Observable subscribe流程介绍

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本文基于 RxJava 2.x 版本

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我们直接看Observable的subscribe方法

代码语言：javascript复制

public final Disposable subscribe() {
    return subscribe(Functions.emptyConsumer(), Functions.ON_ERROR_MISSING, Functions.EMPTY_ACTION, Functions.emptyConsumer());
}
public final Disposable subscribe(Consumer<? super T> onNext) {
    return subscribe(onNext, Functions.ON_ERROR_MISSING, Functions.EMPTY_ACTION, Functions.emptyConsumer());
}
public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError) {
    return subscribe(onNext, onError, Functions.EMPTY_ACTION, Functions.emptyConsumer());
}
public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
                                  Action onComplete) {
    return subscribe(onNext, onError, onComplete, Functions.emptyConsumer());
}
public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
                                  Action onComplete, Consumer<? super Disposable> onSubscribe) {
    ...
    LambdaObserver<T> ls = new LambdaObserver<T>(onNext, onError, onComplete, onSubscribe);
    subscribe(ls);
    return ls;
}
public final void subscribe(Observer<? super T> observer) {
    ObjectHelper.requireNonNull(observer, "observer is null");
    try {
        ...
        subscribeActual(observer);
    } catch (NullPointerException e) { // NOPMD
        throw e;
    } catch (Throwable e) {
        ...
    }
}
protected abstract void subscribeActual(Observer<? super T> observer);

• subscribe()
• subscribe(Consumer<? super T> onNext)
• subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError)
• subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError, Action onComplete)
• subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError, Action onComplete, Consumer<? super Disposable> onSubscribe)

我们可以看到 前面四个方法都是调用了第五个方法，对参数onNext、onError、onComplete、onSubscribe的默认赋值。

然后四参数的方法将onNext、onError、onComplete、onSubscribe构建成一个LambdaObserver对象，传递给了subscribe(Observer<? super T> observer)方法。

而 subscribe(Observer<? super T> observer)则是调用了抽象方法subscribeActual(Observer<? super T> observer)，这个方法由上一个操作符返回的Observer对象重写实现。

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接下来我们先来看看这些参数的默认值：

Functions.emptyConsumer()：accept(Object v)回调的空实现

代码语言：javascript复制

public static <T> Consumer<T> emptyConsumer() {
    return (Consumer<T>)EMPTY_CONSUMER;
}

static final Consumer<Object> EMPTY_CONSUMER = new EmptyConsumer();

static final class EmptyConsumer implements Consumer<Object> {
    @Override
    public void accept(Object v) { }

    @Override
    public String toString() {
        return "EmptyConsumer";
    }
}

Functions.ON_ERROR_MISSING： 输出报错信息

代码语言：javascript复制

public static final Consumer<Throwable> ON_ERROR_MISSING = new OnErrorMissingConsumer();
static final class OnErrorMissingConsumer implements Consumer<Throwable> {
    @Override
    public void accept(Throwable error) {
        RxJavaPlugins.onError(new OnErrorNotImplementedException(error));
    }
}
public static void onError(@NonNull Throwable error) {
    ...
    error.printStackTrace(); // NOPMD
    uncaught(error);
}

Functions.EMPTY_ACTION：run()回调的空实现

代码语言：javascript复制

public static final Action EMPTY_ACTION = new EmptyAction();
static final class EmptyAction implements Action {
    @Override
    public void run() { }

    @Override
    public String toString() {
        return "EmptyAction";
    }
}

---

然后我们来看看LambdaObserver类：

代码语言：javascript复制

public final class LambdaObserver<T> extends AtomicReference<Disposable>
        implements Observer<T>, Disposable, LambdaConsumerIntrospection {

    final Consumer<? super T> onNext;
    final Consumer<? super Throwable> onError;
    final Action onComplete;
    final Consumer<? super Disposable> onSubscribe;

    public LambdaObserver(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
            Action onComplete,
            Consumer<? super Disposable> onSubscribe) {
        super();
        this.onNext = onNext;
        this.onError = onError;
        this.onComplete = onComplete;
        this.onSubscribe = onSubscribe;
    }

    @Override
    public void onSubscribe(Disposable d) {
        if (DisposableHelper.setOnce(this, d)) {
            try {
                onSubscribe.accept(this);
            } catch (Throwable ex) {
                Exceptions.throwIfFatal(ex);
                d.dispose();
                onError(ex);
            }
        }
    }

    @Override
    public void onNext(T t) {
        if (!isDisposed()) {
            try {
                onNext.accept(t);
            } catch (Throwable e) {
                Exceptions.throwIfFatal(e);
                get().dispose();
                onError(e);
            }
        }
    }

    @Override
    public void onError(Throwable t) {
        if (!isDisposed()) {
            lazySet(DisposableHelper.DISPOSED);
            try {
                onError.accept(t);
            } catch (Throwable e) {
                Exceptions.throwIfFatal(e);
                RxJavaPlugins.onError(new CompositeException(t, e));
            }
        } else {
            RxJavaPlugins.onError(t);
        }
    }

    @Override
    public void onComplete() {
        if (!isDisposed()) {
            lazySet(DisposableHelper.DISPOSED);
            try {
                onComplete.run();
            } catch (Throwable e) {
                Exceptions.throwIfFatal(e);
                RxJavaPlugins.onError(e);
            }
        }
    }

    @Override
    public void dispose() {
        DisposableHelper.dispose(this);
    }

    @Override
    public boolean isDisposed() {
        return get() == DisposableHelper.DISPOSED;
    }

    @Override
    public boolean hasCustomOnError() {
        return onError != Functions.ON_ERROR_MISSING;
    }
}

我们只要关注onSubscribe(Disposable d)、onNext(T t)、onError(Throwable t)、onComplete()这几个实现，分别是调用onSubscribe、onNext、onError、onComplete几个对象的回调方法。

以上

action object observable super

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