Android笔记（三十七） 如何停止AsyncTask？

　　当我们加载一张图片的时候，加载的过程中我们想要取消操作，该怎么办呢？调用Asynctask的 cancel() 方法就可以了，我们看代码：

　　先看一个例子：

　　MainAciticty.java

package cn.lixyz.asynctest;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;

public class MainActivity extends Activity {

private Button btStart;
private Button btStop;
private ImageView imageView;
private ProgressBar progressBar;
private String URL = "http://ww3.sinaimg.cn/bmiddle/612c96afjw1ewwftl3uqkj20u00koq48.jpg";

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

//获取组件
btStart = (Button) findViewById(R.id.btStart);
btStop = (Button) findViewById(R.id.btStop);
imageView = (ImageView) findViewById(R.id.imageView);
progressBar = (ProgressBar) findViewById(R.id.progressBar);

//给button设置点击事件，点击按钮，启动异步任务
MyOnClickListener moc = new MyOnClickListener();
btStart.setOnClickListener(moc);
btStop.setOnClickListener(moc);
}

class MyOnClickListener implements View.OnClickListener {

LoadImage li = new LoadImage();
@Override
public void onClick(View v) {
switch (v.getId()){
case R.id.btStart:
li.execute(URL);
break;
case R.id.btStop:
li.cancel(true);
break;
}
}
}

class LoadImage extends AsyncTask<String, Integer, Bitmap> {

//开始执行耗时操作，连接网络获取图片，并且将Bitmap返回
@Override
protected Bitmap doInBackground(String... params) {

int i = 0;
while (true) {
if (i > 100) {
break;
}
i++;
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
Log.d("TTTT","" + i);
publishProgress(i);
}

String url = params[0];
Bitmap bitmap = null;
URLConnection connection;
InputStream is;
try {
connection = new URL(url).openConnection();
is = connection.getInputStream();
BufferedInputStream bis = new BufferedInputStream(is);
bitmap = BitmapFactory.decodeStream(bis);
is.close();
bis.close();
} catch (IOException e) {
e.printStackTrace();
}
return bitmap;
}

//onPreExcute方法是在doInBackGround方法前执行，用于做一些初始化操作，这里将progressBar显示出来
@Override
protected void onPreExecute() {
super.onPreExecute();
progressBar.setVisibility(View.VISIBLE);
}

//doInBackGround方法执行后，会自动执行该方法，获取到doInBackGround返回的对象，然后更改UI
@Override
protected void onPostExecute(Bitmap bitmap) {
super.onPostExecute(bitmap);
progressBar.setVisibility(View.GONE);
imageView.setImageBitmap(bitmap);
}

@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
progressBar.setProgress(values[0]);
}
}
}

　　acticity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">

<RelativeLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_weight="4">
<ImageView
android:id="@+id/imageView"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_centerInParent="true"/>
<ProgressBar
android:id="@+id/progressBar"
android:visibility="gone"
android:layout_centerInParent="true"
style="@style/Widget.AppCompat.ProgressBar.Horizontal"
android:layout_width="wrap_content"
android:layout_height="wrap_content" />
</RelativeLayout>

<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1">
<Button
android:id="@+id/btStart"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1"
android:layout_gravity="center"
android:text="点击载入" />
<Button
android:id="@+id/btStop"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="点击退出"/>

</LinearLayout>
</LinearLayout>

　　执行结果：





　　点击退出按钮之后，进度条停止加载了，似乎已经成功了，但是我们看log：



　　后台进度依旧在更新着，AsyncTask实际上并没有停止，原来，调用了cancel之后，并不是真正的退出，而是将AsyncTask标记为Canceled，你需要在doInBackground中随时查看状态，如果是canceled，就退出：

package cn.lixyz.asynctest;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.ProgressBar;

import java.io.BufferedInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.net.URLConnection;

public class MainActivity extends Activity {

private Button btStart;
private Button btStop;
private ImageView imageView;
private ProgressBar progressBar;
private String URL = "http://ww3.sinaimg.cn/bmiddle/612c96afjw1ewwftl3uqkj20u00koq48.jpg";

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

//获取组件
btStart = (Button) findViewById(R.id.btStart);
btStop = (Button) findViewById(R.id.btStop);
imageView = (ImageView) findViewById(R.id.imageView);
progressBar = (ProgressBar) findViewById(R.id.progressBar);

//给button设置点击事件，点击按钮，启动异步任务
MyOnClickListener moc = new MyOnClickListener();
btStart.setOnClickListener(moc);
btStop.setOnClickListener(moc);
}

class MyOnClickListener implements View.OnClickListener {

LoadImage li = new LoadImage();
@Override
public void onClick(View v) {
switch (v.getId()){
case R.id.btStart:
li.execute(URL);
break;
case R.id.btStop:
li.cancel(true);
break;
}
}
}

class LoadImage extends AsyncTask<String, Integer, Bitmap> {

//开始执行耗时操作，连接网络获取图片，并且将Bitmap返回
@Override
protected Bitmap doInBackground(String... params) {

int i = 0;
while (true) {
if (i > 100) {
break;
}
if (isCancelled()){
break;
}
i++;
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
Log.d("TTTT","" + i);
publishProgress(i);
}

String url = params[0];
Bitmap bitmap = null;
URLConnection connection;
InputStream is;
try {
connection = new URL(url).openConnection();
is = connection.getInputStream();
BufferedInputStream bis = new BufferedInputStream(is);
bitmap = BitmapFactory.decodeStream(bis);
is.close();
bis.close();
} catch (IOException e) {
e.printStackTrace();
}
return bitmap;
}

//onPreExcute方法是在doInBackGround方法前执行，用于做一些初始化操作，这里将progressBar显示出来
@Override
protected void onPreExecute() {
super.onPreExecute();
progressBar.setVisibility(View.VISIBLE);
}

//doInBackGround方法执行后，会自动执行该方法，获取到doInBackGround返回的对象，然后更改UI
@Override
protected void onPostExecute(Bitmap bitmap) {
super.onPostExecute(bitmap);
progressBar.setVisibility(View.GONE);
imageView.setImageBitmap(bitmap);
}

@Override
protected void onProgressUpdate(Integer... values) {
super.onProgressUpdate(values);
progressBar.setProgress(values[0]);
}
}
}

cancel