map基本使用
map是變換操作符,對原始Observable發射的每一項數據應用一個你選擇的函數生成新的結果,然後返回一個發射這些結果Observable。
但從字面上還是比較難以理解,我們可以用代碼示例說明:
Observable.just(1,2,3).map(new Function<Integer, String>() {
@Override
public String apply(Integer integer) throws Exception {
return "This is new result " + integer;
}
}).subscribe(new Consumer<String>() {
@Override
public void accept(String s) throws Exception {
println("accept : " + s +"\n");
}
});
輸出結果:
accept : This is new result 1
accept : This is new result 2
accept : This is new result 3
由上面代碼可知,執行map操作時,首先接收原始Observable發射的數據,然後根據你的操作生成新的數據並將這些新的數據發射,這時觀察者中接收的就是新生成的數據。
下面我們將從源碼的角度來分析下:
這裏我們首先使用just操作符創建一個Observable來發射指定的數據。關於just如何創建Observable對象,我們這裏不做分析,前面文章中已經說明。這裏just創建的具體對象爲ObservableFromArray。我們直接分析map的源碼,我們看到在調用map方法時,我們需要傳入一個Function的對象:
/**
* A functional interface that takes a value and returns another value, possibly with a
* different type and allows throwing a checked exception.
*
* @param <T> the input value type
* @param <R> the output value type
*/
public interface Function<T, R> {
/**
* Apply some calculation to the input value and return some other value.
* @param t the input value
* @return the output value
* @throws Exception on error
*/
@NonNull
R apply(@NonNull T t) throws Exception;
}
如上所述,這個接口的功能主要是接收一個值(T),然後返回另一個值(R)。
我們在查看map的方法:
@CheckReturnValue
@SchedulerSupport(SchedulerSupport.NONE)
public final <R> Observable<R> map(Function<? super T, ? extends R> mapper) {
ObjectHelper.requireNonNull(mapper, "mapper is null");
return RxJavaPlugins.onAssembly(new ObservableMap<T, R>(this, mapper));
}
與之前其它操作符一樣的調用邏輯,將當前的Observable對象和生成的Function對象作爲參數,生成一個ObservableMap的對象。
public final class ObservableMap<T, U> extends AbstractObservableWithUpstream<T, U> {
final Function<? super T, ? extends U> function;
public ObservableMap(ObservableSource<T> source, Function<? super T, ? extends U> function) {
super(source);
this.function = function;
}
@Override
public void subscribeActual(Observer<? super U> t) {
source.subscribe(new MapObserver<T, U>(t, function));
}
......
}
完成了Observable對象初始化後,我們開始訂閱觀察者。這裏我們選擇使用的觀察者爲Consumer對象。訂閱觀察者時,執行subscribe方法:
Observable#subscribe
@CheckReturnValue
@SchedulerSupport(SchedulerSupport.NONE)
public final Disposable subscribe(Consumer<? super T> onNext) {
return subscribe(onNext, Functions.ON_ERROR_MISSING, Functions.EMPTY_ACTION, Functions.emptyConsumer());
}
@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;
}
同樣的,將在subscribe方法中執行subscribeActual(observer)方法:
@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;
}
}
這裏其實執行的是ObservableMap中的subscribeActual方法。在subscribeActual方法中,首先會創建一個MapObserver對象,參數t對應的是LambdaObserver。
然後執行source.subscribe方法,source代表的是之前的Observable對象,也就是just創建的ObservableFromArray對象,所以再次調用Observable中subscribe方法,執行subscribeActual,而這次執行的對象是ObservableFromArray,而參數observer具體實現是MapObserver的對象:
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();
}
......
}
到這裏執行的邏輯與之前分析fromArray操作符用法相同,不再做具體分析。在FromArrayDisposable中具體執行的run方法中:
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();
}
}
如上此時actual是由MapObserver實現的,我們看下MapObserver的onNext方法:
MapObserver#onNext:
@Override
public void onNext(T t) {
if (done) {
return;
}
if (sourceMode != NONE) {
actual.onNext(null);
return;
}
U v;
try {
v = ObjectHelper.requireNonNull(mapper.apply(t), "The mapper function returned a null value.");
} catch (Throwable ex) {
fail(ex);
return;
}
actual.onNext(v);
}
當調用onNext方法時,通過Function接口回調apply方法獲得轉換後的數據,然後再通過 actual.onNext(v)方法發射出去。此時的actual中的onNext方法就可以接收新的參數了,而actual就是之前初始化的LambdaObserver對象。通過它可以讓Consumer的accept方法中接收該數據。這裏的執行邏輯之前已經分析過,這裏不再詳述。