在ListView上的item顯示網絡圖片，首先考慮的是2個因素
1.防止內存溢出
2.異步加載網絡圖片，防止ANR
針對以上2個問題，第一個問題解決方案是1.使用inSampleSize縮小圖片，2.且及時回收使用的Bitmap。第二個問題的解決方案是使用AsyncTask，下面討論這2個方法的具體實現，大部分代碼來自於Google的ApiDemo：DisplayingBitmapsSample，本人只是做了少量修改。
使用inSampleSize，參考這篇：BitMap高效顯示策略（一）：大圖的縮放和加載
回收Bitmap：Bitmap的回收使用recycle（）方法。爲了讓代碼能更好的複用，不建議在代碼中手動調用recycle，因爲Imageview通過BitmapDrawable去顯示Bitmap，所以可以定義一個BitmapDrawable的子類去實現這個回收邏輯
public class RecyclingBitmapDrawable extends BitmapDrawable {
private int mCacheRefCount = 0;
private int mDisplayRefCount = 0;
private boolean mHasBeenDisplayed;
public RecyclingBitmapDrawable(Resources res, Bitmap bitmap) {
super(res, bitmap);
}
public void setIsDisplayed(boolean isDisplayed) {
synchronized (this) {
if (isDisplayed) {
mDisplayRefCount++;
mHasBeenDisplayed = true;
} else {
mDisplayRefCount--;
}
}
checkState();
}
public void setIsCached(boolean isCached) {
synchronized (this) {
if (isCached) {
mCacheRefCount++;
} else {
mCacheRefCount--;
}
}
checkState();
}
private synchronized void checkState() {
if (mCacheRefCount <= 0 && mDisplayRefCount <= 0 && mHasBeenDisplayed
&& hasValidBitmap()) {
getBitmap().recycle();
}
}
private synchronized boolean hasValidBitmap() {
Bitmap bitmap = getBitmap();
return bitmap != null && !bitmap.isRecycled();
}
}
這個RecyclingBitmapDrawable的setIsDisplayed方法在有ImageView調用setImageDrawable的時被調用，這個過程也不需要手動調用，可以通過實現ImageView的子類去實現。
public class RecyclingImageView extends ImageView {
public RecyclingImageView(Context context) {
super(context);
}
public RecyclingImageView(Context context, AttributeSet attrs) {
super(context, attrs);
}
@Override
protected void onDetachedFromWindow() {
setImageDrawable(null);
super.onDetachedFromWindow();
}
@Override
public void setImageDrawable(Drawable drawable) {
final Drawable previousDrawable = getDrawable();
super.setImageDrawable(drawable);
notifyDrawable(drawable, true);
notifyDrawable(previousDrawable, false);
}
private static void notifyDrawable(Drawable drawable, final boolean isDisplayed) {
if (drawable instanceof RecyclingBitmapDrawable) {
((RecyclingBitmapDrawable) drawable).setIsDisplayed(isDisplayed);
} else if (drawable instanceof LayerDrawable) {
LayerDrawable layerDrawable = (LayerDrawable) drawable;
for (int i = 0, z = layerDrawable.getNumberOfLayers(); i < z; i++) {
notifyDrawable(layerDrawable.getDrawable(i), isDisplayed);
}
}
}
}以上通過實現2個類，可以做到bitmap的recycle方法隱式調用，不用手動管理，只要在佈局文件中使用RecyclingImageView顯示圖片即可。
使用AsyncTask：
爲了避免ANR，應該避免在主線程中進行網絡下載／讀取文件的操作，ListView上Item下載圖片應使用AsyncTask。
在Adapter的getView中，爲每個ImageView去啓動一個AsyncTask用於下載圖片。
class BitmapWorkerTask extends AsyncTask<Void, Void, BitmapDrawable> {
private final WeakReference imageViewReference;
private int mUrl = 0;
public BitmapWorkerTask(ImageView imageView，String url) {
imageViewReference = new WeakReference(imageView);
mUrl = url;
}
@Override
protected Bitmap doInBackground(Void... params) {
bitmap = processBitmap(mUrl); //子類實現
return bitmap;
}
@Override
protected void onPostExecute(Bitmap bitmap) {
if (imageViewReference != null && bitmap != null) {
final ImageView imageView = imageViewReference.get();
if (imageView != null) {
imageView.setImageBitmap(bitmap);
}
}
}
}以上的代碼，在ListView不進行上下滑動的時候，是沒問題的，但是如果在ListView進行滑動時，會出現問題：
爲了內存的優化，ListView／GridView等組件，在Item被滑動到屏幕上看不到的位置時，會被回收，用作下個新的可視Item，如果每個Item上的ImageView都啓動一個AsyncTask，則有可能出現當ListView向下划動時，新出現的Item是之前被回收的，那麼這個Item加載圖片的Url是上一個Item的Url，這樣會造成圖片顯示順序被打亂的情況。
針對這種情況的解決方法是：
定義一個AsyncDrawable繼承BitmapDrawable，作爲ImageView的佔位Drawable，在加載圖片的AsyncTask執行完畢後，在ImageView上顯示圖片。它保存一個最近使用的AsyncTask引用，加載圖片的AsyncTask中也保存ImageVIew的引用。
在加載圖片到指定的ImageView的任務開始前，判斷和ImageView關聯的，當前執行的任務是否之前已經在被執行（下面的cancelPotentialWork方法）。
在加載圖片的任務的doInBackgound和onPostExecute中，再次通過AsyncDrawable中引用的 AsyncTask 判斷當前的加載任務是否和之前的匹配（下面的getAttachedImageView方法）
代碼：
ImageWorker.java:異步加載網絡圖片的核心類
/***
*
* ListView上每個item啓動一個AsyncTask去下載圖片，AsyncTask在容量爲2的線程池上運行。
* 每個AsyncTask和item上的下載url和imageview綁定
*
*/
public abstract class ImageWorker {
private boolean mExitTasksEarly = false; // 是否提前退出的標誌
protected boolean mPauseWork = false;
private final Object mPauseWorkLock = new Object();
protected Resources mResources;
protected ImageWorker(Context context) {
mResources = context.getResources();
}
public void loadImage(String url, ImageView imageView) {
if (url == null) {
return;
}
if (cancelPotentialWork(url, imageView)) {
final BitmapWorkerTask task = new BitmapWorkerTask(url, imageView);
final AsyncDrawable asyncDrawable = new AsyncDrawable(mResources,
task);
imageView.setImageDrawable(asyncDrawable);
task.executeOnExecutor(ImageAsyncTask.DUAL_THREAD_EXECUTOR);
}
}
/**
*
*
* 當item進行圖片下載時，這個item有可能是以前回收的item，此時item上的imageView還和一個task關聯着，
* 檢查當前的task和imageview綁定的以前的task是不是在下載同一個圖片， 不匹配則終止以前運行的task，否則繼續之前的下載
*
* @param data
* @param imageView
* @return true表示需要重新下載
*/
public static boolean cancelPotentialWork(String currentUrl,
ImageView imageView) {
BitmapWorkerTask oldBitmapWorkerTask = getBitmapWorkerTask(imageView);
if (oldBitmapWorkerTask != null) {
String oldUrl = oldBitmapWorkerTask.mUrl;
if (oldUrl == null || !oldUrl.equals(currentUrl)) {
oldBitmapWorkerTask.cancel(true);
} else {
return false;
}
}
return true;
}
/***
* 下載圖片的task。 ListView上Item的ImageView被回收重用後，綁定的下載圖片task可能和之前的不一樣，
* Item回收可能發生在:task執行中，task執行後。這2種情況都需要校驗，通過{@link #getAttachedImageView()}
* 方法
*
*/
private class BitmapWorkerTask extends
ImageAsyncTask<Void, Void, BitmapDrawable> {
private String mUrl;
private final WeakReference<ImageView> imageViewReference;
public BitmapWorkerTask(String url, ImageView imageView) {
mUrl = url;
imageViewReference = new WeakReference<ImageView>(imageView);
}
@Override
protected BitmapDrawable doInBackground(Void... params) {
Bitmap bitmap = null;
BitmapDrawable drawable = null;
synchronized (mPauseWorkLock) {
while (mPauseWork && !isCancelled()) {
try {
mPauseWorkLock.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
if (bitmap == null && !isCancelled()
&& getAttachedImageView() != null && !mExitTasksEarly) {
bitmap = processBitmap(mUrl);
}
if (bitmap != null) {
drawable = new BitmapDrawable(mResources, bitmap);
}
return drawable;
}
@Override
protected void onPostExecute(BitmapDrawable result) {
if (isCancelled() || mExitTasksEarly) {
result = null;
}
ImageView imageView = getAttachedImageView();
if (result != null && imageView != null) {
setImageDrawable(imageView, result);
}
}
@Override
protected void onCancelled(BitmapDrawable value) {
super.onCancelled(value);
synchronized (mPauseWorkLock) {
mPauseWorkLock.notifyAll();
}
}
/***
* 返回當前綁定task的ImageView，如果ImageView綁定的task不是自己，則返回null
*
* @return
*/
private ImageView getAttachedImageView() {
ImageView imageView = imageViewReference.get();
BitmapWorkerTask bitmapWorkerTask = getBitmapWorkerTask(imageView);
if (this == bitmapWorkerTask) {
return imageView;
}
return null;
}
}
/***
* 圖片下載完畢後設置ImageView
*
* @param imageView
* @param drawable
*/
private void setImageDrawable(ImageView imageView, Drawable drawable) {
imageView.setImageDrawable(drawable);
}
/***
* 返回當前ImageView綁定的task
*
* @param imageView
* @return
*/
private static BitmapWorkerTask getBitmapWorkerTask(ImageView imageView) {
if (imageView != null) {
Drawable drawable = imageView.getDrawable();
if (drawable instanceof AsyncDrawable) {
final AsyncDrawable asyncDrawable = (AsyncDrawable) drawable;
return asyncDrawable.getBitmapWorkerTask();
}
}
return null;
}
private static class AsyncDrawable extends BitmapDrawable {
private final WeakReference<BitmapWorkerTask> bitmapWorkerTaskReference;
public AsyncDrawable(Resources res, BitmapWorkerTask bitmapWorkerTask) {
super(res);
bitmapWorkerTaskReference = new WeakReference<ImageWorker.BitmapWorkerTask>(
bitmapWorkerTask);
}
public BitmapWorkerTask getBitmapWorkerTask() {
return bitmapWorkerTaskReference.get();
}
}
public void setPauseWork(boolean pauseWork) {
synchronized (mPauseWorkLock) {
mPauseWork = pauseWork;
if (!mPauseWork) {
mPauseWorkLock.notifyAll();
}
}
}
public void setExitTasksEarly(boolean exitTasksEarly) {
mExitTasksEarly = exitTasksEarly;
setPauseWork(false);
}
/***
* 下載圖片的代碼由子類實現
*
* @param url
* @return
*/
protected abstract Bitmap processBitmap(String url);
}它們都調用了 getBitmapWorkerTask 方法。getBitmapWorkerTask獲得與這個ImageView綁定的BitmapWorkerTask，用於和當前的BitmapWorkerTask比對
ImageResizer：繼承ImageWorker，處理圖片縮放。
這樣相當於在 BitmapWorkerTask開始前，執行中，執行後都做了檢查，所以避免了在這3個階段的某個階段中，ListView回收Item導致圖片順序打亂的問題。
public class ImageResizer extends ImageWorker{
private static final String TAG = "ImageResizer";
private int mImageWidth;
private int mImageHeight;
public ImageResizer(Context context, int imageWidth, int imageHeight) {
super(context);
setImageSize(imageWidth, imageHeight);
}
public ImageResizer(Context context, int imageSize) {
super(context);
setImageSize(imageSize, imageSize);
}
protected ImageResizer(Context context) {
super(context);
}
public void setImageSize(int width, int height) {
mImageWidth = width;
mImageHeight = height;
}
public int getmImageWidth() {
return mImageWidth;
}
public int getmImageHeight() {
return mImageHeight;
}
@Override
protected Bitmap processBitmap(String url) {
return null;
}
public static BitmapFactory.Options getSampledBitmapOptionsFromStream(
InputStream is, int reqWidth, int reqHeight) {
final BitmapFactory.Options options = new BitmapFactory.Options();
options.inJustDecodeBounds = true;
BitmapFactory.decodeStream(is, null, options);
// 計算縮放比例
options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
options.inJustDecodeBounds = false;
return options;
}
public static Bitmap decodeSampledBitmapFromStream(
InputStream is, BitmapFactory.Options options) {
return BitmapFactory.decodeStream(is, null, options);
}
/**
* @Description: 計算圖片縮放比例
* @param @param options
* @param @param reqWidth
* @param @param reqHeight
* @param @return
* @return int 縮小的比例
* @throws
*/
public static int calculateInSampleSize(BitmapFactory.Options options,
int reqWidth, int reqHeight) {
final int height = options.outHeight;
final int width = options.outWidth;
int inSampleSize = 1;
if (height > reqHeight || width > reqWidth) {
final int halfHeight = height / 2;
final int halfWidth = width / 2;
while ((halfHeight / inSampleSize) > reqHeight
&& (halfWidth / inSampleSize) > reqWidth) {
inSampleSize *= 2;
}
}
return inSampleSize;
}
}
public class ImageDownloader extends ImageResizer {
public ImageDownloader(Context context) {
super(context);
}
public ImageDownloader(Context context, int imageSize) {
super(context, imageSize);
}
@Override
protected Bitmap processBitmap(String url) {
return downLoadByUrl(url);
}
public Bitmap downLoadByUrl(String urlString) {
disableConnectionReuseIfNecessary();
HttpURLConnection urlConnection = null;
InputStream in = null;
try {
final URL url = new URL(urlString);
urlConnection = (HttpURLConnection) url.openConnection();
in = urlConnection.getInputStream();
final BitmapFactory.Options options =
getSampledBitmapOptionsFromStream(in, getmImageWidth(), getmImageHeight());
in.close();
urlConnection.disconnect();
//重新獲得輸入流
urlConnection = (HttpURLConnection) url.openConnection();
in = urlConnection.getInputStream();
Bitmap bitmap = decodeSampledBitmapFromStream(in, options);
return bitmap;
} catch (final IOException e) {
Log.e("text", "Error in downloadBitmap - " + e);
} finally {
if (urlConnection != null) {
urlConnection.disconnect();
}
try {
if (in != null) {
in.close();
}
} catch (final IOException e) {}
}
return null;
}
public static void disableConnectionReuseIfNecessary() {
if (Build.VERSION.SDK_INT < Build.VERSION_CODES.FROYO) {
System.setProperty("http.keepAlive", "false");
}
}
}
ImageGridFragment：
public class ImageGridFragment extends Fragment {
private static final String TAG = "ImageGridFragment";
private ImageWorker mImageWorker;
private ImageAdapter mAdapter;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mImageWorker = new ImageFetcher(getActivity(), 50);
}
@Override
public View onCreateView(LayoutInflater inflater,
@Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.list, container, false);
ListView listView = (ListView) v.findViewById(R.id.image_list);
mAdapter = new ImageAdapter(getActivity());
listView.setAdapter(mAdapter);
listView.setOnScrollListener(new AbsListView.OnScrollListener() {
@Override
public void onScrollStateChanged(AbsListView absListView, int scrollState) {
if (scrollState == AbsListView.OnScrollListener.SCROLL_STATE_FLING) {
if (Build.VERSION.SDK_INT >= VERSION_CODES.HONEYCOMB) {
mImageWorker.setPauseWork(true);
}
} else {
mImageWorker.setPauseWork(false);
}
}
@Override
public void onScroll(AbsListView absListView, int firstVisibleItem,
int visibleItemCount, int totalItemCount) {
}
});
return v;
}
@Override
public void onResume() {
super.onResume();
mImageWorker.setExitTasksEarly(false);
mAdapter.notifyDataSetChanged();
}
@Override
public void onPause() {
super.onPause();
mImageWorker.setPauseWork(false);
mImageWorker.setExitTasksEarly(true);
}
private class ImageAdapter extends BaseAdapter {
private final Context mContext;
private LayoutInflater mInflater;
public ImageAdapter(Context context) {
this.mContext = context;
this.mInflater = LayoutInflater.from(mContext);
}
@Override
public int getCount() {
return Images.imageThumbUrls.length;
}
@Override
public Object getItem(int position) {
return null;
}
@Override
public long getItemId(int position) {
return 0;
}
@Override
public View getView(int position, View convertView, ViewGroup parent) {
ImageView imageView;
if (convertView == null) {
convertView = mInflater.inflate(R.layout.list_item, parent, false);
}
imageView = (RecyclingImageView) convertView.findViewById(R.id.img);
//參數：url,imageview
mImageWorker.loadImage(Images.imageThumbUrls[position], imageView);
return convertView;
}
}
}public class MainActivity extends ActionBarActivity {
private static final String TAG = "ImageGridActivity";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
if (getSupportFragmentManager().findFragmentByTag(TAG) == null) {
final FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
ft.add(android.R.id.content, new ImageGridFragment(), TAG);
ft.commit();
}
}
}佈局：
list.xml：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<ListView
android:id="@+id/image_list"
android:layout_width="match_parent"
android:layout_height="match_parent"
></ListView>
</LinearLayout>
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<com.example.displayingbitamapdemov1.image.RecyclingImageView
android:id="@+id/img"
android:layout_width="60dp"
android:layout_height="60dp"
android:background="@drawable/ic_launcher"
android:scaleType="centerCrop"
/>
</LinearLayout>
Demo下載地址：
http://download.csdn.net/detail/ohehehou/8110965