Rxjava2切换线程原理

版本如下：

    implementation "io.reactivex.rxjava2:rxjava:2.2.5"
    implementation 'io.reactivex.rxjava2:rxandroid:2.1.0'

建议一边看博客，一边打开Android studio看代码

首先分析 AndroidSchedulers.mainThread() ，后面分析Schedulers.io()，思路有些类似，在具体的Scheduler.scheduleDirect()方法实现上不一样，因为android的主线程使用了Handler比较简单，所以先看Android主线程。

常用代码如下

Observable observable = new Observable() {
            @Override
            protected void subscribeActual(Observer observer) {
                observer.onNext("1");
                observer.onNext("2");
            }
        };

        observable
                .subscribeOn(AndroidSchedulers.mainThread())
                .subscribe(new Observer() {
            @Override
            public void onSubscribe(Disposable d) {

            }

            @Override
            public void onNext(Object o) {
                Log.e("Tag" , "recive : " + o + "  thread :" + Thread.currentThread().getName());
            }

            @Override
            public void onError(Throwable e) {

            }

            @Override
            public void onComplete() {

            }
        });

从subscribeOn方法上手

    @CheckReturnValue
    @SchedulerSupport(SchedulerSupport.CUSTOM)
    public final Observable subscribeOn(Scheduler scheduler) {
        //非空判断，若为空会直接抛出异常
        ObjectHelper.requireNonNull(scheduler, "scheduler is null");
        //这里将Observable和Scheduler封装成了ObservableSubscribeOn
        //onAssembly方法返回了这个new ObservableSubscribeOn(this, scheduler)对象
        return RxJavaPlugins.onAssembly(new ObservableSubscribeOn(this, scheduler));
    }

所以为了方便理解，我们可以将原来的代码分割成这样：

接下来是subscribe方法，点进去直接看：

    //这个方法在Observable中，ObservableSubscribeOn是Observable的子类
public final void subscribe(Observer observer) {
        //非空判断
        ObjectHelper.requireNonNull(observer, "observer is null");
        try {
            //这里直接返回了observer
            observer = RxJavaPlugins.onSubscribe(this, observer);
            //非空判断
            ObjectHelper.requireNonNull(observer, "The RxJavaPlugins.onSubscribe hook returned a null Observer. Please change the handler provided to RxJavaPlugins.setOnObservableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins");
            //这里是重点方法
            subscribeActual(observer);
        } catch (NullPointerException e) { // NOPMD
            省略
        } catch (Throwable e) {
            省略
        }
    }

到了subscribeActual(observer);习惯性直接Ctrl + 左键就想看源码，点进入发现是Observable类的抽象方法，回看上面我们分割过的代码，就可以看出来实现类是ObservableSubscribeOn，所以直接看ObservableSubscribeOn.subscribeActual（）方法

    //ObservableSubscribeOn类的subscribeActual方法
    @Override
    public void subscribeActual(final Observer observer) {
        final SubscribeOnObserver parent = new SubscribeOnObserver(observer);
        //这里就调用的Observer方法，就是我们自己实现的方法
        observer.onSubscribe(parent);
        //scheduler.scheduleDirect(new SubscribeTask(parent))是重点
        parent.setDisposable(scheduler.scheduleDirect(new SubscribeTask(parent)));
    }

看到这里，scheduler.scheduleDirect(new SubscribeTask(parent))是重点的方法，scheduler还是在我们分割出来的observable.subscribeOn(AndroidSchedulers.mainThread());方法中赋值的，所以现在再回头去看AndroidSchedulers.mainThread()

这里调用的方法很多，但其实并不繁琐，所以大家可以直接自己看，我文字应该可以说一下，如果你自己还是找不到那我也会贴出代码。

文字版：

AndroidSchedulers.mainThread()点进mainThread()方法，里面直接返回的是RxAndroidPlugins.onMainThreadScheduler(MAIN_THREAD)

onMainThreadScheduler（）方法返回了MAIN_THREAD

MAIN_THREAD = RxAndroidPlugins.initMainThreadScheduler(）

RxAndroidPlugins.initMainThreadScheduler(）返回了new Callable().call()，也就是MainHolder.DEFAULT

static final Scheduler DEFAULT = new HandlerScheduler(new Handler(Looper.getMainLooper()), false);

HandlerScheduler类中就是Scheduler的子类，并且重写了scheduleDirect（）方法。

代码版：

    
    
//在AndroidSchedulers类中 ，顺序调用了下列方法
    public static Scheduler mainThread() {
        //这里直接返回了MAIN_THREAD
        return RxAndroidPlugins.onMainThreadScheduler(MAIN_THREAD);
    }


//定义了MAIN_THREAD
private static final Scheduler MAIN_THREAD =
    //调用了initMainThreadScheduler方法         
    RxAndroidPlugins.initMainThreadScheduler(
            new Callable() {
                @Override public Scheduler call() throws Exception {
                    return MainHolder.DEFAULT;
                }
            });


//方法里最终调用了callRequireNonNull
public static Scheduler initMainThreadScheduler(Callable         scheduler) {
        if (scheduler == null) {
            throw new NullPointerException("scheduler == null");
        }
        Function, Scheduler> f = onInitMainThreadHandler;
        if (f == null) {
            return callRequireNonNull(scheduler);
        }
        return applyRequireNonNull(f, scheduler);
    }


//最终返回了s.call()，也就是MainHolder.DEFAULT
static Scheduler callRequireNonNull(Callable s) {
        try {
            Scheduler scheduler = s.call();
            if (scheduler == null) {
                throw new NullPointerException("Scheduler Callable returned null");
            }
            return scheduler;
        } catch (Throwable ex) {
            throw Exceptions.propagate(ex);
        }
    }
    
    
private static final class MainHolder {
        static final Scheduler DEFAULT
            = new HandlerScheduler(new Handler(Looper.getMainLooper()), false);
    }

下面直接看HandlerScheduler中的scheduleDirect方法：

    @Override
    public Disposable scheduleDirect(Runnable run, long delay, TimeUnit unit) {
        if (run == null) throw new NullPointerException("run == null");
        if (unit == null) throw new NullPointerException("unit == null");

        run = RxJavaPlugins.onSchedule(run);
        ScheduledRunnable scheduled = new ScheduledRunnable(handler, run);
        //ScheduledRunnable 实现了 Runnable接口 ，handler 是new Handler(Looper.getMainLooper())，这样就实现在切换到主线程
        handler.postDelayed(scheduled, unit.toMillis(delay));
        return scheduled;
    }

handler 是new Handler(Looper.getMainLooper())，这样就实现在切换到主线程

 

 

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接下来分析Schedulers.io()，其实之前的逻辑是一样的，只是在调用scheduler.scheduleDirect（）时，是由不同子类实现的，所以我们先找到实现scheduleDirect方法的类再分析。

    @NonNull
    public static Scheduler io() {
//        这里直接返回了IO
        return RxJavaPlugins.onIoScheduler(IO);
    }
//    这里帮大家整理成这样，源码里IO = RxJavaPlugins.initIoScheduler(new IOTask());是写在static{}里的
//    initIoScheduler最终返回了IOTask的Call()方法，即Schedulers.IoHolder.DEFAULT
    static final Scheduler IO = RxJavaPlugins.initIoScheduler(new IOTask());

    static final class IOTask implements Callable {
        @Override
        public Scheduler call() throws Exception {
            return Schedulers.IoHolder.DEFAULT;
        }
    }
    static final class IoHolder {
        static final Scheduler DEFAULT = new IoScheduler();
    }

这里已经找到了Scheduler的子类IoScheduler，但是发现IoScheduler并未重写scheduleDirect方法，所以是直接调用了父类Scheduler的scheduleDirect方法

    @NonNull
    public Disposable scheduleDirect(@NonNull Runnable run) {
        return scheduleDirect(run, 0L, TimeUnit.NANOSECONDS);
    }

    @NonNull
    public Disposable scheduleDirect(@NonNull Runnable run, long delay, @NonNull TimeUnit unit) {
//      createWorker()是子类实现的，在Schedulers.io()中是IoScheduler类
        final Worker w = createWorker();
        final Runnable decoratedRun = RxJavaPlugins.onSchedule(run);
        DisposeTask task = new DisposeTask(decoratedRun, w);
//      这里是重点
        w.schedule(task, delay, unit);
        return task;
    }

 

IoScheduler重写的createWorker如下：

public Worker createWorker() {
        return new EventLoopWorker(pool.get());
    }

所以w.schedule(task, delay, unit);就可以直接去EventLoopWorker中找，代码如下

    static final class EventLoopWorker extends Scheduler.Worker {
        private final CompositeDisposable tasks;
        private final IoScheduler.CachedWorkerPool pool;
        private final IoScheduler.ThreadWorker threadWorker;

        EventLoopWorker(IoScheduler.CachedWorkerPool pool) {
            this.pool = pool;
            this.tasks = new CompositeDisposable();
            this.threadWorker = pool.get();
        }

//        省略一些方法
        
        @NonNull
        @Override
        public Disposable schedule(@NonNull Runnable action, long delayTime, @NonNull TimeUnit unit) {
  //        省略
            return threadWorker.scheduleActual(action, delayTime, unit, tasks);
        }
    }

发现threadWorker.scheduleActual(action, delayTime, unit, tasks);而threadWorker = pool.get();所以还得去找pool.get

点进入后可以找到贴出部分代码：

    ThreadWorker get() {
//        检查缓存队列是否为空，不为空则直接取出来用
        while (!expiringWorkerQueue.isEmpty()) {
            ThreadWorker threadWorker = expiringWorkerQueue.poll();
            if (threadWorker != null) {
                return threadWorker;
            }
        }
//      如果缓存是空的则new ThreadWorker
        // No cached worker found, so create a new one.
        ThreadWorker w = new ThreadWorker(threadFactory);
        allWorkers.add(w);
        return w;
    }

返回了ThreadWorker，再查看ThreadWorker代码，发现其中并没有scheduleActual方法，发现它的父类已经写好了，代码如下：

    public ScheduledRunnable scheduleActual(final Runnable run, long delayTime, @NonNull TimeUnit unit, @Nullable DisposableContainer parent) {
        Runnable decoratedRun = RxJavaPlugins.onSchedule(run);

        ScheduledRunnable sr = new ScheduledRunnable(decoratedRun, parent);

        Future f;
        try {
            if (delayTime <= 0) {
//              提交了到了线程池中
                f = executor.submit((Callable