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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：

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。
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：

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
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：

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。
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。
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);
    }
}