rxJava 源码 原理分析

文章主要简单结合源码分析rxjava的思想和处理方式,很多功能用法不错列举~

rxJava基本用法 :

int drawableRes = ...;
ImageView imageView = ...;
Observable.create(new OnSubscribe() {
    @Override
    public void call(Subscriber subscriber) {
        Drawable drawable = getTheme().getDrawable(drawableRes));
        subscriber.onNext(drawable);
        subscriber.onCompleted();
    }
}).subscribe(new Observer() {
    @Override
    public void onNext(Drawable drawable) {
        imageView.setImageDrawable(drawable);
    }

    @Override
    public void onCompleted() {
    }

    @Override
    public void onError(Throwable e) {
        Toast.makeText(activity, "Error!", Toast.LENGTH_SHORT).show();
    }
});

 (rxJava 也提供了action funcrion的简单用法 原理类似,大家都知道 只有调用了Observable.subscribe() 方法 那create的逻辑才会执行.那我们就看看subscribe() 做了什么 )

源码摘取

A  

  public final static Observable create(OnSubscribe f) {
        return new Observable(hook.onCreate(f));
   }

B 

public class Observable {

    final OnSubscribe onSubscribe;

    /**
     * Creates an Observable with a Function to execute when it is subscribed to.
     * 

     * Note: Use {@link #create(OnSubscribe)} to create an Observable, instead of this constructor,
     * unless you specifically have a need for inheritance.
     * 
     * @param f
     *            {@link OnSubscribe} to be executed when {@link #subscribe(Subscriber)} is called
     */
    protected Observable(OnSubscribe f) {
        this.onSubscribe = f;
    }

C 

public final Subscription subscribe(Subscriber subscriber) {
        // validate and proceed
        if (subscriber == null) {
            throw new IllegalArgumentException("observer can not be null");
        }
        if (onSubscribe == null) {
            throw new IllegalStateException("onSubscribe function can not be null.");
            /*
             * the subscribe function can also be overridden but generally that's not the appropriate approach
             * so I won't mention that in the exception
             */
        }
        
        // new Subscriber so onStart it
        subscriber.onStart();
        
        /*
         * See https://github.com/ReactiveX/RxJava/issues/216 for discussion on "Guideline 6.4: Protect calls
         * to user code from within an Observer"
         */
        // if not already wrapped
        if (!(subscriber instanceof SafeSubscriber)) {
            // assign to `observer` so we return the protected version
            subscriber = new SafeSubscriber(subscriber);
        }

        // The code below is exactly the same an unsafeSubscribe but not used because it would add a sigificent depth to alreay huge call stacks.
        try {
            // allow the hook to intercept and/or decorate
            hook.onSubscribeStart(this, onSubscribe).call(subscriber);
            return hook.onSubscribeReturn(subscriber);
        } catch (Throwable e) {
            // special handling for certain Throwable/Error/Exception types
            Exceptions.throwIfFatal(e);
            // if an unhandled error occurs executing the onSubscribe we will propagate it
            try {
                subscriber.onError(hook.onSubscribeError(e));
            } catch (OnErrorNotImplementedException e2) {
                // special handling when onError is not implemented ... we just rethrow
                throw e2;
            } catch (Throwable e2) {
                // if this happens it means the onError itself failed (perhaps an invalid function implementation)
                // so we are unable to propagate the error correctly and will just throw
                RuntimeException r = new RuntimeException("Error occurred attempting to subscribe [" + e.getMessage() + "] and then again while trying to pass to onError.", e2);
                // TODO could the hook be the cause of the error in the on error handling.
                hook.onSubscribeError(r);
                // TODO why aren't we throwing the hook's return value.
                throw r;
            }
            return Subscriptions.unsubscribed();
        }
    }

分析：（hook这个类设计没有看明白 基本都是传递什么进取就返回什么 ）

Observable 内部持有了一个Onsubscribe 的引用并且是final 的.. 

Observable.create(Onsubscribe  )后给引用赋值,然后subscribe 时候回调它的call 方法.并且把自己引用传递过去

再回过头看例子,rxjava的机制就没那么神秘了,发布订阅模型, create 创建了一个发布者.subscribe 创建一个或者多个订阅者. create call逻辑执行完后直接把结果传递给下一个订阅者. 然后后续还有很多复杂的处理.

但rxjava想要表现的也就是通过这种方式,来解决随着编码的增多代码逻辑仍然保持最初的简洁性.

再看另一个亮点Scheduler 线程的切换

源码摘要 

 public final Observable subscribeOn(Scheduler scheduler) {
        return nest().lift(new OperatorSubscribeOn(scheduler));
 }

 public final Observable> nest() {
        return just(this);
 }

 public final static  Observable just(final T value) {
        return ScalarSynchronousObservable.create(value);
 }
 public final class ScalarSynchronousObservable extends Observable {

    public static final  ScalarSynchronousObservable create(T t) {
        return new ScalarSynchronousObservable(t);
    }

    private final T t;

    protected ScalarSynchronousObservable(final T t) {
        super(new OnSubscribe() {

            @Override
            public void call(Subscriber s) {
                s.onNext(t);
                s.onCompleted();
            }

        });
        this.t = t;
    }

    public T get() {
        return t;
    }

}
-----------------------------------------------------------------------------------------------------------------
 public final  Observable lift(final Operator lift) {
        return new Observable(new OnSubscribe() {
            @Override
            public void call(Subscriber o) {
                try {
                    Subscriber st = hook.onLift(lift).call(o);
                    try {
                        // new Subscriber created and being subscribed with so 'onStart' it
                        st.onStart();
                        onSubscribe.call(st);
                    } catch (Throwable e) {
                        // localized capture of errors rather than it skipping all operators 
                        // and ending up in the try/catch of the subscribe method which then
                        // prevents onErrorResumeNext and other similar approaches to error handling
                        if (e instanceof OnErrorNotImplementedException) {
                            throw (OnErrorNotImplementedException) e;
                        }
                        st.onError(e);
                    }
                } catch (Throwable e) {
                    if (e instanceof OnErrorNotImplementedException) {
                        throw (OnErrorNotImplementedException) e;
                    }
                    // if the lift function failed all we can do is pass the error to the final Subscriber
                    // as we don't have the operator available to us
                    o.onError(e);
                }
            }
        });
    }

-------------------------------------------------------------------------------------------------------------------

public class OperatorSubscribeOn implements Operator> {

    private final Scheduler scheduler;

    public OperatorSubscribeOn(Scheduler scheduler) {
        this.scheduler = scheduler;
    }

    @Override
    public Subscriber> call(final Subscriber subscriber) {
        final Worker inner = scheduler.createWorker();
        subscriber.add(inner);
        return new Subscriber>(subscriber) {

            @Override
            public void onCompleted() {
                // ignore because this is a nested Observable and we expect only 1 Observable emitted to onNext
            }

            @Override
            public void onError(Throwable e) {
                subscriber.onError(e);
            }

            @Override
            public void onNext(final Observable o) {
                inner.schedule(new Action0() {
                    @Override
                    public void call() {
                        final Thread t = Thread.currentThread();
                        o.unsafeSubscribe(new Subscriber(subscriber) {
                            @Override
                            public void onCompleted() {
                                subscriber.onCompleted();
                            }
                            @Override
                            public void onError(Throwable e) {
                                subscriber.onError(e);
                            }
                            @Override
                            public void onNext(T t) {
                                subscriber.onNext(t);
                            }
                            @Override
                            public void setProducer(final Producer producer) {
                                subscriber.setProducer(new Producer() {
                                    @Override
                                    public void request(final long n) {
                                        if (Thread.currentThread() == t) {
                                            // don't schedule if we're already on the thread (primarily for first setProducer call)
                                            // see unit test 'testSetProducerSynchronousRequest' for more context on this
                                            producer.request(n);
                                        } else {
                                            inner.schedule(new Action0() {

                                                @Override
                                                public void call() {
                                                    producer.request(n);
                                                }
                                            });
                                        }
                                    }
                                });
                            }

                        });
                    }
                });
            }
        };
    }
}
然后支持的Schedulers 类型 有Schedulers.io | Schedulers.newThread | Schedulers.computation | (默认Schedulers.immediate | Schedulers.trampoline)
主要介绍前三个
io 对应 CachedThreadScheduler  可复用线程池
newThread对应NewThreadScheduler 单个线程
computation对应EventLoopsScheduler 根据cpu计算得到的线程池
nest方法创建了一个新的发布者,lift 变换 接受之前的发布者处理完后的事件,然后包装交给后面订阅者.就在这个过程中使用Schedulers在指定的线程调度器重执行后回调给
发布者中最后交由订阅者处理