AsyncTask是android提供的轻量级的异步类,是针对子线程中更新UI,封装简化异步操作的封装好的组件。Android的耗时操作如:网络操作、读取文件等如果放在主线程中会出现ANR(Application Not Responding)错误,使用AsyncTask可以保证Android的单线程模型,也避免了ANR。
AsyncTask
是一个抽象类,通常用于被继承,继承AsyncTask需要指定如下三个泛型参数:
package com.example.asynctask;
import android.os.AsyncTask;
public class MyAsyncTask extends AsyncTask<Void, Void, Void> {
@Override
protected Void doInBackground(Void... params) {
System.out.println("=========doInBackground===========");
return null;
}
@Override
protected void onPreExecute() {
System.out.println("=========onPreExecute===========");
super.onPreExecute();
}
@Override
protected void onPostExecute(Void result) {
System.out.println("=========onPostExecute===========");
super.onPostExecute(result);
}
@Override
protected void onProgressUpdate(Void... values) {
System.out.println("=========onProgressUpdate===========");
super.onProgressUpdate(values);//获取进度值,更新进度条
}
}
执行结果为:
在doInBackground中添加publishProgress()方法
@Override
protected Void doInBackground(Void... params) {
System.out.println("=========doInBackground===========");
publishProgress();//传入进度值
return null;
}
布局文件activity_image.xml
<RelativeLayout 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="${relativePackage}.${activityClass}" >
<ImageView
android:id="@+id/image"
android:layout_width="match_parent"
android:layout_height="match_parent" />
<ProgressBar
android:id="@+id/progressbar"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"
android:visibility="gone" />
RelativeLayout>
ImageActivity
package com.example.asynctask;
import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
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.widget.ImageView;
import android.widget.ProgressBar;
public class ImageActivity extends Activity {
private ImageView mImageView;
private ProgressBar mProgressBar;
private static String URL = "https://img-my.csdn.net/uploads/avatar/6/A/7/1_leoleohan.jpg";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_image);
mImageView = (ImageView) findViewById(R.id.image);
mProgressBar = (ProgressBar) findViewById(R.id.progressbar);
// 设置传递进去的参数
new MyAsyncTask().execute(URL);
}
/**
* Params:启动任务时输入参数的类型,这里传入url,是String类型 ;Progress:后台任务执行中返回进度值的类型。这里填Void
* Result:后台执行任务完成后返回结果的类型。这里需要返回一个Bitmap
*
* @author LeoLeoHan
*
*/
class MyAsyncTask extends AsyncTask {
@Override
protected void onPreExecute() {
super.onPreExecute();
mProgressBar.setVisibility(View.VISIBLE);// 显示控件
}
/**
* 获取返回的结果,运行在主线程,所以可以对UI进行操作
*/
@Override
protected void onPostExecute(Bitmap bitmap) {
super.onPostExecute(bitmap);
mProgressBar.setVisibility(View.GONE);
mImageView.setImageBitmap(bitmap);
}
/**
* 运行在子线程中,开始真正的异步操作
*/
@Override
protected Bitmap doInBackground(String... params) {
// String... params是可变长的数组,第0位就是传进来的url
String url = params[0];
Bitmap bitmap = null;
// 定义网络连接对象
URLConnection connection;
// 定义用于获取数据的输入流
InputStream is;
try {
connection = new URL(url).openConnection();
is = connection.getInputStream();
BufferedInputStream bis = new BufferedInputStream(is);
bitmap = BitmapFactory.decodeStream(bis);// 将输入流解析成bitmap
is.close();
bis.close();
} catch (MalformedURLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
// 将bitmap作为参数返回
return bitmap;
}
}
}
布局文件activity_image.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:padding="16dp"
android:layout_height="match_parent"
tools:context="${relativePackage}.${activityClass}" >
<ProgressBar
android:id="@+id/pg"
style="?android:attr/progressBarStyleHorizontal"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_centerHorizontal="true"
android:layout_centerVertical="true" />
RelativeLayout>
ProgressBarActivity
package com.example.asynctask;
import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.widget.ProgressBar;
public class ProgressBarActivity extends Activity {
private ProgressBar mProgressBar;
private MyAsyncTask myAsyncTask;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_progress_bar);
mProgressBar = (ProgressBar) findViewById(R.id.pg);
myAsyncTask = new MyAsyncTask();
myAsyncTask.execute();
}
@Override
protected void onPause() {
super.onPause();
if (myAsyncTask != null
&& myAsyncTask.getStatus() == AsyncTask.Status.RUNNING) {
// cancel方法只是将对应的asynctask标记为cancel状态,并不是真正的取消线程的执行
myAsyncTask.cancel(true);
}
}
class MyAsyncTask extends AsyncTask {
@Override
protected Void doInBackground(Void... params) {
/**
* 模拟进度的更新
*/
for (int i = 0; i < 100; i++) {
if (isCancelled()) {
break;
}
publishProgress(i);
try {
Thread.sleep(300);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
return null;
}
@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
if (isCancelled()) {
return;
}
// 获取进度更新值
mProgressBar.setProgress(values[0]);
}
}
}