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
与ViewPager
组件,并且学习处理并发与配置改变问题。GridView
- [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
组件来实现这个效果. 然而,一个更加合适的Adapter是PagerAdapter 的子类ViewPager
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FragmentStatePagerAdapter:
的状态。这样能够保持消耗更少的内存。Fragments
- 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
或PagerAdapter
会更加合适。FragmentPagerAdapter
- 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
- 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);
}
}
}
-
里面包含了Fragment
的子组件:ImageView
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
会比较好。AsyncTask
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
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:BitmapWorkerTask
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
会循环使用子视图)。GridView
- 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!