【Android Training - 09】高效地显示Bitmap图片 [ Lesson 5 - 使用ViewPager与GridView显示图片 ]

Displaying Bitmaps in Your UI [在UI上呈现Bitmap]

• 这一课会演示如何运用前面三节课的内容，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，例如网易阅读，在阅读文章时可以左右滑动来切换文章，在Google Reader里面文章的切换也是这样，不过最终网易还是做的稍微好些，因为GReader对手势的判断有点过了，一不小心左右碰到就可能文章被切换掉，这点让我很苦恼，而网易阅读避免了这样的问题，需要用户动作稍大才会出现切换的效果] . 
• 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Lesson 2: Processing Bitmaps Off the UI Thread 。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。
• 对于完整的代码示例，请参考Lesson 0：章节概览，下载完整示例。

学习自：http://developer.android.com/training/displaying-bitmaps/display-bitmap.html，请多指教，谢谢！

转载请注明出处 ： http://blog.csdn.net/kesenhoo ，谢谢配合！