错过 ACTION_DRAG_STARTED 事件的拖放、ListView 和 item View [英] Drag and drop, ListView and item Views that miss the ACTION_DRAG_STARTED event
问题描述
在 Android 上，我使用 ListView 并且我希望能够使用拖放对其项目重新排序.我知道拖放列表视图"有不同的实现，但是我想使用 拖放框架 自 API 级别 11 起推出.
On Android, I use a ListView and I want to be able to reorder its items using drag and drop. I know there are different implementation of a "drag and drop listview", however I want to use the Drag and Drop framework coming since API level 11.
它开始得很好，直到我想在拖放时滚动我的 ListView.正如在下面的示例中所写，现在，我检查我在哪个列表元素之上，所以如果它的位置不在 ListView.getLastVisiblePosition() 和 ListView.getFirstVisiblePosition() 我使用 ListView.smoothScrollToPosition() 来查看其他列表项.
It started very well until I wanted to scroll my ListView while doing a drag and drop. As it is written in the example below, for now, I check on top of which list element I am, so if its position is not between ListView.getLastVisiblePosition() and ListView.getFirstVisiblePosition() I use a ListView.smoothScrollToPosition() to view the other list items.
这是第一个实现，但效果很好.
It is a first implementation but it works quite well.
滚动时出现问题:当我在它们之上时，一些元素不响应拖放事件 - DragEvent.ACTION_DRAG_ENTERED 和其他元素.这是由于 ListView 管理其项目视图的方式:它试图回收不再可见的项目视图.
The problem arises while scrolling: some elements do not answer to the drag and drop events - DragEvent.ACTION_DRAG_ENTERED and the others - when I am on top of them. It is due to the way the ListView manages its item views: it tries to recycle the item views that are not visible any more.
没关系，它可以工作，但有时 ListAdapter 的 getView() 返回一个新对象.由于它是新对象，因此该对象错过了 DragEvent.ACTION_DRAG_STARTED，因此它不会响应其他 DragEvent 事件！
It is all right and it works, but sometimes the getView() of the ListAdapter returns a new object. Since it is new, this object missed the DragEvent.ACTION_DRAG_STARTED so it does not answer to the other DragEvent events!
这是一个例子.在这种情况下，如果我通过长按列表项开始拖放，如果我拖动它，如果我在它们上面，大多数项目将具有绿色背景；但有些没有.
Here is an example. In this case, if I start a drag and drop with a long click on a list item and if I drag it, the majority of items will have a green background if I am on top of them ; but some don't.
有什么想法让他们订阅拖放事件机制，即使他们错过了DragEvent.ACTION_DRAG_STARTED?
Any idea about making them subscribe to the Drag and drop event mechanism even if they missed DragEvent.ACTION_DRAG_STARTED?
// Somewhere I have a ListView that use the MyViewAdapter
// MyListView _myListView = ...
// _myListView.setAdapter(new MyViewAdapter(getActivity(), ...));
_myListView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
@Override
public boolean onItemLongClick(AdapterView<?> parent, View view, int position, long id) {
DragShadowBuilder shadowBuilder = new View.DragShadowBuilder(view);
view.startDrag(null, shadowBuilder, _myListView.getItemAtPosition(position), 0);
return true;
}
});
class MyViewAdapter extends ArrayAdapter<MyElement> {
public MyViewAdapter(Context context, List<TimedElement> objects) {
super(context, 0, objects);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View myElementView = convertView;
if (myElementView == null) {
/* If the code is executed here while scrolling with a drag and drop,
* the new view is not associated to the current drag and drop events */
Log.d("app", "Object created!");
// Create view
// myElementView = ...
// Prepare drag and drop
myElementView.setOnDragListener(new MyElementDragListener());
}
// Associates view and position in ListAdapter, needed for drag and drop
myElementView.setTag(R.id.item_position, position);
// Continue to prepare view
// ...
return timedElementView;
}
private class MyElementDragListener implements View.OnDragListener {
@Override
public boolean onDrag(View v, DragEvent event) {
final int action = event.getAction();
switch(action) {
case DragEvent.ACTION_DRAG_STARTED:
return true;
case DragEvent.ACTION_DRAG_ENTERED:
v.setBackgroundColor(Color.GREEN);
v.invalidate();
return true;
case DragEvent.ACTION_DRAG_LOCATION:
int targetPosition = (Integer)v.getTag(R.id.item_position);
if (event.getY() < v.getHeight()/2 ) {
Log.i("app", "top "+targetPosition);
}
else {
Log.i("app", "bottom "+targetPosition);
}
// To scroll in ListView while doing drag and drop
if (targetPosition > _myListView.getLastVisiblePosition()-2) {
_myListView.smoothScrollToPosition(targetPosition+2);
}
else if (targetPosition < _myListView.getFirstVisiblePosition()+2) {
_myListView.smoothScrollToPosition(targetPosition-2);
}
return true;
case DragEvent.ACTION_DRAG_EXITED:
v.setBackgroundColor(Color.BLUE);
v.invalidate();
return true;
case DragEvent.ACTION_DROP:
case DragEvent.ACTION_DRAG_ENDED:
default:
break;
}
return false;
}
}
}
推荐答案
我没有解决这个回收问题，但我找到了一个可能的解决方法，仍然使用 Drag &删除框架.这个想法是改变视角:而不是在列表中的每个 View 上使用 OnDragListener，它可以直接在 ListView 上使用.
I did not solved this recycling problem, but I found a possible workaround still using the Drag & Drop framework. The idea is to change of perspective: instead of using a OnDragListener on each View in the list, it can be used on the ListView directly.
然后我们的想法是在执行 Drag & 时找到手指在哪个项目上.拖放，并在ListView的ListAdapter中编写相关的显示代码.然后，诀窍是找到我们在哪个项目视图之上，以及放置完成的位置.
Then the idea is to find on top of which item the finger is while doing the Drag & Drop, and to write the related display code in the ListAdapter of the ListView. The trick is then to find on top of which item view we are, and where the drop is done.
为了做到这一点，我将适配器创建的每个视图的 ListView 位置设置为 id - 使用 View.setId()，所以我以后可以使用 ListView.pointToPosition() 和 ListView.findViewById() 的组合找到它.
In order to do that, I set as an id to each view created by the adapter its ListView position - with View.setId(), so I can find it later using a combination of ListView.pointToPosition() and ListView.findViewById().
作为一个拖动监听器的例子(我提醒你，应用在 ListView 上)，它可以是这样的:
As a drag listener example (which is, I remind you, applied on the ListView), it can be something like that:
// Initalize your ListView
private ListView _myListView = new ListView(getContext());
// Start drag when long click on a ListView item
_myListView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
@Override
public boolean onItemLongClick(AdapterView<?> parent, View view, int position, long id) {
DragShadowBuilder shadowBuilder = new View.DragShadowBuilder(view);
view.startDrag(null, shadowBuilder, _myListView.getItemAtPosition(position), 0);
return true;
}
});
// Set the adapter and drag listener
_myListView.setOnDragListener(new MyListViewDragListener());
_myListView.setAdapter(new MyViewAdapter(getActivity()));
// Classes used above
private class MyViewAdapter extends ArrayAdapter<Object> {
public MyViewAdapter (Context context, List<TimedElement> objects) {
super(context, 0, objects);
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
View myView = convertView;
if (myView == null) {
// Instanciate your view
}
// Associates view and position in ListAdapter, needed for drag and drop
myView.setId(position);
return myView;
}
}
private class MyListViewDragListener implements View.OnDragListener {
@Override
public boolean onDrag(View v, DragEvent event) {
final int action = event.getAction();
switch(action) {
case DragEvent.ACTION_DRAG_STARTED:
return true;
case DragEvent.ACTION_DRAG_DROP:
// We drag the item on top of the one which is at itemPosition
int itemPosition = _myListView.pointToPosition((int)event.getX(), (int)event.getY());
// We can even get the view at itemPosition thanks to get/setid
View itemView = _myListView.findViewById(itemPosition );
/* If you try the same thing in ACTION_DRAG_LOCATION, itemView
* is sometimes null; if you need this view, just return if null.
* As the same event is then fired later, only process the event
* when itemView is not null.
* It can be more problematic in ACTION_DRAG_DROP but for now
* I never had itemView null in this event. */
// Handle the drop as you like
return true;
}
}
}
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