这是AsyncTask的另一个示例，该示例使用Fragment来处理的运行时配置更改（如用户旋转屏幕时）setRetainInstance(true)。还显示了确定的（定期更新）进度条。
该示例部分基于官方文档“ 在配置更改期间保留对象”。
在此示例中，需要后台线程的工作仅是将图像从Internet加载到UI中。
Alex Lockwood似乎是对的，当涉及使用AsyncTasks使用“保留的片段”来处理运行时配置更改时，这是最佳实践。onRetainNonConfigurationInstance()在Android Studio中的Lint中被弃用。官方文档警告我们android:configChanges不要使用“自行处理配置更改” ...
自己处理配置更改会使使用替代资源变得更加困难，因为系统不会自动为您应用它们。当您必须避免由于配置更改而导致重启时，应将这种技术视为万不得已的方法，建议不要将其用于大多数应用程序。
然后是一个问题，即是否应该对后台线程完全使用AsyncTask。
AsyncTask的官方参考警告...
理想情况下，应该将AsyncTasks用于较短的操作（最多几秒钟）。如果需要使线程长时间运行，则强烈建议您使用java.util.concurrent pacakge提供的各种API，例如执行程序，ThreadPoolExecutor和FutureTask。
或者，可以使用服务，加载程序（使用CursorLoader或AsyncTaskLoader）或内容提供程序来执行异步操作。
我将其余的帖子分解为：
步骤
从基本的AsyncTask开始，作为活动的内部类（它不一定是内部类，但可能很方便）。在此阶段，AsyncTask不处理运行时配置更改。
public class ThreadsActivity extends ActionBarActivity {
private ImageView mPictureImageView;
private class LoadImageFromNetworkAsyncTask
extends AsyncTask<String, Void, Bitmap> {
@Override
protected Bitmap doInBackground(String... urls) {
return loadImageFromNetwork(urls[0]);
}
@Override
protected void onPostExecute(Bitmap bitmap) {
mPictureImageView.setImageBitmap(bitmap);
}
}
/**
* Requires in AndroidManifext.xml
* <uses-permission android:name="android.permission.INTERNET" />
*/
private Bitmap loadImageFromNetwork(String url) {
Bitmap bitmap = null;
try {
bitmap = BitmapFactory.decodeStream((InputStream)
new URL(url).getContent());
} catch (Exception e) {
e.printStackTrace();
}
return bitmap;
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_threads);
mPictureImageView =
(ImageView) findViewById(R.id.imageView_picture);
}
public void getPicture(View view) {
new LoadImageFromNetworkAsyncTask()
.execute("http://i.imgur.com/SikTbWe.jpg");
}
}
添加一个嵌套类RetainedFragment，该类扩展了Fragement类，并且没有自己的UI。将setRetainInstance（true）添加到此Fragment的onCreate事件中。提供设置和获取数据的过程。
public class ThreadsActivity extends Activity {
private ImageView mPictureImageView;
private RetainedFragment mRetainedFragment = null;
...
public static class RetainedFragment extends Fragment {
private Bitmap mBitmap;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// The key to making data survive
// runtime configuration changes.
setRetainInstance(true);
}
public Bitmap getData() {
return this.mBitmap;
}
public void setData(Bitmap bitmapToRetain) {
this.mBitmap = bitmapToRetain;
}
}
private class LoadImageFromNetworkAsyncTask
extends AsyncTask<String, Integer,Bitmap> {
....
在最外层的Activity类的onCreate（）中处理RetainedFragment：如果已经存在，则对其进行引用（以防Activity重新启动）。创建并添加它（如果不存在）；然后，如果已经存在，请从RetainedFragment中获取数据，并使用该数据设置UI。
public class ThreadsActivity extends Activity {
...
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_threads);
final String retainedFragmentTag = "RetainedFragmentTag";
mPictureImageView =
(ImageView) findViewById(R.id.imageView_picture);
mLoadingProgressBar =
(ProgressBar) findViewById(R.id.progressBar_loading);
// Find the RetainedFragment on Activity restarts
FragmentManager fm = getFragmentManager();
// The RetainedFragment has no UI so we must
// reference it with a tag.
mRetainedFragment =
(RetainedFragment) fm.findFragmentByTag(retainedFragmentTag);
// if Retained Fragment doesn't exist create and add it.
if (mRetainedFragment == null) {
// Add the fragment
mRetainedFragment = new RetainedFragment();
fm.beginTransaction()
.add(mRetainedFragment, retainedFragmentTag).commit();
// The Retained Fragment exists
} else {
mPictureImageView
.setImageBitmap(mRetainedFragment.getData());
}
}
从UI初始化AsyncTask
public void getPicture(View view) {
new LoadImageFromNetworkAsyncTask().execute(
"http://i.imgur.com/SikTbWe.jpg");
}
添加并编写一个确定的进度条：
- 将进度条添加到UI布局；
- 在Activity oncreate（）;中获得对它的引用；
- 在过程的开始和结束时使其可见且不可见；
- 定义进度以在onProgressUpdate中报告给UI。
- 将AsyncTask 2nd Generic参数从Void更改为可以处理进度更新的类型（例如Integer）。
- 在doInBackground（）中的常规点处进行publishProgress。
以上过程的所有代码
活动布局。
<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.mysecondapp.ThreadsActivity">
<RelativeLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin">
<ImageView
android:id="@+id/imageView_picture"
android:layout_width="300dp"
android:layout_height="300dp"
android:background="@android:color/black" />
<Button
android:id="@+id/button_get_picture"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_below="@id/imageView_picture"
android:onClick="getPicture"
android:text="Get Picture" />
<Button
android:id="@+id/button_clear_picture"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignBottom="@id/button_get_picture"
android:layout_toEndOf="@id/button_get_picture"
android:layout_toRightOf="@id/button_get_picture"
android:onClick="clearPicture"
android:text="Clear Picture" />
<ProgressBar
android:id="@+id/progressBar_loading"
style="?android:attr/progressBarStyleHorizontal"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_below="@id/button_get_picture"
android:progress="0"
android:indeterminateOnly="false"
android:visibility="invisible" />
</RelativeLayout>
</ScrollView>
Activity具有：子类化的AsyncTask内部类；RetainedFragment内部类的子类，用于处理运行时配置更改（例如，当用户旋转屏幕时）；并定期更新确定的进度条。...
public class ThreadsActivity extends Activity {
private ImageView mPictureImageView;
private RetainedFragment mRetainedFragment = null;
private ProgressBar mLoadingProgressBar;
public static class RetainedFragment extends Fragment {
private Bitmap mBitmap;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// The key to making data survive runtime configuration changes.
setRetainInstance(true);
}
public Bitmap getData() {
return this.mBitmap;
}
public void setData(Bitmap bitmapToRetain) {
this.mBitmap = bitmapToRetain;
}
}
private class LoadImageFromNetworkAsyncTask extends AsyncTask<String,
Integer, Bitmap> {
@Override
protected Bitmap doInBackground(String... urls) {
// Simulate a burdensome load.
int sleepSeconds = 4;
for (int i = 1; i <= sleepSeconds; i++) {
SystemClock.sleep(1000); // milliseconds
publishProgress(i * 20); // Adjust for a scale to 100
}
return com.example.standardapplibrary.android.Network
.loadImageFromNetwork(
urls[0]);
}
@Override
protected void onProgressUpdate(Integer... progress) {
mLoadingProgressBar.setProgress(progress[0]);
}
@Override
protected void onPostExecute(Bitmap bitmap) {
publishProgress(100);
mRetainedFragment.setData(bitmap);
mPictureImageView.setImageBitmap(bitmap);
mLoadingProgressBar.setVisibility(View.INVISIBLE);
publishProgress(0);
}
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_threads);
final String retainedFragmentTag = "RetainedFragmentTag";
mPictureImageView = (ImageView) findViewById(R.id.imageView_picture);
mLoadingProgressBar = (ProgressBar) findViewById(R.id.progressBar_loading);
// Find the RetainedFragment on Activity restarts
FragmentManager fm = getFragmentManager();
// The RetainedFragment has no UI so we must reference it with a tag.
mRetainedFragment = (RetainedFragment) fm.findFragmentByTag(
retainedFragmentTag);
// if Retained Fragment doesn't exist create and add it.
if (mRetainedFragment == null) {
// Add the fragment
mRetainedFragment = new RetainedFragment();
fm.beginTransaction().add(mRetainedFragment,
retainedFragmentTag).commit();
// The Retained Fragment exists
} else {
mPictureImageView.setImageBitmap(mRetainedFragment.getData());
}
}
public void getPicture(View view) {
mLoadingProgressBar.setVisibility(View.VISIBLE);
new LoadImageFromNetworkAsyncTask().execute(
"http://i.imgur.com/SikTbWe.jpg");
}
public void clearPicture(View view) {
mRetainedFragment.setData(null);
mPictureImageView.setImageBitmap(null);
}
}
在此示例中，库函数（上面使用显式包前缀com.example.standardapplibrary.android.Network引用）确实有效。
public static Bitmap loadImageFromNetwork(String url) {
Bitmap bitmap = null;
try {
bitmap = BitmapFactory.decodeStream((InputStream) new URL(url)
.getContent());
} catch (Exception e) {
e.printStackTrace();
}
return bitmap;
}
将您的后台任务所需的任何权限添加到AndroidManifest.xml中。
<manifest>
...
<uses-permission android:name="android.permission.INTERNET" />
将您的活动添加到AndroidManifest.xml ...
<manifest>
...
<application>
<activity
android:name=".ThreadsActivity"
android:label="@string/title_activity_threads"
android:parentActivityName=".MainActivity">
<meta-data
android:name="android.support.PARENT_ACTIVITY"
android:value="com.example.mysecondapp.MainActivity" />
</activity>