欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页

RxJava2.0中fromArray操作符用法和源码分析(三)

程序员文章站 2024-02-29 08:28:34
...

fromArray基本使用

fromArray用来创建一个Observable对象,可以将一个数组转化为可被观察的序列并且将它的数据逐个发射。
RxJava2.0中fromArray操作符用法和源码分析(三)
fromArray与just相似,都可以用来发射单个或一组数据,但是区别是当fromArray发射一组数据到观察序列中来时,它会先进行遍历,然后再逐个发射。而just发射一组数据时,会把它当成一个整体,一次性发射。字面意思难以理解,我们用代码说明:

Integer[] items = { 0, 1, 2, 3, 4, 5 };

Observable.fromArray(items).subscribe(new Consumer<Integer>() {
    @Override
    public void accept(Integer integer) throws Exception {
        println("onNext : fromArray : " + integer + "\n");
    }
});

Observable.just(items).subscribe(new Consumer<Integer[]>() {
    @Override
    public void accept(Integer[] integers) throws Exception {
        for (int i: integers) {
            println("onNext : just : " + i + "\n");
        }
    }
});

输出结果:

onNext : fromArray : 0
onNext : fromArray : 1
onNext : fromArray : 2
onNext : fromArray : 3
onNext : fromArray : 4
onNext : fromArray : 5

onNext : just : 0
onNext : just : 1
onNext : just : 2
onNext : just : 3
onNext : just : 4
onNext : just : 5

上面输出结果是不一样的,使用fromArray时,接收的数据是逐个打印出来,而使用just时,直接接收的是一个数组。所以如上面所说fromArray接收的数据源是逐个发射的,而just是将数据作为一个完整的对象一次性发射的。

下面我们将继续从源码的角度进行分析:
老规矩我们还是从fromArray方法中着手分析:
Observable#fromArray

@CheckReturnValue
@SchedulerSupport(SchedulerSupport.NONE)
public static <T> Observable<T> fromArray(T... items) {
    ObjectHelper.requireNonNull(items, "items is null");
    if (items.length == 0) {
        return empty();
    } else
    if (items.length == 1) {
        return just(items[0]);
    }
    return RxJavaPlugins.onAssembly(new ObservableFromArray<T>(items));
}

fromArray方法接收的是一个可变参数,该参数又作为创建ObservableFromArray对象的参数。最后返回的是ObservableFromArray对象,作为Observable的具体实现类。

然后开始订阅观察者对象,这里我使用Consumer作为观察者。继续查看subscribe方法:
Observable#subscribe

......

@CheckReturnValue
@SchedulerSupport(SchedulerSupport.NONE)
public final Disposable subscribe(Consumer<? super T> onNext, Consumer<? super Throwable> onError,
        Action onComplete, Consumer<? super Disposable> onSubscribe) {
    ObjectHelper.requireNonNull(onNext, "onNext is null");
    ObjectHelper.requireNonNull(onError, "onError is null");
    ObjectHelper.requireNonNull(onComplete, "onComplete is null");
    ObjectHelper.requireNonNull(onSubscribe, "onSubscribe is null");

    LambdaObserver<T> ls = new LambdaObserver<T>(onNext, onError, onComplete, onSubscribe);

    subscribe(ls);

    return ls;
}

@SchedulerSupport(SchedulerSupport.NONE)
@Override
public final void subscribe(Observer<? super T> observer) {
    ObjectHelper.requireNonNull(observer, "observer is null");
    try {
        observer = RxJavaPlugins.onSubscribe(this, observer);

        ObjectHelper.requireNonNull(observer, "Plugin returned null Observer");

        subscribeActual(observer);
    } catch (NullPointerException e) { // NOPMD
        throw e;
    } catch (Throwable e) {
        Exceptions.throwIfFatal(e);
        // can't call onError because no way to know if a Disposable has been set or not
        // can't call onSubscribe because the call might have set a Subscription already
        RxJavaPlugins.onError(e);

        NullPointerException npe = new NullPointerException("Actually not, but can't throw other exceptions due to RS");
        npe.initCause(e);
        throw npe;
    }
}

这里其实和just操作符调用的逻辑一样,都是重载的subscribe方法,最后执行的是接收四个参数的subscribe方法,如上。这里的创建LambdaObserver对象,不再做重复说明。
subscribeActual其实调用的是Observable实现类中的方法,也就是ObservableFromArray中的方法。我们直接进入该类中查看:
ObservableFromArray

public final class ObservableFromArray<T> extends Observable<T> {
    final T[] array;
    public ObservableFromArray(T[] array) {
        this.array = array;
    }
    @Override
    public void subscribeActual(Observer<? super T> s) {
        FromArrayDisposable<T> d = new FromArrayDisposable<T>(s, array);

        s.onSubscribe(d);

        if (d.fusionMode) {
            return;
        }

        d.run();
    }

    ......
}

subscribeActual中,创建了一个FromArrayDisposable类对象,执行了Observer中的onSubscribe方法,也就是LambdaObserver中的onSubscribe方法。我们在进入这个方法中看一下:

LambdaObserver#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 s) {
    if (DisposableHelper.setOnce(this, s)) {
        try {
            onSubscribe.accept(this);
        } catch (Throwable ex) {
            Exceptions.throwIfFatal(ex);
            s.dispose();
            onError(ex);
        }
    }
}

通过第四个参数 onSubscribe的接口回调方法,将FromArrayDisposable对象暴露给外部调用。这里也不再说明。
我们重点来看看FromArrayDisposable中的run方法:

FromArrayDisposable#run

void run() {
    T[] a = array;
    int n = a.length;

    for (int i = 0; i < n && !isDisposed(); i++) {
        T value = a[i];
        if (value == null) {
            actual.onError(new NullPointerException("The " + i + "th element is null"));
            return;
        }
        actual.onNext(value);
    }
    if (!isDisposed()) {
        actual.onComplete();
    }
}

array 代表我们刚才传入的数组的对象。在for循环中对数组进行遍历,然后将数组中的值据逐个通过actual.onNext(value)方法发送出去。如果出现错误时,则调用actual.onError。完成发送后,在调用actual.onComplete()方法。而我们知道这里面分别调用的是LambdaObserver对象中的方法。

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

    private static final long serialVersionUID = -7251123623727029452L;
    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 s) {
        if (DisposableHelper.setOnce(this, s)) {
            try {
                onSubscribe.accept(this);
            } catch (Throwable ex) {
                Exceptions.throwIfFatal(ex);
                s.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));
            }
        }
    }

    @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;
    }
}

如上,在LambdaObserver类中onNext,onError,onComplete分别对应的不同的参数回调,从而将接口中的数据传递给外部。这样便完成了调用。

下面列出fromArray和just操作符的简单demo:

List<String> list = new ArrayList<>();
list.add("hello");
list.add("world");

Observable.fromArray(list).subscribe(new Consumer<List<String>>() {
    @Override
    public void accept(List<String> strings) throws Exception {
        println("onNext : fromArray : " + strings + "\n");
    }
});

Observable.just(list).subscribe(new Consumer<List<String>>() {
    @Override
    public void accept(List<String> strings) throws Exception {
        println("onNext : just : " + strings + "\n");
    }
});

输出结果:

onNext : fromArray : [hello, world]

onNext : just : [hello, world]