Android：AsyncTask 使用

1. Task 的实例必须在 UI thread 中创建

2. execute 方法必须在 UI thread 中调用

3. 不要手动的调用 onPreExecute(), onPostExecute(Result)，doInBackground(Params...), onProgressUpdate(Progress...)这几个方法

4. 该 task 只能被执行一次，否则多次调用时将会出现异常

一个简单进度条的例子：

<?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/progress_bar"

android:layout_width="200dip" android

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:layout_height="10dip"

android:layout_gravity="center"

android:max="100" style="?android:attr/progressBarStyleHorizontal"

android:progress="0">

</ProgressBar>

</LinearLayout>

import android.app.Activity;

import android.os.AsyncTask;

import android.os.Bundle;

import android.widget.ProgressBar;

public class Double extends Activity {

public ProgressBar pBar;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.main);

pBar = (ProgressBar) findViewById(R.id.progress_bar);

//AsyncTask.execute()要在主线程里调用

new AsyncLoader().execute((Void)null);

}

public void initProgress() {

pBar.setProgress(0);

try {

Thread.sleep(1000);

} catch (InterruptedException e) {

e.printStackTrace();

}

pBar.setProgress(50);

try {

Thread.sleep(1000);

} catch (InterruptedException e) {

e.printStackTrace();

}

pBar.setProgress(100);

}

//AsyncTask

class AsyncLoader extends AsyncTask<Void, Void, Integer>{

@Override

protected Integer doInBackground(Void... params) {

initProgress();

return null;

}

}

}

获取网页的例子：

//设置三种类型参数分别为 String,Integer,String

class PageTask extends AsyncTask<String, Integer, String> {

// 可变长的输入参数，与 AsyncTask.exucute()对应

@Override

protected String doInBackground(String... params) {

try {

HttpClient client = new DefaultHttpClient();

// params[0] 代表连接的 url

HttpGet get = new HttpGet(params[0]);

HttpResponse response = client.execute(get);

HttpEntity entity = response.getEntity();

long length = entity.getContentLength();

InputStream is = entity.getContent();

String s = null;

if (is != null) {

ByteArrayOutputStream baos = new ByteArrayOutputStream();

byte[] buf = new byte[128];

int ch = -1;

int count = 0;

while ((ch = is.read(buf)) != -1) {

baos.write(buf, 0, ch);

count += ch;

if (length > 0) {

// 如果知道响应的长度，调用 publishProgress（）更新进度

publishProgress((int) ((count / (float) length) * 100));

}

// 为了在模拟器中清楚地看到进度，让线程休眠 100ms

Thread.sleep(100);

}

s = new String(baos.toByteArray()); }

// 返回结果

return s;

} catch (Exception e) {

e.printStackTrace();

}

return null;

}

@Override