Android - 使用GridView在LongClick上拖动 - 无法启动拖动

Question

我正在开发SdCard上所有图片的缩略图马赛克。 如果用户在图片上长按，我想开始拖放过程。

我清楚地设法获取所有图像文件并在GridView中使用我自己的BaseAdapter显示它们，但过程很长，因为我以这种方式创建每个图像的预览：

uri = Uri.fromFile(file);
                try {
                    bitmap = BitmapFactory.decodeStream(getContentResolver().openInputStream(uri));
                    if (bitmap != null) {
                        newBitmap = Bitmap.createScaledBitmap(bitmap, 200, 200, true);
                        bitmap.recycle();
                        // here code to add the bitmap to the adapter (see after)
                    }
                } catch (IOException e) {
                    //Error fetching image, try to recover
                }

因此，为了避免漫长的等待，我在AsyncTask中加载图像。 该过程运行良好，图像逐个显示，但现在LongClick上出现错误，在此代码中：

mGrid.setOnItemLongClickListener(new OnItemLongClickListener()
    {
        public boolean onItemLongClick(AdapterView<?> adapter, View view, int position, long id) 
        {
            ClipData data = ClipData.newPlainText("AdapterPosition", String.valueOf(position) );
            view.startDrag(data, new DragShadowBuilder(view), null, 0);

            return true;
        }                       
    }); 

和错误：

    12-27 16:54:14.980: E/View(8089): Unable to initiate drag
12-27 16:54:14.980: E/View(8089): java.lang.NullPointerException
12-27 16:54:14.980: E/View(8089):   at android.view.View.startDrag(View.java:11470)
12-27 16:54:14.980: E/View(8089):   at renault.limandroid.widget.PartageWidgetActivity$9.onItemLongClick(PartageWidgetActivity.java:547)

有人有想法吗？

以下是我的代码的很大一部分：

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_partage_widget);

        // ... //

        mImageAdapter = new ImageAdapter(this);
        loadImages();
        mGrid.setOnItemLongClickListener(new OnItemLongClickListener()
        {
            public boolean onItemLongClick(AdapterView<?> adapter, View view, int position, long id) 
            {
                ClipData data = ClipData.newPlainText("AdapterPosition", String.valueOf(position) );
                view.startDrag(data, new DragShadowBuilder(view), null, 0);

                return true;
            }                       
        }); 
        mGrid.setAdapter(mImageAdapter);
}

        private void loadImages() 
    {
        // si c'est la première fois qu'on passe ici
        if (mLoadImageTask.getStatus() == AsyncTask.Status.PENDING)
        {
            mLoadImageTask.execute();
        }
        // si le scan est terminé mais qu'on veut le remettre à jour
        // on va recréer la tâche
        else if (mLoadImageTask.getStatus() == AsyncTask.Status.FINISHED)
        {
            mLoadImageTask = new LoadImagesFromSDCard();
            mLoadImageTask.execute();
        }
        }

@Override
        protected Object doInBackground(Object... params) {

            setProgressBarIndeterminateVisibility(true); 
            Log.d(TAG,"LoadImages - do in background");

            Bitmap bitmap = null;
            Bitmap newBitmap = null;
            Uri uri = null;            

            List<File> listFiles = new ArrayList<File>();
            listFiles = StorageManager.findImageFiles();

            for (File file : listFiles) 
            {
                            // mImageListFile contains the files that have already been loaded
                if (!mImageListFile.contains(file))
                {   
                    uri = Uri.fromFile(file);
                    try {
                        bitmap = BitmapFactory.decodeStream(getContentResolver().openInputStream(uri));
                        if (bitmap != null) {
                            newBitmap = Bitmap.createScaledBitmap(bitmap, 200, 200, true);
                            bitmap.recycle();
                            if (newBitmap != null) {
                                publishProgress(new LoadedImage(newBitmap));
                            }
                        }
                    } catch (IOException e) {
                        //Error fetching image, try to recover
                    }

                    mImageListFile.add(file);
                }
            }

            return null;
        }
        /**
         * Add a new LoadedImage in the images grid.
         *
         * @param value The image.
         */
        @Override
        public void onProgressUpdate(LoadedImage... value) 
        {
            mImageAdapter.addPhoto(value[0]); 
            mImageAdapter.notifyDataSetChanged();   
        }
        /**
         * Set the visibility of the progress bar to false.
         * 
         * @see android.os.AsyncTask#onPostExecute(java.lang.Object)
         */
        @Override
        protected void onPostExecute(Object result) 
        {
            setProgressBarIndeterminateVisibility(false);
            mGrid.invalidate();
        }
    }

class ImageAdapter extends BaseAdapter {

        private Context mContext; 
        private ArrayList<LoadedImage> photos = new ArrayList<LoadedImage>();

        public ImageAdapter(Context context) {
            mContext = context;
            photos.add(new LoadedImage(BitmapFactory.decodeResource(context.getResources(),
                    R.drawable.click)));
        }

        public void addPhoto(LoadedImage photo) 
        { 
            photos.add(photo); 
        } 

        public int getCount() { 
            return photos.size(); 
        } 


        public long getItemId(int position) { 
            return position; 
        } 

        public View getView(int position, View convertView, ViewGroup parent) 
        { 
            final ImageView imageView;  
            final int pos = position;

            if (convertView == null) { 
                imageView = new ImageView(mContext); 
            } else { 
                imageView = (ImageView) convertView; 
            } 

            imageView.setScaleType(ImageView.ScaleType.CENTER_INSIDE);
            imageView.setPadding(2, 2, 2, 2);
            imageView.setLayoutParams(new GridView.LayoutParams(parent.getWidth()/4, parent.getWidth()/4));
            imageView.setImageBitmap(photos.get(position).getBitmap());

            return imageView; 
        }

        public Object getItem(int position) 
        {
            return photos.get(position);
        } 

    }

    /**
     * A LoadedImage contains the Bitmap loaded for the image.
     */
    private static class LoadedImage {
        Bitmap mBitmap;

        LoadedImage(Bitmap bitmap) {
            mBitmap = bitmap;
        }

        public Bitmap getBitmap() {
            return mBitmap;
        }
    }