当按下键时，GWT onPreviewNativeEvent运行2次

Question

当我按下ESC键时，我使用该事件显示消息，但它会显示警报两次。错误在哪里？

Event.addNativePreviewHandler(new Event.NativePreviewHandler() {
    @Override
    public void onPreviewNativeEvent(Event.NativePreviewEvent event) {
        NativeEvent nativeEvent = event.getNativeEvent();
            if( nativeEvent.getKeyCode() == 27){
            Windows.alert("HELLO");
        }
        }
    });

Answer 1

它会触发KeyDown和KeyUp事件。你应该这样做：

public void onPreviewNativeEvent(NativePreviewEvent event) {
    NativeEvent nativeEvent = event.getNativeEvent();
    if ("keydown".equals(nativeEvent.getType())) {
        if (nativeEvent.getKeyCode() == KeyCodes.KEY_ESCAPE) {
            Windows.alert("HELLO");
        }
    }
}