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