GWT 2.4 中的拖放

Question

我有一个实际上是图像的自定义小部件，我希望能够将它拖到 AbsolutePanel 中并每次都获取它的坐标。我想使用 GWT 2.4 中的新 DND API，但我很难实现它。有人可以提出我必须采取的基本步骤吗？

Answer 1

GWT 2.4 引入的新 DnD API 目前不支持AbsolutePanel（请参阅HasAllDragAndDropHandlers 接口的实现）。查看FocusPanel 的实现，为其他面板启用它应该不会太难。

以下是关于如何解决问题的快速概念证明：

public void onModuleLoad() {
    DragImage image = new DragImage();
    image.setUrl(Resources.INSTANCE.someImage().getSafeUri());
    final DropAbsolutePanel target = new DropAbsolutePanel();
    target.getElement().getStyle().setBorderWidth(1.0, Unit.PX);
    target.getElement().getStyle().setBorderStyle(BorderStyle.SOLID);
    target.getElement().getStyle().setBorderColor("black");
    target.setSize("200px", "200px");

    // show drag over effect
    target.addDragOverHandler(new DragOverHandler() {

        @Override
        public void onDragOver(DragOverEvent event) {
            target.getElement().getStyle().setBackgroundColor("#ffa");
        }
    });

    // clear drag over effect
    target.addDragLeaveHandler(new DragLeaveHandler() {

        @Override
        public void onDragLeave(DragLeaveEvent event) {
            target.getElement().getStyle().clearBackgroundColor();
        }
    });

    // enable as drop target
    target.addDropHandler(new DropHandler() {

        @Override
        public void onDrop(DropEvent event) {
            event.preventDefault();
            // not sure if the calculation is right, didn't test it really
            int x = (event.getNativeEvent().getClientX() - target.getAbsoluteLeft()) + Window.getScrollLeft();
            int y = (event.getNativeEvent().getClientY() - target.getAbsoluteTop()) + Window.getScrollTop();
            target.getElement().getStyle().clearBackgroundColor();
            Window.alert("x: " + x + ", y:" + y);
            // add image with same URL as the dropped one to absolute panel at given coordinates
            target.add(new Image(event.getData("text")), x, y);
        }
    });

    RootPanel.get().add(image);
    RootPanel.get().add(target);
}

这里是自定义面板

public class DropAbsolutePanel extends AbsolutePanel implements HasDropHandlers, HasDragOverHandlers,
        HasDragLeaveHandlers {

    @Override
    public HandlerRegistration addDropHandler(DropHandler handler) {
        return addBitlessDomHandler(handler, DropEvent.getType());
    }

    @Override
    public HandlerRegistration addDragOverHandler(DragOverHandler handler) {
        return addBitlessDomHandler(handler, DragOverEvent.getType());
    }

    @Override
    public HandlerRegistration addDragLeaveHandler(DragLeaveHandler handler) {
        return addBitlessDomHandler(handler, DragLeaveEvent.getType());
    }
}

和图像类：

public class DragImage extends Image {

    public DragImage() {
        super();
        initDnD();
    }

    private void initDnD() {
        // enables dragging if browser supports html5
        getElement().setDraggable(Element.DRAGGABLE_TRUE);
        addDragStartHandler(new DragStartHandler() {

            @Override
            public void onDragStart(DragStartEvent event) {
                // attach image URL to drag data
                event.setData("text", getUrl());
            }
        });
    }
}