简短版:

• 有没有办法让新创建的 View 接收已在运行的拖放操作的 DragEvent？

有 How to register a DragEvent while already inside one and have it listen in the current DragEvent? ，但我真的想要一个更清洁的解决方案。

建议的 GONE->VISIBLE 解决方法要“正确”是相当复杂的，因为您需要确保仅在列表项变得可见时才使用它，而不是无条件地在所有当前 ListView 项上使用它。在此 hack 略有漏洞，没有更多的解决方法代码来使其正确。

长版:

我有一个 ListView。 ListView 的元素是包含可拖动符号(小框)的自定义 View ，例如类似这样:

可以在 ListView 的项目之间拖动小框，就像将元素排序到框中一样。列表项上的拖动处理程序或多或少是微不足道的:

@Override
public boolean onDragEvent(DragEvent event)
{
    if ((event.getLocalState() instanceof DragableSymbolView)) {
        final DragableSymbolView draggedView = (DragableSymbolView) event.getLocalState();
        if (draggedView.getTag() instanceof SymbolData) {
            final SymbolData symbol = (SymbolData) draggedView.getTag();
            switch (event.getAction()) {
            case DragEvent.ACTION_DRAG_STARTED:
                return true;

            case DragEvent.ACTION_DRAG_ENTERED:
                setSelected(true);
                return true;

            case DragEvent.ACTION_DRAG_ENDED:
            case DragEvent.ACTION_DRAG_EXITED:
                setSelected(false);
                return true;

            case DragEvent.ACTION_DROP:
                setSelected(false);
                // [...] remove symbol from soruce box and add to current box
                requestFocus();
                break;
            }
        }
    }

    return super.onDragEvent(event);
}

将指针悬停在符号上并开始拖动(即将其移动到一个小阈值之外)时，拖动开始。

但是，现在屏幕尺寸可能不足以包含所有框，因此 ListView 需要滚动。我发现我需要自己实现滚动的困难方法，因为 ListView 在拖动时不会自动滚动。

进来ListViewScrollingDragListener:

public class ListViewScrollingDragListener
    implements View.OnDragListener {

    private final ListView _listView;

    public static final int DEFAULT_SCROLL_BUFFER_DIP = 96;
    public static final int DEFAULT_SCROLL_DELTA_UP_DIP = 48;
    public static final int DEFAULT_SCROLL_DELTA_DOWN_DIP = 48;

    private int _scrollDeltaUp;
    private int _scrollDeltaDown;

    private boolean _doScroll = false;
    private boolean _scrollActive = false;

    private int _scrollDelta = 0;

    private int _scrollDelay = 250;
    private int _scrollInterval = 100;

    private int _scrollBuffer;

    private final Rect _visibleRect = new Rect();

    private final Runnable _scrollHandler = new Runnable() {

        @Override
        public void run()
        {
            if (_doScroll && (_scrollDelta != 0) && _listView.canScrollVertically(_scrollDelta)) {
                _scrollActive = true;
                _listView.smoothScrollBy(_scrollDelta, _scrollInterval);
                _listView.postDelayed(this, _scrollInterval);
            } else {
                _scrollActive = false;
            }
        }
    };

    public ListViewScrollingDragListener(final ListView listView, final boolean attach)
    {
        _scrollBuffer = UnitUtil.dipToPixels(listView, DEFAULT_SCROLL_BUFFER_DIP);
        _scrollDeltaUp = -UnitUtil.dipToPixels(listView, DEFAULT_SCROLL_DELTA_UP_DIP);
        _scrollDeltaDown = UnitUtil.dipToPixels(listView, DEFAULT_SCROLL_DELTA_DOWN_DIP);

        _listView = listView;
        if (attach) {
            _listView.setOnDragListener(this);
        }
    }

    public ListViewScrollingDragListener(final ListView listView)
    {
        this(listView, true);
    }

    protected void handleDragLocation(final float x, final float y)
    {
        _listView.getGlobalVisibleRect(_visibleRect);
        if (_visibleRect.contains((int) x, (int) y)) {
            if (y < _visibleRect.top + _scrollBuffer) {
                _scrollDelta = _scrollDeltaUp;
                _doScroll = true;
            } else if (y > _visibleRect.bottom - _scrollBuffer) {
                _scrollDelta = _scrollDeltaDown;
                _doScroll = true;
            } else {
                _doScroll = false;
                _scrollDelta = 0;
            }
            if ((_doScroll) && (!_scrollActive)) {
                _scrollActive = true;
                _listView.postDelayed(_scrollHandler, _scrollDelay);
            }
        }
    }

    public ListView getListView()
    {
        return _listView;
    }

    @Override
    public boolean onDrag(View v, DragEvent event)
    {
        /* hide sequence controls during drag */
        switch (event.getAction()) {
        case DragEvent.ACTION_DRAG_ENTERED:
            _doScroll = true;
            break;

        case DragEvent.ACTION_DRAG_EXITED:
        case DragEvent.ACTION_DRAG_ENDED:
        case DragEvent.ACTION_DROP:
            _doScroll = false;
            break;

        case DragEvent.ACTION_DRAG_LOCATION:
            handleDragLocation(event.getX(), event.getY());
            break;
        }
        return true;
    }
}

当您在其可见区域的上边界或下边界附近拖动时，这基本上会滚动 ListView。它并不完美，但已经足够好了。

但是，有一个问题:

当列表滚动到之前不可见的元素时，该元素不会接收到 DragEvent。在其上拖动符号时，它不会被选中(突出显示)，也不接受掉落。

关于如何使“卷入” View 从已经激活的拖放操作接收 DragEvent 有什么想法吗？

最佳答案

所以根本问题是 ViewGroup(即 ListView 扩展)缓存了一个子级列表以通知 DragEvent。此外，它仅在接收到 ACTION_DRAG_STARTED 时填充此缓存。有关更多详细信息，请阅读源代码 here .

关于解决方案!我们不是在 ListView 的各个行上监听放置事件，而是在 ListView 本身上监听它们。然后，根据事件的坐标，我们将找出被拖动的 View 从哪一行拖动到哪一行或悬停在哪一行。当删除发生时，我们将执行从前一行中删除并添加到新行的事务。

private void init(Context context) {
    setAdapter(new RandomIconAdapter()); // Adapter that contains our data set
    setOnDragListener(new ListDragListener());
    mListViewScrollingDragListener = new ListViewScrollingDragListener(this, false);
}

ListViewScrollingDragListener mListViewScrollingDragListener;

private class ListDragListener implements OnDragListener {
    // The view that our dragged view would be dropped on
    private View mCurrentDropZoneView = null;
    private int mDropStartRowIndex = -1;

    @Override
    public boolean onDrag(View v, DragEvent event) {
        switch (event.getAction()) {
            case DragEvent.ACTION_DRAG_LOCATION:
                // Update the active drop zone based on the position of the event
                updateCurrentDropZoneView(event);

                // Funnel drag events to separate listener to handle scrolling near edges
                mListViewScrollingDragListener.onDrag(v, event);

                if( mDropStartRowIndex == -1 )          // Only initialize once per drag->drop gesture
                {
                    mDropStartRowIndex = indexOfChild(mCurrentDropZoneView) + getFirstVisiblePosition();
                    log("mDropStartRowIndex %d", mDropStartRowIndex);
                }
                break;
            case DragEvent.ACTION_DRAG_ENDED:
            case DragEvent.ACTION_DRAG_EXITED:
                mCurrentDropZoneView = null;
                mDropStartRowIndex = -1;
                break;
            case DragEvent.ACTION_DROP:
                // Update our data set based on the row that the dragged view was dropped in
                int finalDropRow = indexOfChild(mCurrentDropZoneView) + getFirstVisiblePosition();
                updateDataSetWithDrop(mDropStartRowIndex, finalDropRow);

                // Let adapter update ui
                ((BaseAdapter)getAdapter()).notifyDataSetChanged();

                break;
        }

        // The ListView handles ALL drag events all the time. Fine for now since we don't need to
        // drag -> drop outside of the ListView.
        return true;
    }

    private void updateDataSetWithDrop(int fromRow, int toRow) {
        log("updateDataSetWithDrop fromRow %d and toRow %d", fromRow, toRow);
        sIconsForListItems[fromRow]--;
        sIconsForListItems[toRow]++;
    }

    // NOTE: The DragEvent in local to DragDropListView, as are children coordinates
    private void updateCurrentDropZoneView(DragEvent event) {
        View previousDropZoneView = mCurrentDropZoneView;
        mCurrentDropZoneView = findFrontmostDroppableChildAt(event.getX(), event.getY());
        log("mCurrentDropZoneView updated to %d for x/y : %f/%f with action %d",
                mCurrentDropZoneView == null ? -1 : indexOfChild(mCurrentDropZoneView) + getFirstVisiblePosition(),
                event.getX(), event.getY(), event.getAction());

        if (mCurrentDropZoneView != previousDropZoneView) {
            if (previousDropZoneView != null) previousDropZoneView.setSelected(false);
            if (mCurrentDropZoneView != null) mCurrentDropZoneView.setSelected(true);
        }
    }
}

/**
 * The next four methods are utility methods taken from Android Source Code. Most are package-private on View
 * or ViewGroup so I'm forced to replicate them here. Original source can be found:
 * http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/5.1.0_r1/android/view/ViewGroup.java#ViewGroup.findFrontmostDroppableChildAt%28float%2Cfloat%2Candroid.graphics.PointF%29
 */
private View findFrontmostDroppableChildAt(float x, float y) {
    int childCount = this.getChildCount();
    for(int i=0; i<childCount; i++)
    {
        View child = getChildAt(i);
        if (isTransformedTouchPointInView(x, y, child)) {
            return child;
        }
    }

    return null;
}

static public boolean isTransformedTouchPointInView(float x, float y, View child) {
    PointF point = new PointF(x, y);
    transformPointToViewLocal(point, child);
    return pointInView(child, point.x, point.y);
}

static public void transformPointToViewLocal(PointF pointToModify, View child) {
    pointToModify.x -= child.getLeft();
    pointToModify.y -= child.getTop();
}

static public boolean pointInView(View v, float localX, float localY) {
    return localX >= 0 && localX < (v.getRight() - v.getLeft())
                && localY >= 0 && localY < (v.getBottom() - v.getTop());
}

static final int[] sIconsForListItems;
static final int NUM_LIST_ITEMS = 50;
static final int MAX_NUM_ICON_PER_ELEMENT = 8;
static {
    sIconsForListItems = new int[NUM_LIST_ITEMS];
    for (int i=0; i < NUM_LIST_ITEMS; i++)
    {
        sIconsForListItems[i] = (getRand(MAX_NUM_ICON_PER_ELEMENT));
    }
}

private static final String TAG = DragDropListView.class.getSimpleName();
private static void log(String format, Object... args) {
    Log.d(TAG, String.format(format, args));
}

很多评论，希望代码是自文档化的。一些注意事项:

• RandomIconAdapter 只是一个扩展 BaseAdapter 并由 sIconsForListItems 支持的基本适配器。
• ListViewScrollingDragListener 与提示中的相同。
• 在 GS6 5.0.2 上测试

关于android - 如何使列表元素在滚动后获得 DragEvents，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/29825676/