Android线程专题:AsynchTask的使用场景及源码分析

AsynchTask简介

   AsyncTask是个抽象类，支持UI线程的正确和轻松使用，允许你执行后台操作并且把结果推到UI线程，且不需要操作Thread和Handler

AsynchTask使用场景

  理想状态下，AsynchTask应该用于短时操作(最多几秒钟)。如果你需要保留线程长时间运行，强烈建议你使ThreadPoolExecutor

AsynchTask示例

   class MyAsyncTask extends AsyncTask<String,Integer,String>{

        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            Log.e(TAG, "onPreExecute: "+Thread.currentThread().getName());
        }

        @Override
        protected String doInBackground(String... strings) {
            Log.e(TAG, "doInBackground: "+Thread.currentThread().getName()+"//"+strings[0]+"//"+strings[1]);
            int progress=0;
            for (int i=0;i<=5;i++){
                progress+=20;
                publishProgress(progress);
            }
            return "执行结束";
        }

        @Override
        protected void onProgressUpdate(Integer... values) {
            super.onProgressUpdate(values);
            Log.e(TAG, "onProgressUpdate: "+Thread.currentThread().getName()+"//"+values[0]);
        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            Log.e(TAG, "onPostExecute: "+Thread.currentThread().getName()+s);
        }
    }



 private MyAsyncTask asyncTask;
        asyncTask = new MyAsyncTask();
        asyncTask.execute("开始了啊","第二个");

//运行结果
DemoActivity: onPreExecute: main
DemoActivity: doInBackground: AsyncTask #1//开始了啊//第二个
DemoActivity: onProgressUpdate: main//20
DemoActivity: onProgressUpdate: main//40
DemoActivity: onProgressUpdate: main//60
DemoActivity: onProgressUpdate: main//80
DemoActivity: onProgressUpdate: main//100
DemoActivity: onProgressUpdate: main//120
DemoActivity: onPostExecute: main执行结束

1. AsynchTask是抽象类，使用它必须继承，且指定三个泛型参数，分别是Params, Progress, Result；Params是执行AsyncTask时需要传入的参数，可用于在后台任务中使用；Progress是后台任务执行时，如果需要在界面上显示当前的进度，则使用这里指定的泛型作为进度单位；Result当任务执行完毕后，如果需要对结果进行返回，则使用这里指定的泛型作为返回值类型
2. onPreExecute、onProgressUpdate、onPostExecute运行在主线程​​​​​；onPreExecute最先执行，做准备工作；onProgressUpdate用来更新UI进度；onPostExecute时间节点是耗时操作执行结束后返回内容
3. doInBackground运行在子线程，做耗时操作；可以通过publishProgress发送进度给onProgressUpdate，

源码分析

查看AsynchTask的构造函数

  private final WorkerRunnable<Params, Result> mWorker;
    private final FutureTask<Result> mFuture; 

public AsyncTask(@Nullable Looper callbackLooper) {
        mHandler = callbackLooper == null || callbackLooper == Looper.getMainLooper()
            ? getMainHandler()
            : new Handler(callbackLooper);

        mWorker = new WorkerRunnable<Params, Result>() {
            public Result call() throws Exception {
                mTaskInvoked.set(true);
                Result result = null;
                try {
                    Process.setThreadPriority(Process.THREAD_PRIORITY_BACKGROUND);
                    //noinspection unchecked
                    result = doInBackground(mParams);
                    Binder.flushPendingCommands();
                } catch (Throwable tr) {
                    mCancelled.set(true);
                    throw tr;
                } finally {
                    postResult(result);
                }
                return result;
            }
        };

        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);
                }
            }
        };
    }


 private static abstract class WorkerRunnable<Params, Result> implements Callable<Result> {
        Params[] mParams;
    }

• 初始化中mWorker是Callable接口的实现类，跟FutureTask共同定义了线程，Callable线程详情

下一步是执行execute()

 private static volatile Executor sDefaultExecutor = SERIAL_EXECUTOR;
public static final Executor SERIAL_EXECUTOR = new SerialExecutor();


 @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) {
        if (mStatus != Status.PENDING) {
            switch (mStatus) {
                case RUNNING:
                    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;
    }

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

  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;
            }
        }
    }

 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);
            }
        }
    }  

 public static final Executor THREAD_POOL_EXECUTOR;

    static {
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(
                CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE_SECONDS, TimeUnit.SECONDS,
                sPoolWorkQueue, sThreadFactory);
        threadPoolExecutor.allowCoreThreadTimeOut(true);
        THREAD_POOL_EXECUTOR = threadPoolExecutor;
    }

1. excute调用executeOnExecutor，首先执行onPreExecute()
2. exec.execute(mFuture)，是通过线程池执行是执行构造函数的变量mWorker内的call(）方法，执行其中的doInBackground(mParams)，最后执行postResult(result);
3. 在AsynchTask实现类里的doInBackground中通过执行publishProgress传递进度；而传递是通过Handler发送，完成了子线程到主线程的切换

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

 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;
            }
        }
    }

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


  private static class AsyncTaskResult<Data> {
        final AsyncTask mTask;
        final Data[] mData;

        AsyncTaskResult(AsyncTask task, Data... data) {
            mTask = task;
            mData = data;
        }
    }

1. doBackground执行后，最终调用AsynchTask的finish方法，然后调用onPostExecute完成整个流程

总结:使用场景是短时异步执行，UI更新操作

本文地址：https://blog.csdn.net/qq_30359699/article/details/107186534