关于异步任务AsyncTask的解析
2013-05-17 19:28
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在Android中实现异步任务机制有两种方式,Handler和AsyncTask。
Handler模式需要为每一个任务创建一个新的线程,任务完成后通过Handler实例向UI线程发送消息,完成界面的更新,这种方式对于整个过程的控制比较精细,但也是有缺点的,例如代码相对臃肿,在多个任务同时执行时,不易对线程进行精确的控制。关于Handler的相关知识,前面也有所介绍,不清楚的朋友们可以参照一下。
为了简化操作,Android1.5提供了工具类android.os.AsyncTask,它使创建异步任务变得更加简单,不再需要编写任务线程和Handler实例即可完成相同的任务。
先来看看AsyncTask的定义:
[java]
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public abstract class AsyncTask<Params, Progress, Result> {
布局文件main.xml代码如下:
[xhtml]
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<?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">
<Button
android:id="@+id/execute"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="execute"/>
<Button
android:id="@+id/cancel"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:enabled="false"
android:text="cancel"/>
<ProgressBar
android:id="@+id/progress_bar"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:progress="0"
android:max="100"
style="?android:attr/progressBarStyleHorizontal"/>
<ScrollView
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<TextView
android:id="@+id/text_view"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
</ScrollView>
</LinearLayout>
我们来看一下运行时的界面:
以上几个截图分别是初始界面、执行异步任务时界面、执行成功后界面、取消任务后界面。执行成功后,整个过程日志打印如下:
如果我们在执行任务时按下了“cancel”按钮,日志打印如下:
可以看到onCancelled()方法将会被调用,onPostExecute(Result result)方法将不再被调用。
上面介绍了AsyncTask的基本应用,有些朋友也许会有疑惑,AsyncTask内部是怎么执行的呢,它执行的过程跟我们使用Handler又有什么区别呢?答案是:AsyncTask是对Thread+Handler良好的封装,在android.os.AsyncTask代码里仍然可以看到Thread和Handler的踪迹。下面就向大家详细介绍一下AsyncTask的执行原理。
我们先看一下AsyncTask的大纲视图:
我们可以看到关键几个步骤的方法都在其中,doInBackground(Params... params)是一个抽象方法,我们继承AsyncTask时必须覆写此方法;onPreExecute()、onProgressUpdate(Progress... values)、onPostExecute(Result result)、onCancelled()这几个方法体都是空的,我们需要的时候可以选择性的覆写它们;publishProgress(Progress... values)是final修饰的,不能覆写,只能去调用,我们一般会在doInBackground(Params...
params)中调用此方法;另外,我们可以看到有一个Status的枚举类和getStatus()方法,Status枚举类代码段如下:
[java]
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//初始状态
private volatile Status mStatus = Status.PENDING;
public enum Status {
/**
* Indicates that the task has not been executed yet.
*/
PENDING,
/**
* Indicates that the task is running.
*/
RUNNING,
/**
* Indicates that {@link AsyncTask#onPostExecute} has finished.
*/
FINISHED,
}
/**
* Returns the current status of this task.
*
* @return The current status.
*/
public final Status getStatus() {
return mStatus;
}
代码中涉及到三个陌生的变量:mWorker、sExecutor、mFuture,我们也会看一下他们的庐山真面目:
关于sExecutor,它是java.util.concurrent.ThreadPoolExecutor的实例,用于管理线程的执行。代码如下:
[java]
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private static final int CORE_POOL_SIZE = 5;
private static final int MAXIMUM_POOL_SIZE = 128;
private static final int KEEP_ALIVE = 10;
//新建一个队列用来存放线程
private static final BlockingQueue<Runnable> sWorkQueue =
new LinkedBlockingQueue<Runnable>(10);
//新建一个线程工厂
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);
//新建一个线程
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
//新建一个线程池执行器,用于管理线程的执行
private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);
而mFuture实际上是java.util.concurrent.FutureTask的实例,下面是它的FutureTask类的相关信息:
[java]
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/**
* A cancellable asynchronous computation.
* ...
*/
public class FutureTask<V> implements RunnableFuture<V> {
可以看到FutureTask是一个可以中途取消的用于异步计算的类。
下面是mWorker和mFuture实例在AsyncTask中的体现:
[java]
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private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
//call方法被调用后,将设置优先级为后台级别,然后调用AsyncTask的doInBackground方法
public Result call() throws Exception {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
return doInBackground(mParams);
}
};
//在mFuture实例中,将会调用mWorker做后台任务,完成后会调用done方法
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) {
//发送取消任务的消息
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);
}
//发送显示结果的消息
message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(AsyncTask.this, result));
message.sendToTarget();
}
};
}
我们看到,在处理消息时,遇到“MESSAGE_POST_RESULT”时,它会调用AsyncTask中的finish()方法,我们来看一下finish()方法的定义:
[java]
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private void finish(Result result) {
if (isCancelled()) result = null;
onPostExecute(result); //调用onPostExecute显示结果
mStatus = Status.FINISHED; //改变状态为FINISHED
}
这个AsyncTaskResult究竟是什么呢,它又包含什么内容呢?其实它也是AsyncTask的一个内部类,是用来包装执行结果的一个类,让我们来看一下它的代码结构:
[java]
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@SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
[java]
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//发送显示结果的消息
message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(AsyncTask.this, result));
message.sendToTarget();
[java]
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result.mTask.onProgressUpdate(result.mData);
result.mTask.onProgressUpdate(result.mData);
概括来说,当我们调用execute(Params... params)方法后,execute方法会调用onPreExecute()方法,然后由ThreadPoolExecutor实例sExecutor执行一个FutureTask任务,这个过程中doInBackground(Params... params)将被调用,如果被开发者覆写的doInBackground(Params... params)方法中调用了publishProgress(Progress...
values)方法,则通过InternalHandler实例sHandler发送一条MESSAGE_POST_PROGRESS消息,更新进度,sHandler处理消息时onProgressUpdate(Progress... values)方法将被调用;如果遇到异常,则发送一条MESSAGE_POST_CANCEL的消息,取消任务,sHandler处理消息时onCancelled()方法将被调用;如果执行成功,则发送一条MESSAGE_POST_RESULT的消息,显示结果,sHandler处理消息时onPostExecute(Result
result)方法被调用。
经过上面的介绍,相信朋友们都已经认识到AsyncTask的本质了,它对Thread+Handler的良好封装,减少了开发者处理问题的复杂度,提高了开发效率,希望朋友们能多多体会一下。本文转自http://blog.csdn.net/liuhe688/article/details/6532519
Handler模式需要为每一个任务创建一个新的线程,任务完成后通过Handler实例向UI线程发送消息,完成界面的更新,这种方式对于整个过程的控制比较精细,但也是有缺点的,例如代码相对臃肿,在多个任务同时执行时,不易对线程进行精确的控制。关于Handler的相关知识,前面也有所介绍,不清楚的朋友们可以参照一下。
为了简化操作,Android1.5提供了工具类android.os.AsyncTask,它使创建异步任务变得更加简单,不再需要编写任务线程和Handler实例即可完成相同的任务。
先来看看AsyncTask的定义:
[java]
view plaincopyprint?
public abstract class AsyncTask<Params, Progress, Result> {
[java] view plaincopyprint? package com.scott.async; import java.io.ByteArrayOutputStream; import java.io.InputStream; import org.apache.http.HttpEntity; import org.apache.http.HttpResponse; import org.apache.http.HttpStatus; import org.apache.http.client.HttpClient; import org.apache.http.client.methods.HttpGet; import org.apache.http.impl.client.DefaultHttpClient; import android.app.Activity; import android.os.AsyncTask; import android.os.Bundle; import android.util.Log; import android.view.View; import android.widget.Button; import android.widget.ProgressBar; import android.widget.TextView; public class MainActivity extends Activity { private static final String TAG = "ASYNC_TASK"; private Button execute; private Button cancel; private ProgressBar progressBar; private TextView textView; private MyTask mTask; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); execute = (Button) findViewById(R.id.execute); execute.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { //注意每次需new一个实例,新建的任务只能执行一次,否则会出现异常 mTask = new MyTask(); mTask.execute("http://www.baidu.com"); execute.setEnabled(false); cancel.setEnabled(true); } }); cancel = (Button) findViewById(R.id.cancel); cancel.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { //取消一个正在执行的任务,onCancelled方法将会被调用 mTask.cancel(true); } }); progressBar = (ProgressBar) findViewById(R.id.progress_bar); textView = (TextView) findViewById(R.id.text_view); } private class MyTask extends AsyncTask<String, Integer, String> { //onPreExecute方法用于在执行后台任务前做一些UI操作 @Override protected void onPreExecute() { Log.i(TAG, "onPreExecute() called"); textView.setText("loading..."); } //doInBackground方法内部执行后台任务,不可在此方法内修改UI @Override protected String doInBackground(String... params) { Log.i(TAG, "doInBackground(Params... params) called"); try { HttpClient client = new DefaultHttpClient(); HttpGet get = new HttpGet(params[0]); HttpResponse response = client.execute(get); if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) { HttpEntity entity = response.getEntity(); InputStream is = entity.getContent(); long total = entity.getContentLength(); ByteArrayOutputStream baos = new ByteArrayOutputStream(); byte[] buf = new byte[1024]; int count = 0; int length = -1; while ((length = is.read(buf)) != -1) { baos.write(buf, 0, length); count += length; //调用publishProgress公布进度,最后onProgressUpdate方法将被执行 publishProgress((int) ((count / (float) total) * 100)); //为了演示进度,休眠500毫秒 Thread.sleep(500); } return new String(baos.toByteArray(), "gb2312"); } } catch (Exception e) { Log.e(TAG, e.getMessage()); } return null; } //onProgressUpdate方法用于更新进度信息 @Override protected void onProgressUpdate(Integer... progresses) { Log.i(TAG, "onProgressUpdate(Progress... progresses) called"); progressBar.setProgress(progresses[0]); textView.setText("loading..." + progresses[0] + "%"); } //onPostExecute方法用于在执行完后台任务后更新UI,显示结果 @Override protected void onPostExecute(String result) { Log.i(TAG, "onPostExecute(Result result) called"); textView.setText(result); execute.setEnabled(true); cancel.setEnabled(false); } //onCancelled方法用于在取消执行中的任务时更改UI @Override protected void onCancelled() { Log.i(TAG, "onCancelled() called"); textView.setText("cancelled"); progressBar.setProgress(0); execute.setEnabled(true); cancel.setEnabled(false); } } } package com.scott.async; import java.io.ByteArrayOutputStream; import java.io.InputStream; import org.apache.http.HttpEntity; import org.apache.http.HttpResponse; import org.apache.http.HttpStatus; import org.apache.http.client.HttpClient; import org.apache.http.client.methods.HttpGet; import org.apache.http.impl.client.DefaultHttpClient; import android.app.Activity; import android.os.AsyncTask; import android.os.Bundle; import android.util.Log; import android.view.View; import android.widget.Button; import android.widget.ProgressBar; import android.widget.TextView; public class MainActivity extends Activity { private static final String TAG = "ASYNC_TASK"; private Button execute; private Button cancel; private ProgressBar progressBar; private TextView textView; private MyTask mTask; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); execute = (Button) findViewById(R.id.execute); execute.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { //注意每次需new一个实例,新建的任务只能执行一次,否则会出现异常 mTask = new MyTask(); mTask.execute("http://www.baidu.com"); execute.setEnabled(false); cancel.setEnabled(true); } }); cancel = (Button) findViewById(R.id.cancel); cancel.setOnClickListener(new View.OnClickListener() { @Override public void onClick(View v) { //取消一个正在执行的任务,onCancelled方法将会被调用 mTask.cancel(true); } }); progressBar = (ProgressBar) findViewById(R.id.progress_bar); textView = (TextView) findViewById(R.id.text_view); } private class MyTask extends AsyncTask<String, Integer, String> { //onPreExecute方法用于在执行后台任务前做一些UI操作 @Override protected void onPreExecute() { Log.i(TAG, "onPreExecute() called"); textView.setText("loading..."); } //doInBackground方法内部执行后台任务,不可在此方法内修改UI @Override protected String doInBackground(String... params) { Log.i(TAG, "doInBackground(Params... params) called"); try { HttpClient client = new DefaultHttpClient(); HttpGet get = new HttpGet(params[0]); HttpResponse response = client.execute(get); if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) { HttpEntity entity = response.getEntity(); InputStream is = entity.getContent(); long total = entity.getContentLength(); ByteArrayOutputStream baos = new ByteArrayOutputStream(); byte[] buf = new byte[1024]; int count = 0; int length = -1; while ((length = is.read(buf)) != -1) { baos.write(buf, 0, length); count += length; //调用publishProgress公布进度,最后onProgressUpdate方法将被执行 publishProgress((int) ((count / (float) total) * 100)); //为了演示进度,休眠500毫秒 Thread.sleep(500); } return new String(baos.toByteArray(), "gb2312"); } } catch (Exception e) { Log.e(TAG, e.getMessage()); } return null; } //onProgressUpdate方法用于更新进度信息 @Override protected void onProgressUpdate(Integer... progresses) { Log.i(TAG, "onProgressUpdate(Progress... progresses) called"); progressBar.setProgress(progresses[0]); textView.setText("loading..." + progresses[0] + "%"); } //onPostExecute方法用于在执行完后台任务后更新UI,显示结果 @Override protected void onPostExecute(String result) { Log.i(TAG, "onPostExecute(Result result) called"); textView.setText(result); execute.setEnabled(true); cancel.setEnabled(false); } //onCancelled方法用于在取消执行中的任务时更改UI @Override protected void onCancelled() { Log.i(TAG, "onCancelled() called"); textView.setText("cancelled"); progressBar.setProgress(0); execute.setEnabled(true); cancel.setEnabled(false); } } }
布局文件main.xml代码如下:
[xhtml]
view plaincopyprint?
<?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">
<Button
android:id="@+id/execute"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="execute"/>
<Button
android:id="@+id/cancel"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:enabled="false"
android:text="cancel"/>
<ProgressBar
android:id="@+id/progress_bar"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:progress="0"
android:max="100"
style="?android:attr/progressBarStyleHorizontal"/>
<ScrollView
android:layout_width="fill_parent"
android:layout_height="wrap_content">
<TextView
android:id="@+id/text_view"
android:layout_width="fill_parent"
android:layout_height="wrap_content"/>
</ScrollView>
</LinearLayout>
[xhtml] view plaincopyprint? <uses-permission android:name="android.permission.INTERNET"/> <uses-permission android:name="android.permission.INTERNET"/>
我们来看一下运行时的界面:
以上几个截图分别是初始界面、执行异步任务时界面、执行成功后界面、取消任务后界面。执行成功后,整个过程日志打印如下:
如果我们在执行任务时按下了“cancel”按钮,日志打印如下:
可以看到onCancelled()方法将会被调用,onPostExecute(Result result)方法将不再被调用。
上面介绍了AsyncTask的基本应用,有些朋友也许会有疑惑,AsyncTask内部是怎么执行的呢,它执行的过程跟我们使用Handler又有什么区别呢?答案是:AsyncTask是对Thread+Handler良好的封装,在android.os.AsyncTask代码里仍然可以看到Thread和Handler的踪迹。下面就向大家详细介绍一下AsyncTask的执行原理。
我们先看一下AsyncTask的大纲视图:
我们可以看到关键几个步骤的方法都在其中,doInBackground(Params... params)是一个抽象方法,我们继承AsyncTask时必须覆写此方法;onPreExecute()、onProgressUpdate(Progress... values)、onPostExecute(Result result)、onCancelled()这几个方法体都是空的,我们需要的时候可以选择性的覆写它们;publishProgress(Progress... values)是final修饰的,不能覆写,只能去调用,我们一般会在doInBackground(Params...
params)中调用此方法;另外,我们可以看到有一个Status的枚举类和getStatus()方法,Status枚举类代码段如下:
[java]
view plaincopyprint?
//初始状态
private volatile Status mStatus = Status.PENDING;
public enum Status {
/**
* Indicates that the task has not been executed yet.
*/
PENDING,
/**
* Indicates that the task is running.
*/
RUNNING,
/**
* Indicates that {@link AsyncTask#onPostExecute} has finished.
*/
FINISHED,
}
/**
* Returns the current status of this task.
*
* @return The current status.
*/
public final Status getStatus() {
return mStatus;
}
[java] view plaincopyprint? public final AsyncTask<Params, Progress, Result> execute(Params... params) { if (mStatus != Status.PENDING) { switch (mStatus) { case RUNNING: //如果该任务正在被执行则抛出异常 //值得一提的是,在调用cancel取消任务后,状态仍未RUNNING throw new IllegalStateException("Cannot execute task:" + " the task is already running."); case FINISHED: //如果该任务已经执行完成则抛出异常 throw new IllegalStateException("Cannot execute task:" + " the task has already been executed " + "(a task can be executed only once)"); } } //改变状态为RUNNING mStatus = Status.RUNNING; //调用onPreExecute方法 onPreExecute(); mWorker.mParams = params; sExecutor.execute(mFuture); return this; } public final AsyncTask<Params, Progress, Result> execute(Params... params) { if (mStatus != Status.PENDING) { switch (mStatus) { case RUNNING: //如果该任务正在被执行则抛出异常 //值得一提的是,在调用cancel取消任务后,状态仍未RUNNING throw new IllegalStateException("Cannot execute task:" + " the task is already running."); case FINISHED: //如果该任务已经执行完成则抛出异常 throw new IllegalStateException("Cannot execute task:" + " the task has already been executed " + "(a task can be executed only once)"); } } //改变状态为RUNNING mStatus = Status.RUNNING; //调用onPreExecute方法 onPreExecute(); mWorker.mParams = params; sExecutor.execute(mFuture); return this; }
代码中涉及到三个陌生的变量:mWorker、sExecutor、mFuture,我们也会看一下他们的庐山真面目:
关于sExecutor,它是java.util.concurrent.ThreadPoolExecutor的实例,用于管理线程的执行。代码如下:
[java]
view plaincopyprint?
private static final int CORE_POOL_SIZE = 5;
private static final int MAXIMUM_POOL_SIZE = 128;
private static final int KEEP_ALIVE = 10;
//新建一个队列用来存放线程
private static final BlockingQueue<Runnable> sWorkQueue =
new LinkedBlockingQueue<Runnable>(10);
//新建一个线程工厂
private static final ThreadFactory sThreadFactory = new ThreadFactory() {
private final AtomicInteger mCount = new AtomicInteger(1);
//新建一个线程
public Thread newThread(Runnable r) {
return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
}
};
//新建一个线程池执行器,用于管理线程的执行
private static final ThreadPoolExecutor sExecutor = new ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS, sWorkQueue, sThreadFactory);
[java] view plaincopyprint? private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { Params[] mParams; } private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> { Params[] mParams; }
而mFuture实际上是java.util.concurrent.FutureTask的实例,下面是它的FutureTask类的相关信息:
[java]
view plaincopyprint?
/**
* A cancellable asynchronous computation.
* ...
*/
public class FutureTask<V> implements RunnableFuture<V> {
[java] view plaincopyprint? public interface RunnableFuture<V> extends Runnable, Future<V> { /** * Sets this Future to the result of its computation * unless it has been cancelled. */ void run(); } public interface RunnableFuture<V> extends Runnable, Future<V> { /** * Sets this Future to the result of its computation * unless it has been cancelled. */ void run(); }
可以看到FutureTask是一个可以中途取消的用于异步计算的类。
下面是mWorker和mFuture实例在AsyncTask中的体现:
[java]
view plaincopyprint?
private final WorkerRunnable<Params, Result> mWorker;
private final FutureTask<Result> mFuture;
public AsyncTask() {
mWorker = new WorkerRunnable<Params, Result>() {
//call方法被调用后,将设置优先级为后台级别,然后调用AsyncTask的doInBackground方法
public Result call() throws Exception {
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
return doInBackground(mParams);
}
};
//在mFuture实例中,将会调用mWorker做后台任务,完成后会调用done方法
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) {
//发送取消任务的消息
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);
}
//发送显示结果的消息
message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(AsyncTask.this, result));
message.sendToTarget();
}
};
}
[java] view plaincopyprint? private static final int MESSAGE_POST_RESULT = 0x1; //显示结果 private static final int MESSAGE_POST_PROGRESS = 0x2; //更新进度 private static final int MESSAGE_POST_CANCEL = 0x3; //取消任务 private static final InternalHandler sHandler = new InternalHandler(); 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 //调用AsyncTask.finish方法 result.mTask.finish(result.mData[0]); break; case MESSAGE_POST_PROGRESS: //调用AsyncTask.onProgressUpdate方法 result.mTask.onProgressUpdate(result.mData); break; case MESSAGE_POST_CANCEL: //调用AsyncTask.onCancelled方法 result.mTask.onCancelled(); break; } } } private static final int MESSAGE_POST_RESULT = 0x1; //显示结果 private static final int MESSAGE_POST_PROGRESS = 0x2; //更新进度 private static final int MESSAGE_POST_CANCEL = 0x3; //取消任务 private static final InternalHandler sHandler = new InternalHandler(); 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 //调用AsyncTask.finish方法 result.mTask.finish(result.mData[0]); break; case MESSAGE_POST_PROGRESS: //调用AsyncTask.onProgressUpdate方法 result.mTask.onProgressUpdate(result.mData); break; case MESSAGE_POST_CANCEL: //调用AsyncTask.onCancelled方法 result.mTask.onCancelled(); break; } } }
我们看到,在处理消息时,遇到“MESSAGE_POST_RESULT”时,它会调用AsyncTask中的finish()方法,我们来看一下finish()方法的定义:
[java]
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private void finish(Result result) {
if (isCancelled()) result = null;
onPostExecute(result); //调用onPostExecute显示结果
mStatus = Status.FINISHED; //改变状态为FINISHED
}
[java] view plaincopyprint? AsyncTaskResult result = (AsyncTaskResult) msg.obj; AsyncTaskResult result = (AsyncTaskResult) msg.obj;
这个AsyncTaskResult究竟是什么呢,它又包含什么内容呢?其实它也是AsyncTask的一个内部类,是用来包装执行结果的一个类,让我们来看一下它的代码结构:
[java]
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@SuppressWarnings({"RawUseOfParameterizedType"})
private static class AsyncTaskResult<Data> {
final AsyncTask mTask;
final Data[] mData;
AsyncTaskResult(AsyncTask task, Data... data) {
mTask = task;
mData = data;
}
}
[java] view plaincopyprint? //发送取消任务的消息 message = sHandler.obtainMessage(MESSAGE_POST_CANCEL, new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null)); message.sendToTarget(); //发送取消任务的消息 message = sHandler.obtainMessage(MESSAGE_POST_CANCEL, new AsyncTaskResult<Result>(AsyncTask.this, (Result[]) null)); message.sendToTarget();
[java]
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//发送显示结果的消息
message = sHandler.obtainMessage(MESSAGE_POST_RESULT,
new AsyncTaskResult<Result>(AsyncTask.this, result));
message.sendToTarget();
[java] view plaincopyprint? result.mTask.finish(result.mData[0]); result.mTask.finish(result.mData[0]);
[java]
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result.mTask.onProgressUpdate(result.mData);
result.mTask.onProgressUpdate(result.mData);
概括来说,当我们调用execute(Params... params)方法后,execute方法会调用onPreExecute()方法,然后由ThreadPoolExecutor实例sExecutor执行一个FutureTask任务,这个过程中doInBackground(Params... params)将被调用,如果被开发者覆写的doInBackground(Params... params)方法中调用了publishProgress(Progress...
values)方法,则通过InternalHandler实例sHandler发送一条MESSAGE_POST_PROGRESS消息,更新进度,sHandler处理消息时onProgressUpdate(Progress... values)方法将被调用;如果遇到异常,则发送一条MESSAGE_POST_CANCEL的消息,取消任务,sHandler处理消息时onCancelled()方法将被调用;如果执行成功,则发送一条MESSAGE_POST_RESULT的消息,显示结果,sHandler处理消息时onPostExecute(Result
result)方法被调用。
经过上面的介绍,相信朋友们都已经认识到AsyncTask的本质了,它对Thread+Handler的良好封装,减少了开发者处理问题的复杂度,提高了开发效率,希望朋友们能多多体会一下。本文转自http://blog.csdn.net/liuhe688/article/details/6532519
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