这是另一个 AsyncTask 示例，它使用 Fragment 处理运行时配置更改（如当用户旋转屏幕时）和 setRetainInstance(true)。还演示了一个确定的（定期更新的）进度条。
示例部分基于官方文档Retaining an Object During a Configuration Change。
在此示例中，需要后台线程的工作仅仅是将图像从 Internet 加载到 UI 中。
Alex Lockwood 似乎是对的，在使用“保留片段”处理 AsyncTasks 的运行时配置更改时，这是最佳实践。 onRetainNonConfigurationInstance() 在 Android Studio 的 Lint 中已弃用。官方文档警告我们不要使用android:configChanges，来自Handling the Configuration Change Yourself，...
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重新启动并且不建议用于大多数应用程序时，应将此技术视为最后的手段。
还有一个问题是是否应该为后台线程使用 AsyncTask。
official reference for AsyncTask 警告...
AsyncTasks 最好用于短时间的操作（最多几秒钟）。如果您需要保持线程长时间运行，强烈建议您使用 java.util.concurrent 提供的各种 API Executor、ThreadPoolExecutor、FutureTask等包。
也可以使用服务、加载程序（使用 CursorLoader 或 AsyncTaskLoader）或内容提供程序来执行异步操作。
我将帖子的其余部分分解为：
程序
-
从一个基本的 AsyncTask 作为 Activity 的内部类开始（它不需要是内部类，但它可能会很方便）。在这个阶段，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，它扩展了 Fragment 类并且没有它自己的 UI。将 setRetainInstance(true) 添加到该 Fragment 的 onCreate 事件中。提供设置和获取数据的程序。
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：如果已经存在就引用它（以防Activity正在重启）；如果它不存在，则创建并添加它；然后，如果它已经存在，则从 RetainedFragment 获取数据并使用该数据设置您的 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 启动 AsyncTask
public void getPicture(View view) {
new LoadImageFromNetworkAsyncTask().execute(
"http://i.imgur.com/SikTbWe.jpg");
}
-
添加并编码一个确定的进度条：
- 在 UI 布局中添加进度条；
- 在Activity oncreate()中获取对它的引用；
- 使其在进程开始和结束时可见和不可见；
- 在 onProgressUpdate 中定义要向 UI 报告的进度。
- 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() 中的常规点发布进度。
上述过程的所有代码
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 带有：子类 AsyncTask 内部类；处理运行时配置更改的子类 RetainedFragment 内部类（例如，当用户旋转屏幕时）；以及定期更新的确定进度条。 ...
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);
}
}
在这个例子中，库函数（上面引用了显式的包前缀 com.example.standardapplibrary.android.Network）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 ...
<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>