Android中的异步处理技术之AsyncTask

目录

定义和作用

AsyncTask是在Executor框架的基础上进行的封装，它实现将耗时任务移动到工作线程中进行，同时提供了方便的接口实现了工作线程和主线程的通信。

AsyncTask主要的方法

使用AsyncTask一般会用到如下方法

private static class MyTask extends AsyncTask<String,Integer,String>{
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
        }

        @Override
        protected void onProgressUpdate(Integer... values) {
            super.onProgressUpdate(values);
        }

        @Override
        protected void onCancelled(String s) {
            super.onCancelled(s);
        }

        @Override
        protected String doInBackground(String... strings) {
            return null;
        }
}

AsyncTask的使用方法

继承AsyncTask并实现其核心方法然后手动调用excute()方法。

public class MainActivity extends AppCompatActivity {
    private Button btStart;
    private ProgressBar pb;
    private TextView tvState;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        btStart = (Button) findViewById(R.id.bt_start);
        pb = (ProgressBar) findViewById(R.id.pb);
        tvState = (TextView) findViewById(R.id.tv_state);
        final MyTask myTask=new MyTask(pb,tvState);
        btStart.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                myTask.execute();
            }
        });
    }
    private static class MyTask extends AsyncTask<String,Integer,String>{
        @SuppressLint("StaticFieldLeak")
        private TextView textView;
        @SuppressLint("StaticFieldLeak")
        private ProgressBar progressBar;
        public MyTask(ProgressBar progressBar,TextView textView) {
            this.progressBar=progressBar;
            this.textView=textView;
        }

        @Override
        protected void onPreExecute() {
            textView.setText("开始加载");
        }

        @Override
        protected void onPostExecute(String s) {
            textView.setText("加载完毕");
        }

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

        @Override
        protected void onCancelled(String s) {
            textView.setText("已取消");
        }

        @Override
        protected String doInBackground(String... strings) {
            int count=0;
            try {
                while (count<=100){
                    count+=1;
                    Thread.sleep(50);
                    publishProgress(count);
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return "";
        }
    }
}

xml布局

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:orientation="vertical"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:layout_width="wrap_content"
        android:text="开始"
        android:id="@+id/bt_start"
        android:layout_gravity="center"
        android:layout_height="wrap_content" />
    <ProgressBar
        android:layout_width="match_parent"
        android:layout_marginTop="20dp"
        android:max="100"
        android:id="@+id/pb"
        style="@style/Widget.AppCompat.ProgressBar.Horizontal"
        android:layout_height="wrap_content" />
    <TextView
        android:layout_width="wrap_content"
        android:layout_marginTop="20dp"
        android:text="状态"
        android:id="@+id/tv_state"
        android:layout_gravity="center"
        android:layout_height="wrap_content" />
</LinearLayout>

注意

• AsyncTask必须在UI线程中启动
• 同一个AsyncTask实例对象只能执行1次，若执行第2次将会抛出异常
• AsyncTask最好声明为静态内部类

• 在不同版本的Android系统的 AsyncTask的execute和executeOnExecutor方法的运行有些许差别

  
  
  
  

  可以看到，如果想要AsyncTask并行执行任务的话，那么在API大于13的版本中建议使用executeOnExecutor代替execute。另外如果 AsyncTask是异步执行，最多也只能有四个任务可以同时进行，其他任务需要在队列中排队，等待空闲线程。之所以会出现这种情况是由于AsyncTask的源码决定的。

public abstract class AsyncTask<Params, Progress, Result> {
    private static final String LOG_TAG = "AsyncTask";

    private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
    // We want at least 2 threads and at most 4 threads in the core pool,
    // preferring to have 1 less than the CPU count to avoid saturating
    // the CPU with background work
    private static final int CORE_POOL_SIZE = Math.max(2, Math.min(CPU_COUNT - 1, 4));
    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
    private static final int KEEP_ALIVE_SECONDS = 30;

    private static final ThreadFactory sThreadFactory = new ThreadFactory() {
        private final AtomicInteger mCount = new AtomicInteger(1);

        public Thread newThread(Runnable r) {
            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
        }
    };
    ...

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