一.概述

AsyncTask就是一个封装了 Handler+线程池 的对象。 能够自由在主线程和子线程中来回切换   ----是串行执行任务的。

二.基本概念

CPU_COUNT = Runtime.getRunTime().getAvailableProcessors() --CPU核心数

1. THREAD_POOL_EXECUTOR: 内部封装的一个线程池.  核心线程数:Math.max(2,Math.min(CPU_COUNT-1,4));

                                               最大线程数:CPU_COUNT*2+1             存活时间:30s

2. SerialExecutor： 内部封装的线程池.

3.InterHandler:   子线程与主线程的通信Handler

三.运行流程

  1.先调用execute()方法

@MainThread  public final AsyncTask
      
        execute(Params... params) {      return executeOnExecutor(sDefaultExecutor, params);  }
      

2. sDefaultExecutor --一个SerialExecutor对象

private static class SerialExecutor implements Executor {     final ArrayDeque
      
        mTasks = new ArrayDeque
       
        ();     Runnable mActive;    public synchronized void execute(final Runnable r) {         mTasks.offer(new Runnable() {     //offer --添加任务             public void run() {                 try {                     r.run();                 } finally {                     scheduleNext();                 }             }         });         if (mActive == null) {             scheduleNext();         }     }    protected synchronized void scheduleNext() {         if ((mActive = mTasks.poll()) != null) {     //poll --取出任务             THREAD_POOL_EXECUTOR.execute(mActive);         }     } }     --AsyncTask串行的关键;  ArrayDeque --一个可扩展的双向队列
       
      

3.调用executeOnExecutor方法

@MainThread  public final AsyncTask
      
        executeOnExecutor(Executor exec,          Params... params) {      if (mStatus != Status.PENDING) {                                                           //PENDING：任务还未执行过          switch (mStatus) {                                                                      //RUNING：任务正在执行              case RUNNING:                                                                       //任务已经完成了                  throw new IllegalStateException("Cannot execute task:"                          + " the task is already running.");              case FINISHED:                  throw new IllegalStateException("Cannot execute task:"                          + " the task has already been executed "                          + "(a task can be executed only once)");          }      }     mStatus = Status.RUNNING;     onPreExecute();     mWorker.mParams = params;      exec.execute(mFuture);     return this;  }
      

4.调用onPreExecute()方法

我们自己可以在这里做一些耗时操作的准备

5.调用了 doInBackGround(Params …params)函数

mWorker = new WorkerRunnable
      
       () {     public Result call() throws Exception {         mTaskInvoked.set(true);         Result result = null;         try {             Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);             //noinspection unchecked             result = doInBackground(mParams);         //是在子线程中执行的，在这里我们执行自己的耗时操作。             Binder.flushPendingCommands();         } catch (Throwable tr) {             mCancelled.set(true);             throw tr;         } finally {             postResult(result);         }         return result;     } };
      

6.postResult()和publishProgress()方法

将得到的结果返回给主线程private Result postResult(Result result) {         @SuppressWarnings("unchecked")         Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,                 new AsyncTaskResult
      
       (this, result));         message.sendToTarget();         return result;     }将任务下载的进度传给主线程  @WorkerThread    protected final void publishProgress(Progress... values) {        if (!isCancelled()) {            getHandler().obtainMessage(MESSAGE_POST_PROGRESS,                    new AsyncTaskResult
       (this, values)).sendToTarget();        }    }
      

7.InterHandler

private static class InternalHandler extends Handler {     public InternalHandler(Looper looper) {         super(looper);     }    @SuppressWarnings({"unchecked", "RawUseOfParameterizedType"})     @Override     public void handleMessage(Message msg) {         AsyncTaskResult
       result = (AsyncTaskResult
      ) msg.obj;         switch (msg.what) {             case MESSAGE_POST_RESULT:                 // There is only one result                 result.mTask.finish(result.mData[0]);                 break;             case MESSAGE_POST_PROGRESS:                 result.mTask.onProgressUpdate(result.mData);                 break;         }     } }

8.onProgressUpdate()可以更新进度

需要手动在doInBackGround()方法里面调用publishProgress()方法

9.onPostResult()方法，任务下载完毕之后得到的结果

自己重写，可以在任务下载之后进行一些操作

四.实际操作

class DownAsyncTask extends AsyncTask
      
       {         protected Result doInBackGround(Params … params){           down()//耗时操作        }        //可重写 主要发生在任务之前 主线程中        protected void onPreExecute(){        }         //主要表现任务的完成进度 需要在doInBackGround中调用 publishProgress()方法      protected void onProgressUpdate(Param param){      }     //任务完成之后的调用方法    protected void onPostExecute(Result result){       }}