使用的版本：
implementation 'io.reactivex:rxjava:1.3.8'
implementation 'io.reactivex:rxandroid:1.2.1'
map操作符简单示例
先看一个简单的例子如下：
Observable.create(new Observable.OnSubscribe<String>() {
@Override
public void call(Subscriber<? super String> subscriber) {
if (!subscriber.isUnsubscribed()) {
subscriber.onNext("123");
subscriber.onNext("345");
subscriber.onCompleted();
}
}
}).map(new Func1<String, Integer>() {
@Override
public Integer call(String s) {
return Integer.valueOf(s);
}
}).subscribe(new Observer<Integer>() {
@Override
public void onCompleted() {
System.out.println("onCompleted");
}
@Override
public void onError(Throwable e) {
System.out.println("onError");
}
@Override
public void onNext(Integer i) {
System.out.println("onNext:" + i);
}
});
在这里以map操作符为例并且我假设你已经知道map操作符是干什么的。create方法和subscribe方法之前已经分析过了，直接贴出map操作符相关的源码。
public final <R> Observable<R> map(Func1<? super T, ? extends R> func) {
//这里其实就是返回了一个新的Observable，
//他的onSubscribe参数就是这里的OnSubscribeMap
return unsafeCreate(new OnSubscribeMap<T, R>(this, func));
}
public static <T> Observable<T> unsafeCreate(OnSubscribe<T> f) {
return new Observable<T>(RxJavaHooks.onCreate(f));
}
可以看出map方法就是创建并返回了一个新的Observable对象，他的他的onSubscribe参数就是这里的OnSubscribeMap。我们这里贴一下它的源码。
public final class OnSubscribeMap<T, R> implements OnSubscribe<R> {
final Observable<T> source;
final Func1<? super T, ? extends R> transformer;
public OnSubscribeMap(Observable<T> source, Func1<? super T, ? extends R> transformer) {
this.source = source;
this.transformer = transformer;
}
@Override
public void call(final Subscriber<? super R> o) {
MapSubscriber<T, R> parent = new MapSubscriber<T, R>(o, transformer);
o.add(parent);
source.unsafeSubscribe(parent);
}
static final class MapSubscriber<T, R> extends Subscriber<T> {
final Subscriber<? super R> actual;
final Func1<? super T, ? extends R> mapper;
boolean done;
public MapSubscriber(Subscriber<? super R> actual, Func1<? super T, ? extends R> mapper) {
this.actual = actual;
this.mapper = mapper;
}
@Override
public void onNext(T t) {
R result;
try {
result = mapper.call(t);
} catch (Throwable ex) {
Exceptions.throwIfFatal(ex);
unsubscribe();
onError(OnErrorThrowable.addValueAsLastCause(ex, t));
return;
}
actual.onNext(result);
}
@Override
public void onError(Throwable e) {
if (done) {
RxJavaHooks.onError(e);
return;
}
done = true;
actual.onError(e);
}
@Override
public void onCompleted() {
if (done) {
return;
}
actual.onCompleted();
}
@Override
public void setProducer(Producer p) {
actual.setProducer(p);
}
}
构造方法中的source参数就是原来的Observable，transformer就是变换操作的Func1。这个类中的重点就是这个call方法，在里面可以看到他把传递过来的Subscriber其实就是Observer给重新包装了一下。生成了一个新的Subscriber传递给source也就是最初的Observable，这样就都连接起来了。看一下unsafeSubscribe方法,这个跟上一章的普通调用一样了。只不过这里使用的Subscriber是之前包装的MapSubscriber。
public final Subscription unsafeSubscribe(Subscriber<? super T> subscriber) {
try {
// new Subscriber so onStart it
subscriber.onStart();
// allow the hook to intercept and/or decorate
RxJavaHooks.onObservableStart(this, onSubscribe).call(subscriber);
return RxJavaHooks.onObservableReturn(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(RxJavaHooks.onObservableError(e));
} catch (Throwable e2) {
Exceptions.throwIfFatal(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 OnErrorFailedException("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.
RxJavaHooks.onObservableError(r);
// TODO why aren't we throwing the hook's return value.
throw r; // NOPMD
}
return Subscriptions.unsubscribed();
}
}
这里我们来分析一下MapSubscriber类。他继承自Subscriber，在他的onError和onComplete方法都是直接调用实际的Observer的相应方法。比较需要注意的是onNext方法，我们看一下，他首先拿到mapper.call(t)的返回，也就是map操作符中变换的结果，然后把结果给到actual.onNext(result)，到这里我们就知道Observer中onNext方法收到的就是变换后的结果。
