Android Training - 高效地显示Bitmap(Lesson 5 - 呈现Bitmap到UI上)

这一课会演示如何运用前面几节课的内容，使用后台线程与Cache机制来加载图片到 ViewPager 与 GridView 组件，并且学习处理并发与配置改变问题。(并发问题的处理确实需要好好考虑，大多数App都没有仔细考虑到这个问题，会导致浪费更多的资源，效率不够)

Load Bitmaps into a ViewPager Implementation(实现加载图片到ViewPager)

swipe view pattern是一个用来切换显示不同详情界面的很好的方法。(关于这种效果请先参看Android Design: Swipe Views ，这是一个很好的设计，在很多App里面都可以看到这种设计，例如网易新闻，可以左右滑动来切换分类与文章，在Google Reader里面文章的切换也是这样).

你可以通过 PagerAdapter 与 ViewPager 组件来实现这个效果. 然而，一个更加合适的Adapter是PagerAdapter 的子类 FragmentStatePagerAdapter:它可以在某个ViewPager中的子视图切换出屏幕时自动销毁与保存 Fragments 的状态。这样能够保持消耗更少的内存。
Note: 如果你只有为数不多的图片并且确保不会超出程序内存限制，那么使用 PagerAdapter 或 FragmentPagerAdapter 会更加合适。

下面是一个使用ViewPager与ImageView作为子视图的示例。

public class ImageDetailActivity extends FragmentActivity {
    public static final String EXTRA_IMAGE = "extra_image";

    private ImagePagerAdapter mAdapter;
    private ViewPager mPager;

    // A static dataset to back the ViewPager adapter  
    public final static Integer[] imageResIds = new Integer[] {
            R.drawable.sample_image_1, R.drawable.sample_image_2, R.drawable.sample_image_3,
            R.drawable.sample_image_4, R.drawable.sample_image_5, R.drawable.sample_image_6,
            R.drawable.sample_image_7, R.drawable.sample_image_8, R.drawable.sample_image_9};

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.image_detail_pager); // Contains just a ViewPager  

        mAdapter = new ImagePagerAdapter(getSupportFragmentManager(), imageResIds.length);
        mPager = (ViewPager) findViewById(R.id.pager);
        mPager.setAdapter(mAdapter);
    }

    public static class ImagePagerAdapter extends FragmentStatePagerAdapter {
        private final int mSize;

        public ImagePagerAdapter(FragmentManager fm, int size) {
            super(fm);
            mSize = size;
        }

        @Override
        public int getCount() {
            return mSize;
        }

        @Override
        public Fragment getItem(int position) {
            return ImageDetailFragment.newInstance(position);
        }
    }
}

Fragment 里面包含了ImageView 的子组件:

public class ImageDetailFragment extends Fragment {
    private static final String IMAGE_DATA_EXTRA = "resId";
    private int mImageNum;
    private ImageView mImageView;

    static ImageDetailFragment newInstance(int imageNum) {
        final ImageDetailFragment f = new ImageDetailFragment();
        final Bundle args = new Bundle();
        args.putInt(IMAGE_DATA_EXTRA, imageNum);
        f.setArguments(args);
        return f;
    }

    // Empty constructor, required as per Fragment docs  
    public ImageDetailFragment() {}

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mImageNum = getArguments() != null ? getArguments().getInt(IMAGE_DATA_EXTRA) : -1;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        // image_detail_fragment.xml contains just an ImageView  
        final View v = inflater.inflate(R.layout.image_detail_fragment, container, false);
        mImageView = (ImageView) v.findViewById(R.id.imageView);
        return v;
    }

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        final int resId = ImageDetailActivity.imageResIds[mImageNum];
        mImageView.setImageResource(resId); // Load image into ImageView  
    }
}

希望你有发现上面示例存在的问题：在UI Thread中读取图片可能会导致程序ANR。使用在Lesson 2中学习的 AsyncTask 会比较好。

public class ImageDetailActivity extends FragmentActivity {
    ...

    public void loadBitmap(int resId, ImageView imageView) {
        mImageView.setImageResource(R.drawable.image_placeholder);
        BitmapWorkerTask task = new BitmapWorkerTask(mImageView);
        task.execute(resId);
    }

    ... // include BitmapWorkerTask class  
}

public class ImageDetailFragment extends Fragment {
    ...

    @Override
    public void onActivityCreated(Bundle savedInstanceState) {
        super.onActivityCreated(savedInstanceState);
        if (ImageDetailActivity.class.isInstance(getActivity())) {
            final int resId = ImageDetailActivity.imageResIds[mImageNum];
            // Call out to ImageDetailActivity to load the bitmap in a background thread  
            ((ImageDetailActivity) getActivity()).loadBitmap(resId, mImageView);
        }
    }
}

在 BitmapWorkerTask 中做一些例如resizing or fetching images from the network，不会卡到UI Thread。如果后台线程不仅仅是做个简单的直接加载动作，增加一个内存Cache或者磁盘Cache会比较好[参考Lesson 3] ,下面是一些为了内存Cache而附加的内容:

public class ImageDetailActivity extends FragmentActivity {
    ...
    private LruCache mMemoryCache;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        ...
        // initialize LruCache as per Use a Memory Cache section  
    }

    public void loadBitmap(int resId, ImageView imageView) {
        final String imageKey = String.valueOf(resId);

        final Bitmap bitmap = mMemoryCache.get(imageKey);
        if (bitmap != null) {
            mImageView.setImageBitmap(bitmap);
        } else {
            mImageView.setImageResource(R.drawable.image_placeholder);
            BitmapWorkerTask task = new BitmapWorkerTask(mImageView);
            task.execute(resId);
        }
    }

    ... // include updated BitmapWorkerTask from Use a Memory Cache section  
}

Load Bitmaps into a GridView Implementation(实现加载图片到GridView)

Grid list building block 是一种有效显示大量图片的方式。这样能够一次显示许多图片，而且那些即将被显示的图片也处于准备显示状态。如果你想要实现这种效果，你必须确保UI是流畅的，能够控制内存使用，并且正确的处理并发问题（因为 GridView 会循环使用子视图)。

下面是一个在Fragment里面内置了ImageView作为GridView子视图的示例：

public class ImageGridFragment extends Fragment implements AdapterView.OnItemClickListener {
    private ImageAdapter mAdapter;

    // A static dataset to back the GridView adapter  
    public final static Integer[] imageResIds = new Integer[] {
            R.drawable.sample_image_1, R.drawable.sample_image_2, R.drawable.sample_image_3,
            R.drawable.sample_image_4, R.drawable.sample_image_5, R.drawable.sample_image_6,
            R.drawable.sample_image_7, R.drawable.sample_image_8, R.drawable.sample_image_9};

    // Empty constructor as per Fragment docs  
    public ImageGridFragment() {}

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mAdapter = new ImageAdapter(getActivity());
    }

    @Override
    public View onCreateView(
            LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        final View v = inflater.inflate(R.layout.image_grid_fragment, container, false);
        final GridView mGridView = (GridView) v.findViewById(R.id.gridView);
        mGridView.setAdapter(mAdapter);
        mGridView.setOnItemClickListener(this);
        return v;
    }

    @Override
    public void onItemClick(AdapterView parent, View v, int position, long id) {
        final Intent i = new Intent(getActivity(), ImageDetailActivity.class);
        i.putExtra(ImageDetailActivity.EXTRA_IMAGE, position);
        startActivity(i);
    }

    private class ImageAdapter extends BaseAdapter {
        private final Context mContext;

        public ImageAdapter(Context context) {
            super();
            mContext = context;
        }

        @Override
        public int getCount() {
            return imageResIds.length;
        }

        @Override
        public Object getItem(int position) {
            return imageResIds[position];
        }

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

        @Override
        public View getView(int position, View convertView, ViewGroup container) {
            ImageView imageView;
            if (convertView == null) { // if it's not recycled, initialize some attributes  
                imageView = new ImageView(mContext);
                imageView.setScaleType(ImageView.ScaleType.CENTER_CROP);
                imageView.setLayoutParams(new GridView.LayoutParams(
                        LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
            } else {
                imageView = (ImageView) convertView;
            }
        //请注意下面的代码  
        imageView.setImageResource(imageResIds[position]); // Load image into ImageView  
        return imageView;
    }
}

与前面加载到图片到ViewPager一样，如果setImageResource的操作会比较耗时，有可能会卡到UI Thread。可以使用类似前面异步处理图片与增加缓存的方法来解决那个问题。然而，我们还需要考虑GridView的循环机制所带来的并发问题。为了处理这个问题，请参考前面的课程。下面是一个更新的解决方案：

public class ImageGridFragment extends Fragment implements AdapterView.OnItemClickListener {
    ...

    private class ImageAdapter extends BaseAdapter {
        ...

        @Override
        public View getView(int position, View convertView, ViewGroup container) {
            ...
            loadBitmap(imageResIds[position], imageView)
            return imageView;
        }
    }

    public void loadBitmap(int resId, ImageView imageView) {
        if (cancelPotentialWork(resId, imageView)) {
            final BitmapWorkerTask task = new BitmapWorkerTask(imageView);
            final AsyncDrawable asyncDrawable =
                    new AsyncDrawable(getResources(), mPlaceHolderBitmap, task);
            imageView.setImageDrawable(asyncDrawable);
            task.execute(resId);
        }
    }

    static class AsyncDrawable extends BitmapDrawable {
        private final WeakReference bitmapWorkerTaskReference;

        public AsyncDrawable(Resources res, Bitmap bitmap,
                BitmapWorkerTask bitmapWorkerTask) {
            super(res, bitmap);
            bitmapWorkerTaskReference =
                new WeakReference(bitmapWorkerTask);
        }

        public BitmapWorkerTask getBitmapWorkerTask() {
            return bitmapWorkerTaskReference.get();
        }
    }

    public static boolean cancelPotentialWork(int data, ImageView imageView) {
        final BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);

        if (bitmapWorkerTask != null) {
            final int bitmapData = bitmapWorkerTask.data;
            if (bitmapData != data) {
                // Cancel previous task  
                bitmapWorkerTask.cancel(true);
            } else {
                // The same work is already in progress  
                return false;
            }
        }
        // No task associated with the ImageView, or an existing task was cancelled  
        return true;
    }

    private static BitmapWorkerTask getBitmapWorkerTask(ImageView imageView) {
       if (imageView != null) {
           final Drawable drawable = imageView.getDrawable();
           if (drawable instanceof AsyncDrawable) {
               final AsyncDrawable asyncDrawable = (AsyncDrawable) drawable;
               return asyncDrawable.getBitmapWorkerTask();
           }
        }
        return null;
    }

    ... // include updated BitmapWorkerTask class  

Note:对于 ListView 同样可以套用上面的方法。

上面的方法提供了足够的弹性，使得你可以做从网络加载与Resize大的数码照片等操作而不至于卡到UI Thread。

学习自：http://developer.android.com/training/displaying-bitmaps/display-bitmap.html，请多指教，谢谢！
转载请注明出自http://kesenhoo.github.com，谢谢配合！

胡凯

Android Training - 高效地显示Bitmap(Lesson 5 - 呈现Bitmap到UI上)

Load Bitmaps into a ViewPager Implementation(实现加载图片到ViewPager)

Load Bitmaps into a GridView Implementation(实现加载图片到GridView)

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