Asynctask源码分析

​首先我们使用AsyncTask时，一般是：

new AsyncTask(...).execute()	

我们看new AsyncTask(),它走的是：

  public AsyncTask() {
        this((Looper) null);
    }
    
   public AsyncTask(@Nullable Looper callbackLooper) {
   //创建一个handler到后面将子线程处理的消息传递给主线程，这个handler是基于主线程创建的
     mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
            ? getMainHandler()
            : new Handler(callbackLooper);
//创建了一个WorkerRunnable,它实现了Callable，这个后面会讲到
        mWorker = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);
                Result result = null;
                try {
                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    //在子线程中处理异步任务
                    result = doInBackground(mParams);
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
               
                    postResult(result);
                }
                 //将处理结果传递出去
                return result;
            }
        };

//FutureTask是实现了RunnableFuture，Runnable和Future接口
        mFuture = new FutureTask<Result>(mWorker) {
            @Override
            protected void done() {
                try {
                    postResultIfNotInvoked(get());
                } catch (InterruptedException e) {
                    android.util.Log.w(LOG_TAG, e);
                } catch (ExecutionException e) {
                    throw new RuntimeException("An error occurred while executing doInBackground()",
                            e.getCause());
                } catch (CancellationException e) {
                    postResultIfNotInvoked(null);
                }
            }
        };
     
   }
   

主要是创建了三个对象，mHandler，mWorker和mFuture，mHandler是将后面处理的结果发送到主线程；mWorker主要实现Callable接口，并在call中调用doInBackground()方法处理任务；mFuture则是实现了RunnableFuture，Runnable和Future接口，最后在done中调用postResultIfNotInvoked（）方法将处理结果传递出去。也就是主要弄清楚mWorker.call()以及mFuture.done()。下面我们分析他们分别在什么时候调用。接着往下看AsyncTask的execute（）：

 @MainThread
    public final AsyncTask<Params, Progress, Result> execute(Params... params) {
        return executeOnExecutor(sDefaultExecutor, params);
    }
     @MainThread
    public final AsyncTask<Params, Progress, Result> executeOnExecutor(Executor exec,
            Params... params) {
	......

        mStatus = Status.RUNNING;
        onPreExecute();
        mWorker.mParams = params;
        exec.execute(mFuture);
        return this;
    }

我们可以看到调用AsyncTask.execute()是执行了executeOnExecutor(sDefaultExecutor, params);这个方法,我们看executeOnExecutor()方法，它执行了 onPreExecute()，对应我们应用中的异步任务前的准备操作，接着将params值传递 给mWorker，最后执行了 exec.execute(mFuture);这个exec就是上面传递进来的 sDefaultExecutor，也就是执行了sDefaultExecutor.execute()方法。sDefaultExecutor是什么呢？往下看:

private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();
private static class SerialExecutor implements Executor {
        final ArrayDeque<Runnable> mTasks = new ArrayDeque<Runnable>();
        Runnable mActive;

        public synchronized void execute(final Runnable r) {
            mTasks.offer(new Runnable() {
                public void run() {
                    try {
                        r.run();
                    } finally {
                        scheduleNext();
                    }
                }
            });
            if (mActive == null) {
                scheduleNext();
            }
        }

        protected synchronized void scheduleNext() {
            if ((mActive = mTasks.poll()) != null) {
                THREAD_POOL_EXECUTOR.execute(mActive);
            }
        }
    }

sDefaultExecutor其实就是SerialExecutor,所以就是执行了SerialExecutor的execute(mFuture)方法，然后调用了 r.run()也就是mFuture.run()，我们看FutureTask的run方法:

 public void run() {
        if (state != NEW ||
            !U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    set(result);
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

Callable c = callable中的callable其实就是mWorker,因为我们在AsyncTask的构造方法中创建mFuture对象时将mWorker传递进去了： mFuture = new FutureTask(mWorker) ,我们看看FutureTask的构造方法：

  public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

发现有 this.callable = callable，所以callable其实就是mWorker。我们接着看FutureTask的run方法，它调用了c.run(),也就是mWorker.run(),现在很清晰了，也就回调到最开始mWorker的run,调用doInBackground()方法等等。接着往下看，调用了 set(result)方法：

    protected void set(V v) {
        if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
            outcome = v;
            U.putOrderedInt(this, STATE, NORMAL); // final state
            finishCompletion();
        }
    }

将result赋值给了outcome。继续往下看，调用了finishCompletion()：

  private void finishCompletion() {
    ...
    done();
    ...
  }

调用了done()方法，是不是很熟悉，也就是我们之前mFuture的done()方法，接着在done()中调用postResultIfNotInvoked(get())：

public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
            //返回report（）的结果
        return report(s);
    }
 private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
        //将outcome返回，也就是对应的result
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }
 private void postResultIfNotInvoked(Result result) {
        final boolean wasTaskInvoked = mTaskInvoked.get();
        if (!wasTaskInvoked) {
            postResult(result);
        }
    }

调用get()返回了outcome也就是result最后通过postResult()将结果传递出去:

  private Result postResult(Result result) {
        @SuppressWarnings("unchecked")
        Message message = getHandler().obtainMessage(MESSAGE_POST_RESULT,
                new AsyncTaskResult<Result>(this, result));
        message.sendToTarget();
        return result;
    }
    
    ...
      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;
            }
        }
     ...

结果通过handler将消息传递到主线程，并执行了result.mTask.finish();我们接着看finish方法：

 private void finish(Result result) {
        if (isCancelled()) {
            onCancelled(result);
        } else {
            onPostExecute(result);
        }
        mStatus = Status.FINISHED;
    }

如果取消了就执行onCancelled()，否则执行onPostExecute()将结果回调到我们的应用当中。
接着我们看在doInBackground()中回调处理进度的方法publishProgress():

protected final void publishProgress(Progress... values) {
        if (!isCancelled()) {
            getHandler().obtainMessage(MESSAGE_POST_PROGRESS,
                    new AsyncTaskResult<Progress>(this, values)).sendToTarget();
        }
    }

通过handler将任务进度传到主线程，我们看handleMessage(见上面的处理） result.mTask.onProgressUpdate(result.mData);是不是很熟悉，就是将进度回调到应用中，并且是在主线程中，可直接更新UI。很多很小的细节，可以再慢慢琢磨，先把大概的分析清楚，这就是AsyncTask的大致流程。