【Android Advanced Training - 09】高效地显示Bitmap图片 [ Lesson 4 - 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！