使用AsyncTask异步更新UI界面及原理分析

package com.szy.demo;

import org.apache.http.HttpResponse;
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.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;
import android.widget.Toast;

/**
*@author coolszy
*@date 2012-3-1
*@blog http://blog.92coding.com */
public class AsyncTaskActivity extends Activity
{

private ImageView mImageView;
private Button mBtnDownload;
private ProgressBar mProgressBar;

@Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);

mImageView = (ImageView) findViewById(R.id.imageView);
mBtnDownload = (Button) findViewById(R.id.btnDownload);
mProgressBar = (ProgressBar) findViewById(R.id.progressBar);
mBtnDownload.setOnClickListener(new OnClickListener()
{
@Override
public void onClick(View v)
{
GetImageTask task = new GetImageTask();
task.execute("http://www.baidu.com/img/baidu_sylogo1.gif");
}
});
}

class GetImageTask extends AsyncTask<String, Integer, Bitmap>
{
/**
* 处理后台执行的任务，在后台线程执行
*/
@Override
protected Bitmap doInBackground(String... params)
{
publishProgress(0);// 将会调用onProgressUpdate(Integer... progress)方法
HttpClient httpClient = new DefaultHttpClient();
publishProgress(30);
HttpGet httpGet = new HttpGet(params[0]);// 获取csdn的logo
final Bitmap bitmap;
try
{
HttpResponse httpResponse = httpClient.execute(httpGet);
//获取返回图片
bitmap = BitmapFactory.decodeStream(httpResponse.getEntity().getContent());
} catch (Exception e)
{

return null;
}
publishProgress(100);
return bitmap;
}

/**
* 在调用publishProgress之后被调用，在UI线程执行
*/
protected void onProgressUpdate(Integer... progress)
{
mProgressBar.setProgress(progress[0]);// 更新进度条的进度
}

/**
* 后台任务执行完之后被调用，在UI线程执行
*/
protected void onPostExecute(Bitmap result)
{
if (result != null)
{
Toast.makeText(AsyncTaskActivity.this, "成功获取图片", Toast.LENGTH_LONG).show();
mImageView.setImageBitmap(result);
} else
{
Toast.makeText(AsyncTaskActivity.this, "获取图片失败", Toast.LENGTH_LONG).show();
}
}

/**
* 在 doInBackground(Params...)之前被调用，在UI线程执行
*/
protected void onPreExecute()
{
mImageView.setImageBitmap(null);
mProgressBar.setProgress(0);// 进度条复位
}

/**
* 在UI线程执行
*/
protected void onCancelled()
{
mProgressBar.setProgress(0);// 进度条复位
}
}

}

<?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">
<ProgressBar
android:id="@+id/progressBar"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
style="?android:attr/progressBarStyleHorizontal"></ProgressBar>
<Button
android:id="@+id/btnDownload"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="下载图片"/>
<ImageView
android:id="@+id/imageView"
android:layout_height="wrap_content"
android:layout_width="wrap_content" />
</LinearLayout>

private static final int CORE_POOL_SIZE =5;//5个核心工作线程
private static final int MAXIMUM_POOL_SIZE = 128;//最多128个工作线程
private static final int KEEP_ALIVE = 1;//空闲线程的超时时间为1秒
private static final BlockingQueue<Runnable> sWorkQueue =
new LinkedBlockingQueue<Runnable>(10);//等待队列
private static final ThreadPoolExecutorsExecutor = new
ThreadPoolExecutor(CORE_POOL_SIZE,
MAXIMUM_POOL_SIZE, KEEP_ALIVE, TimeUnit.SECONDS,

sWorkQueue,sThreadFactory);//线程池是静态变量，所有的异步任务都会放到这个线程池的工作线程内执行。

public AsyncTask()
{
mWorker = new WorkerRunnable<Params, Result>()
{
public Result call() throws Exception
{
Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
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();// 取消任务，发送MESSAGE_POST_CANCEL消息
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));// 完成任务，发送MESSAGE_POST_RESULT消息并传递result对象
message.sendToTarget();
}
};
}

1

task.execute(

"http://www.baidu.com/img/baidu_sylogo1.gif"

);

public final AsyncTask<Params, Progress, Result> execute(Params... params)
{
if (mStatus != Status.PENDING)
{
switch (mStatus)
{
case RUNNING:
throw newIllegalStateException("Cannot execute task:" + " the taskis already running.");
case FINISHED:
throw newIllegalStateException("Cannot execute task:" + " the taskhas already been executed " + "(a task canbe executed only once)");
}
}
mStatus = Status.RUNNING;
onPreExecute();// 运行在ui线程，在提交任务到线程池之前执行
mWorker.mParams = params;
sExecutor.execute(mFuture);// 提交任务到线程池
return this;
}

private static class InternalHandler extends Handler
{
@SuppressWarnings({"unchecked","RawUseOfParameterizedType"})
@Override
public voidhandleMessage(Message msg) {
AsyncTaskResult result =(AsyncTaskResult) msg.obj;
switch (msg.what) {
caseMESSAGE_POST_RESULT:
// There is onlyone result
result.mTask.finish(result.mData[0]);//执行任务成功
break;
caseMESSAGE_POST_PROGRESS:
result.mTask.onProgressUpdate(result.mData);//进度更新
break;
caseMESSAGE_POST_CANCEL:
result.mTask.onCancelled();//取消任务
break;
}
}
}