
System.out.println();
System.out.print(enterOrExit + " method handle");
System.out.print(" and debugCounter is " + debugCounter);
System.out.println();
}
}