使用视图实现拖放

您可以通过响应事件，在视图中实现拖放过程 可能会触发拖动开始、响应和使用放下事件的事件。

开始拖动

用户使用手势开始拖拽，通常通过轻触或点击和 按住他们想要拖动的项

如需在 View 中处理这种情况，请创建一个 ClipData 对象和 ClipData.Item 对象： 要移动的数据。在 ClipData 中，请提供符合以下条件的元数据： 存储在 ClipDescription 对象 在 ClipData 内。如果拖放操作不代表 数据移动，则可能需要使用 null 而不是实际对象。

例如，以下代码段展示了，如何通过创建包含 ImageView 的标记（或标签）的 ClipData 对象，来响应 ImageView 上的轻触并按住手势：

Kotlin

// Create a string for the ImageView label.
val IMAGEVIEW_TAG = "icon bitmap"
...
val imageView = ImageView(context).apply {
    // Set the bitmap for the ImageView from an icon bitmap defined elsewhere.
    setImageBitmap(iconBitmap)
    tag = IMAGEVIEW_TAG
    setOnLongClickListener { v ->
        // Create a new ClipData. This is done in two steps to provide
        // clarity. The convenience method ClipData.newPlainText() can
        // create a plain text ClipData in one step.

        // Create a new ClipData.Item from the ImageView object's tag.
        val item = ClipData.Item(v.tag as? CharSequence)

        // Create a new ClipData using the tag as a label, the plain text
        // MIME type, and the already-created item. This creates a new
        // ClipDescription object within the ClipData and sets its MIME type
        // to "text/plain".
        val dragData = ClipData(
            v.tag as? CharSequence,
            arrayOf(ClipDescription.MIMETYPE_TEXT_PLAIN),
            item)

        // Instantiate the drag shadow builder. We use this imageView object
        // to create the default builder.
        val myShadow = View.DragShadowBuilder(view: this)

        // Start the drag.
        v.startDragAndDrop(dragData,  // The data to be dragged.
                            myShadow,  // The drag shadow builder.
                            null,      // No need to use local data.
                            0          // Flags. Not currently used, set to 0.
        )

        // Indicate that the long-click is handled.
        true
    }
}

Java

// Create a string for the ImageView label.
private static final String IMAGEVIEW_TAG = "icon bitmap";
...
// Create a new ImageView.
ImageView imageView = new ImageView(context);

// Set the bitmap for the ImageView from an icon bitmap defined elsewhere.
imageView.setImageBitmap(iconBitmap);

// Set the tag.
imageView.setTag(IMAGEVIEW_TAG);

// Set a long-click listener for the ImageView using an anonymous listener
// object that implements the OnLongClickListener interface.
imageView.setOnLongClickListener( v -> {

    // Create a new ClipData. This is done in two steps to provide clarity. The
    // convenience method ClipData.newPlainText() can create a plain text
    // ClipData in one step.

    // Create a new ClipData.Item from the ImageView object's tag.
    ClipData.Item item = new ClipData.Item((CharSequence) v.getTag());

    // Create a new ClipData using the tag as a label, the plain text MIME type,
    // and the already-created item. This creates a new ClipDescription object
    // within the ClipData and sets its MIME type to "text/plain".
    ClipData dragData = new ClipData(
            (CharSequence) v.getTag(),
            new String[] { ClipDescription.MIMETYPE_TEXT_PLAIN },
            item);

    // Instantiate the drag shadow builder. We use this imageView object
    // to create the default builder.
    View.DragShadowBuilder myShadow = new View.DragShadowBuilder(imageView);

    // Start the drag.
    v.startDragAndDrop(dragData,  // The data to be dragged.
                            myShadow,  // The drag shadow builder.
                            null,      // No need to use local data.
                            0          // Flags. Not currently used, set to 0.
    );

    // Indicate that the long-click is handled.
    return true;
});

响应拖动开始事件

在拖动操作期间，系统会向当前布局中 View 对象的拖动事件监听器发送拖动事件。监听器的回应方式 调用 DragEvent.getAction() 来获取操作类型。拖动开始时， 此方法会返回 ACTION_DRAG_STARTED。

为响应操作类型为 ACTION_DRAG_STARTED 的事件，系统会记录一个拖动事件 监听器必须执行以下操作：

  1. 致电 DragEvent.getClipDescription() 并使用返回的 ClipDescription 中的 MIME 类型方法查看 表示监听器是否可以接受正在拖动的数据。

    如果拖放操作不表示数据移动， 没有必要。

  2. 如果拖动事件监听器可以接受放下操作，则必须返回 true 以告知 系统继续向监听器发送拖动事件。如果监听器 不能接受放下操作，监听器必须返回 false，并且系统停止 向监听器发送拖动事件，直到系统发送 ACTION_DRAG_ENDED即可结束拖放操作。

对于 ACTION_DRAG_STARTED 事件，以下 DragEvent 方法不适用 有效值：getClipData()， getX(), getY()和 getResult()。

在拖动期间处理事件

在拖动操作期间，拖动会返回 true 的事件监听器来响应 ACTION_DRAG_STARTED拖动事件会继续接收拖动事件。类型 监听器在拖动期间接收的拖动事件的数量取决于 拖动阴影以及监听器 View 的可见性。监听器使用 事件主要用于决定是否必须更改其 View 的外观。

在拖动操作期间，DragEvent.getAction() 会返回以下三个值之一：

  • ACTION_DRAG_ENTERED: 当接触点（即 某个点，即可进入 监听器的 View 的边界框。
  • ACTION_DRAG_LOCATION: 监听器收到 ACTION_DRAG_ENTERED 事件后，会在其收到一个新的 ACTION_DRAG_LOCATION事件 收到 ACTION_DRAG_EXITED 事件。getX() 和 getY() 方法 返回接触点的 X 和 Y 坐标。
  • ACTION_DRAG_EXITED: 此事件操作类型发送到之前接收 ACTION_DRAG_ENTERED。当拖动阴影的接触点从监听器的 View 的边界框内移到边界框外时，系统会发送该事件。

拖动事件监听器不需要对上述任何操作类型做出反应。如果 监听器会向系统返回值，该值将被忽略。

以下是响应以上各种操作类型的一些准则：

  • 在响应 ACTION_DRAG_ENTERED 或 ACTION_DRAG_LOCATION 时，监听器可更改 View 的外观，以表明该视图是可能的拖放目标。
  • 操作类型为“ACTION_DRAG_LOCATION”的事件包含 getX() 和 getY()，分别对应接触点的位置。通过 监听器可以使用此信息来更改 View 属性在 来确定用户可将内容放置到哪个位置 内容。
  • 在响应 ACTION_DRAG_EXITED 时，监听器必须重置所有外观 在响应 ACTION_DRAG_ENTERED 或 ACTION_DRAG_LOCATION。以便向用户表明，View 已不再是当下的拖放目标。

响应放下事件

当用户将拖动阴影释放到 View 上时，而先前的 View 可接受正在拖动的内容，则系统会发送一个 将操作类型为 ACTION_DROP 的事件拖动到 View 上。

拖动事件监听器必须执行以下操作：

  1. 调用 getClipData() 以获取最初的 ClipData 对象 在对 startDragAndDrop() 和处理数据如果拖放操作不表示数据 没有必要这样做

  2. 返回布尔值 true，表示已成功处理放下事件； 否则为 false。返回的值将成为 getResult()，适用于最终的 ACTION_DRAG_ENDED 事件。如果系统 未发出 ACTION_DROP 事件，则 getResult() 返回的值 ACTION_DRAG_ENDED事件的触发条件为false。

对于 ACTION_DROP 事件，getX() 和 getY() 会使用 接收放下操作的 View，以返回 所有接触点都有可能出现

而用户能够将拖动阴影释放到其拖动事件的 View 上。 监听器未接收拖动事件、应用界面的空白区域，甚至 应用区域以外的区域，Android 不会发送包含操作的事件 类型ACTION_DROP，并且只会发送ACTION_DRAG_ENDED事件。

响应拖动结束事件

在用户释放拖动阴影后，系统会立即发送拖动 操作类型为 ACTION_DRAG_ENDED 的事件附加到所有拖动事件监听器 。这表示拖动操作已完成。

每个拖动事件监听器都必须执行以下操作：

  1. 如果监听器在操作期间改变了外观，应重置监听器 恢复其默认外观，以视觉指示 操作完成时的状态。
  2. 监听器可以选择调用 getResult()，以了解关于该操作的更多信息。如果监听器在响应操作事件时返回 true 类型 ACTION_DROP，则 getResult() 会返回布尔值 true。在所有其他 在这种情况下，getResult() 会返回布尔值 false，包括当系统 不会发送 ACTION_DROP 事件。
  3. 为了指示放置操作成功完成，监听器 应向系统返回布尔值 true。如果不返回 false， 显示阴影返回到来源的视觉提示 操作失败的用户。

响应拖动事件：示例

所有拖动事件均由拖动事件方法或监听器接收。通过 以下代码段是一个响应拖动事件的示例：

Kotlin

val imageView = ImageView(this)

// Set the drag event listener for the View.
imageView.setOnDragListener { v, e ->

    // Handle each of the expected events.
    when (e.action) {
        DragEvent.ACTION_DRAG_STARTED -> {
            // Determine whether this View can accept the dragged data.
            if (e.clipDescription.hasMimeType(ClipDescription.MIMETYPE_TEXT_PLAIN)) {
                // As an example, apply a blue color tint to the View to
                // indicate that it can accept data.
                (v as? ImageView)?.setColorFilter(Color.BLUE)

                // Invalidate the view to force a redraw in the new tint.
                v.invalidate()

                // Return true to indicate that the View can accept the dragged
                // data.
                true
            } else {
                // Return false to indicate that, during the current drag and
                // drop operation, this View doesn't receive events again until
                // ACTION_DRAG_ENDED is sent.
                false
            }
        }
        DragEvent.ACTION_DRAG_ENTERED -> {
            // Apply a green tint to the View.
            (v as? ImageView)?.setColorFilter(Color.GREEN)

            // Invalidate the view to force a redraw in the new tint.
            v.invalidate()

            // Return true. The value is ignored.
            true
        }

        DragEvent.ACTION_DRAG_LOCATION ->
            // Ignore the event.
            true
        DragEvent.ACTION_DRAG_EXITED -> {
            // Reset the color tint to blue.
            (v as? ImageView)?.setColorFilter(Color.BLUE)

            // Invalidate the view to force a redraw in the new tint.
            v.invalidate()

            // Return true. The value is ignored.
            true
        }
        DragEvent.ACTION_DROP -> {
            // Get the item containing the dragged data.
            val item: ClipData.Item = e.clipData.getItemAt(0)

            // Get the text data from the item.
            val dragData = item.text

            // Display a message containing the dragged data.
            Toast.makeText(this, "Dragged data is $dragData", Toast.LENGTH_LONG).show()

            // Turn off color tints.
            (v as? ImageView)?.clearColorFilter()

            // Invalidate the view to force a redraw.
            v.invalidate()

            // Return true. DragEvent.getResult() returns true.
            true
        }

        DragEvent.ACTION_DRAG_ENDED -> {
            // Turn off color tinting.
            (v as? ImageView)?.clearColorFilter()

            // Invalidate the view to force a redraw.
            v.invalidate()

            // Do a getResult() and display what happens.
            when(e.result) {
                true ->
                    Toast.makeText(this, "The drop was handled.", Toast.LENGTH_LONG)
                else ->
                    Toast.makeText(this, "The drop didn't work.", Toast.LENGTH_LONG)
            }.show()

            // Return true. The value is ignored.
            true
        }
        else -> {
            // An unknown action type is received.
            Log.e("DragDrop Example", "Unknown action type received by View.OnDragListener.")
            false
        }
    }
}

Java

View imageView = new ImageView(this);

// Set the drag event listener for the View.
imageView.setOnDragListener( (v, e) -> {

    // Handle each of the expected events.
    switch(e.getAction()) {

        case DragEvent.ACTION_DRAG_STARTED:

            // Determine whether this View can accept the dragged data.
            if (e.getClipDescription().hasMimeType(ClipDescription.MIMETYPE_TEXT_PLAIN)) {

                // As an example, apply a blue color tint to the View to
                // indicate that it can accept data.
                ((ImageView)v).setColorFilter(Color.BLUE);

                // Invalidate the view to force a redraw in the new tint.
                v.invalidate();

                // Return true to indicate that the View can accept the dragged
                // data.
                return true;

            }

            // Return false to indicate that, during the current drag-and-drop
            // operation, this View doesn't receive events again until
            // ACTION_DRAG_ENDED is sent.
            return false;

        case DragEvent.ACTION_DRAG_ENTERED:

            // Apply a green tint to the View.
            ((ImageView)v).setColorFilter(Color.GREEN);

            // Invalidate the view to force a redraw in the new tint.
            v.invalidate();

            // Return true. The value is ignored.
            return true;

        case DragEvent.ACTION_DRAG_LOCATION:

            // Ignore the event.
            return true;

        case DragEvent.ACTION_DRAG_EXITED:

            // Reset the color tint to blue.
            ((ImageView)v).setColorFilter(Color.BLUE);

            // Invalidate the view to force a redraw in the new tint.
            v.invalidate();

            // Return true. The value is ignored.
            return true;

        case DragEvent.ACTION_DROP:

            // Get the item containing the dragged data.
            ClipData.Item item = e.getClipData().getItemAt(0);

            // Get the text data from the item.
            CharSequence dragData = item.getText();

            // Display a message containing the dragged data.
            Toast.makeText(this, "Dragged data is " + dragData, Toast.LENGTH_LONG).show();

            // Turn off color tints.
            ((ImageView)v).clearColorFilter();

            // Invalidate the view to force a redraw.
            v.invalidate();

            // Return true. DragEvent.getResult() returns true.
            return true;

        case DragEvent.ACTION_DRAG_ENDED:

            // Turn off color tinting.
            ((ImageView)v).clearColorFilter();

            // Invalidate the view to force a redraw.
            v.invalidate();

            // Do a getResult() and displays what happens.
            if (e.getResult()) {
                Toast.makeText(this, "The drop was handled.", Toast.LENGTH_LONG).show();
            } else {
                Toast.makeText(this, "The drop didn't work.", Toast.LENGTH_LONG).show();
            }

            // Return true. The value is ignored.
            return true;

        // An unknown action type is received.
        default:
            Log.e("DragDrop Example","Unknown action type received by View.OnDragListener.");
            break;
    }

    return false;

});

自定义拖动阴影

您可以通过重写 myDragShadowBuilder View.DragShadowBuilder。以下代码段会创建一个 TextView 的灰色矩形拖动阴影：

Kotlin

private class MyDragShadowBuilder(view: View) : View.DragShadowBuilder(view) {

    private val shadow = ColorDrawable(Color.LTGRAY)

    // Define a callback that sends the drag shadow dimensions and touch point
    // back to the system.
    override fun onProvideShadowMetrics(size: Point, touch: Point) {

            // Set the width of the shadow to half the width of the original
            // View.
            val width: Int = view.width / 2

            // Set the height of the shadow to half the height of the original
            // View.
            val height: Int = view.height / 2

            // The drag shadow is a ColorDrawable. Set its dimensions to
            // be the same as the Canvas that the system provides. As a result,
            // the drag shadow fills the Canvas.
            shadow.setBounds(0, 0, width, height)

            // Set the size parameter's width and height values. These get back
            // to the system through the size parameter.
            size.set(width, height)

            // Set the touch point's position to be in the middle of the drag
            // shadow.
            touch.set(width / 2, height / 2)
    }

    // Define a callback that draws the drag shadow in a Canvas that the system
    // constructs from the dimensions passed to onProvideShadowMetrics().
    override fun onDrawShadow(canvas: Canvas) {

            // Draw the ColorDrawable on the Canvas passed in from the system.
            shadow.draw(canvas)
    }
}

Java

private static class MyDragShadowBuilder extends View.DragShadowBuilder {

    // The drag shadow image, defined as a drawable object.
    private static Drawable shadow;

    // Constructor.
    public MyDragShadowBuilder(View view) {

            // Store the View parameter.
            super(view);

            // Create a draggable image that fills the Canvas provided by the
            // system.
            shadow = new ColorDrawable(Color.LTGRAY);
    }

    // Define a callback that sends the drag shadow dimensions and touch point
    // back to the system.
    @Override
    public void onProvideShadowMetrics (Point size, Point touch) {

            // Define local variables.
            int width, height;

            // Set the width of the shadow to half the width of the original
            // View.
            width = getView().getWidth() / 2;

            // Set the height of the shadow to half the height of the original
            // View.
            height = getView().getHeight() / 2;

            // The drag shadow is a ColorDrawable. Set its dimensions to
            // be the same as the Canvas that the system provides. As a result,
            // the drag shadow fills the Canvas.
            shadow.setBounds(0, 0, width, height);

            // Set the size parameter's width and height values. These get back
            // to the system through the size parameter.
            size.set(width, height);

            // Set the touch point's position to be in the middle of the drag
            // shadow.
            touch.set(width / 2, height / 2);
    }

    // Define a callback that draws the drag shadow in a Canvas that the system
    // constructs from the dimensions passed to onProvideShadowMetrics().
    @Override
    public void onDrawShadow(Canvas canvas) {

            // Draw the ColorDrawable on the Canvas passed in from the system.
            shadow.draw(canvas);
    }
}