Android应用程序线程消息循环模型分析（4）

接下来我们再看看应用程序的配置文件AndroidManifest.xml：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="shy.luo.counter"
android:versionCode="1"
android:versionName="1.0">
<application android:icon="@drawable/icon" android:label="@string/app_name">
<activity android:name=".Counter"
android:label="@string/app_name">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
</manifest>

这个配置文件很简单，我们就不介绍了。

再来看应用程序的界面文件，它定义在res/layout/main.xml文件中：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:gravity="center">
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_marginBottom="10px"
android:orientation="horizontal"
android:gravity="center">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginRight="4px"
android:gravity="center"
android:text="@string/counter">
</TextView>
<TextView
android:id="@+id/textview_counter"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="center"
android:text="0">
</TextView>
</LinearLayout>
<LinearLayout
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
android:gravity="center">
<Button
android:id="@+id/button_start"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="center"
android:text="@string/start">
</Button>
<Button
android:id="@+id/button_stop"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="center"
android:text="@string/stop" >
</Button>
</LinearLayout>
</LinearLayout>

这个界面配置文件也很简单，等一下我们在模拟器把这个应用程序启动起来后，就可以看到它的截图了。应用程序用到的字符串资源文件位于res/values/strings.xml文件中：

<?xml version="1.0" encoding="utf-8"?>
<resources>
<string name="app_name">Counter</string>
<string name="counter">Counter: </string>
<string name="start">Start Counter</string>
<string name="stop">Stop Counter</string>
</resources>

最后，我们还要在工程目录下放置一个编译脚本文件Android.mk：

LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)

LOCAL_MODULE_TAGS := optional

LOCAL_SRC_FILES := $(call all-subdir-java-files)

LOCAL_PACKAGE_NAME := Counter

include $(BUILD_PACKAGE)

接下来就要编译了。有关如何单独编译Android源代码工程的模块，以及如何打包system.img，请参考如何单独编译Android源代码中的模块一文。
执行以下命令进行编译和打包：

USER-NAME@MACHINE-NAME:~/Android$ mmm packages/experimental/Counter
USER-NAME@MACHINE-NAME:~/Android$ make snod

这样，打包好的Android系统镜像文件system.img就包含我们前面创建的Counter应用程序了。

再接下来，就是运行模拟器来运行我们的例子了。关于如何在Android源代码工程中运行模拟器，请参考在Ubuntu上下载、编译和安装Android最新源代码一文。
执行以下命令启动模拟器：

USER-NAME@MACHINE-NAME:~/Android$ emulator

最后我们就可以在Launcher中找到Counter应用程序图标，把它启动起来，点击Start按钮，就会看到应用程序界面上的计数器跑起来了：


这样，使用AsyncTask的例子就介绍完了，下面，我们就要根据上面对AsyncTask的使用情况来重点分析它的实现了。

AsyncTask类定义在frameworks/base/core/java/android/os/AsyncTask.java文件中：

public abstract class AsyncTask<Params, Progress, Result> {
......

private static final BlockingQueue<Runnable> sWorkQueue =
new LinkedBlockingQueue<Runnable>(10);

private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);

public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};

......

private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);

private static final int MESSAGE_POST_RESULT = 0x1;
private static final int MESSAGE_POST_PROGRESS = 0x2;
private static final int MESSAGE_POST_CANCEL = 0x3;

private static final InternalHandler sHandler = new InternalHandler();

private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;

......

public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
public Result call() throws Exception {
......
return doInBackground(mParams);
}
};

mFuture = new FutureTask<Result>(mWorker) {
@Override
protected void done() {
Message message;
Result result = null;

try {
result = get();
} catch (InterruptedException e) {
android.util.Log.w(LOG_TAG, e);
} catch (ExecutionException e) {
throw new RuntimeException("An error occured while executing doInBackground()",
e.getCause());
} catch (CancellationException e) {
message = sHandler.obtainMessage(MESSAGE_POST_CANCEL,
new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null));
message.sendToTarget();
return;
} catch (Throwable t) {
throw new RuntimeException("An error occured while executing "
+ "doInBackground()", t);
}

message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(AsyncTask.this, result));
message.sendToTarget();
}
};
}

......

public final Result get() throws InterruptedException, ExecutionException {
return mFuture.get();
}

......

public final AsyncTask<Params, Progress, Result> execute(Params... params) {
......

mWorker.mParams = params;
sExecutor.execute(mFuture);

return this;
}

......

protected final void publishProgress(Progress... values) {
sHandler.obtainMessage(MESSAGE_POST_PROGRESS,
new AsyncTaskResult<Progress>(this, values)).sendToTarget();
}

private void finish(Result result) {
......
onPostExecute(result);
......
}

......

private static class InternalHandler extends Handler {
@SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})
@Override
public void handleMessage(Message msg) {
AsyncTaskResult result = (AsyncTaskResult) msg.obj;
switch (msg.what) {
case MESSAGE_POST_RESULT:
// There is only one result
result.mTask.finish(result.mData[0]);
break;
case MESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);
break;
case MESSAGE_POST_CANCEL:
result.mTask.onCancelled();
break;
}
}
}

private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
Params[] mParams;
}

private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;

AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
}