Observable Utility Operators

This section explains various utility operators for working with Observables.

toList( ) — collect all elements from an Observable and emit as a single List
toSortedList( ) — collect all elements from an Observable and emit as a single, sorted List
materialize( ) — convert an Observable into a list of Notifications
dematerialize( ) — convert a materialized Observable back into its non-materialized form
all( ) — determine whether all items emitted by an Observable meet some criteria
finallyDo( ) — register an action to take when an Observable completes
sequenceEqual( ) — test the equality of pairs of items emitted by two Observables
synchronize( ) — force a poorly-behaving Observable to be well-behaved
timestamp( ) — attach a timestamp to every object emitted by an Observable
cache( ) — generate the sequence once, and remember it for future subscribers
defer( ) —
observeOn( ) — specify on which Scheduler an Observer should observe the Observable
subscribeOn( ) — specify which Scheduler an Observable should use when its subscription is invoked
onErrorResumeNext( ) — instructs an Observable to continue emitting values after it encounters an error
onErrorReturn( ) — instructs an Observable to emit a particular value when it encounters an error

toList( )

collect all elements from an Observable and emit as a single List

Normally, a Observable that emits multiple items will do so by calling its observer’s onNext closure for each such item. You can change this behavior, instructing the Observable to compose a list of these multiple items and then to call the observer’s onNext closure once, passing it the entire list, by calling the Observable object’s toList( ) method prior to calling its subscribe( ) method. For example:

Observable.tolist(myObservable).subscribe([ onNext: { myListOfSomething -> do something useful with the list } ]);

For example, the following rather pointless code takes a list of integers, converts it into a Observable, then converts that Observable into one that emits the original list as a single item:

numbers = Observable.toObservable([1, 2, 3, 4, 5, 6, 7, 8, 9]);

Observable.toList(numbers).subscribe(
  [ onNext:{ myWriter.println(it); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

[1, 2, 3, 4, 5, 6, 7, 8, 9]
Sequence complete

In addition to calling toList( ) as a stand-alone method, you can also call it as a method of a Observable object, so, in the example above, instead of

Observable.toList(numbers) ...

you could instead write

numbers.toList() ...

toSortedList( )

collect all elements emitted by an Observable and emit this as a single sorted List

The toSortedList( ) method behaves much like toList( ) except that it sorts the resulting list. By default it sorts the list naturally in ascending order, but you can also pass in a function that takes two values and returns a number, and toSortedList( ) will use that number instead of the numerical difference between the two values to sort the values.

For example, the following code takes a list of unsorted integers, converts it into a Observable, then converts that Observable into one that emits the original list in sorted form as a single item:

numbers = Observable.toObservable([8, 6, 4, 2, 1, 3, 5, 7, 9]);

Observable.toSortedList(numbers).subscribe(
  [ onNext:{ myWriter.println(it); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

[1, 2, 3, 4, 5, 6, 7, 8, 9]
Sequence complete

In addition to calling toList( ) as a stand-alone method, you can also call it as a method of a Observable object, so, in the example above, instead of

Observable.toSortedList(numbers) ...

you could instead write

numbers.toSortedList() ...

materialize( )

convert an Observable into a list of Notifications

A well-formed Observable will call its observer’s onNext closure zero or more times, and then will call either the onCompleted or onError closure exactly once. The materialize( ) method converts this series of calls into a series of emissions from a Observable, where it represents each such call as a Notification object.

For example:

numbers = Observable.toObservable([1, 2, 3]);

Observable.materialize(numbers).subscribe(
  [ onNext: { if(rx.Notification.Kind.OnNext == it.kind) { myWriter.println("Next: " + it.value); }
              else if(rx.Notification.Kind.OnCompleted == it.kind) { myWriter.println("Completed"); }
              else if(rx.Notification.Kind.OnError == it.kind) { myWriter.println("Error: " + it.exception); } },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

Next: 1
Next: 2
Next: 3
Completed
Sequence complete

In addition to calling materialize( ) as a stand-alone method, you can also call it as a method of a Observable object, so that instead of

Observable.materialize(numbers) ...

in the above example, you could also write

numbers.materialize() ...

dematerialize( )

convert a materialized Observable back into its non-materialized form

You can undo the effects of materialize( ) by means of the dematerialize( ) method, which will emit the items from the Observable as though materialize( ) had not been applied to it. The following example dematerializes the materialized Observable from the previous section:

numbers = Observable.toObservable([1, 2, 3]);

Observable.materialize(numbers).dematerialize().subscribe(
  [ onNext: { myWriter.println(it); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

1
2
3
Sequence complete

all( )

determine whether all items emitted by an Observable meet some criteria

Pass an closure to all( ) that accepts an object emitted by the source Observable and returns a boolean value based on an evaluation of that object, and all( ) will emit true if and only if that closure returned true for every object emitted by the source Observable.

numbers = Observable.toObservable([1, 2, 3, 4, 5]);

myWriter.println("all even?" )
numbers.all({ 0 == (it % 2) }).subscribe([onNext:{ myWriter.println(it); }]);

myWriter.println("all positive?");
numbers.all({ 0 < it }).subscribe([onNext:{ myWriter.println(it); }]);

all even? 
false
all positive? 
true

finallyDo( )

register an action to take when an Observable completes

You can use the finallyDo( ) method of an Observable to register an action (a closure that implements Action0) that RxJava will invoke when that Observable calls either the onCompleted( ) or onError( ) method of its Observer.

class TestFinally
{
  static class myActionClass implements rx.util.functions.Action0 {
    void call() { myWriter.println('Finally'); myWriter.flush(); }
  }
  
  static main() {
    def myAction = new myActionClass();
    def numbers = Observable.toObservable([1, 2, 3, 4, 5]);
    
    numbers.finallyDo(myAction).subscribe(
          [ onNext: { myWriter.println(it); },
            onCompleted:{ myWriter.println("Sequence complete"); },
            onError:{ myWriter.println("Error encountered"); } ]
    );
  }
}
new TestFinally().main();

1
2
3
4
5
Sequence complete
Finally

sequenceEqual( )

test the equality of pairs of items emitted by two Observables

Pass sequenceEqual( ) two Observables, and it will compare the objects emitted by each Observable, and emit true for each pair of objects if and only if both objects are the same. You can optionally pass a third parameter: a closure that accepts two objects and returns true if they are equal according to a standard of your choosing.

def firstfour = Observable.toObservable([1, 2, 3, 4]);
def firstfouragain = Observable.toObservable([1, 2, 3, 4]);
def firstfive = Observable.toObservable([1, 2, 3, 4, 5]);
def firstfourscrambled = Observable.toObservable([3, 2, 1, 4]);

myWriter.println('firstfour == firstfive?');
Observable.sequenceEqual(firstfour, firstfive).subscribe([onNext:{ myWriter.println(it); }]);
myWriter.println('firstfour == firstfouragain?');
Observable.sequenceEqual(firstfour, firstfouragain).subscribe([onNext:{ myWriter.println(it); }]);
myWriter.println('firstfour == firstfourscrambled?');
Observable.sequenceEqual(firstfour, firstfourscrambled).subscribe([onNext:{ myWriter.println(it); }]);

firstfour == firstfive?
true
true
true
true
firstfour == firstfouragain?
true
true
true
true
firstfour == firstfourscrambled?
false
true
false
true

synchronize( )

force a poorly-behaving Observable to be well-behaved

The Observables implemented by RxJava are themselves well-behaved (except for the test observable returned by never( )), which is to say they call an observer's onNext( ) closure zero or more times, and then call either the observer's onCompleted( ) closure or the observer's onError( ) closure (but never both) exactly once, and then call none of these closures thereafter.

It is possible that you may encounter a poorly-behaved Observable, perhaps because it emits values on different threads and one thread continues calling onNext( ) after another thread has terminated with onError( ). You can force such an Observable to be well-behaved by applying the synchronize( ) method to it.

timestamp( )

attach a timestamp to every object emitted by an Observable

The timestamp( ) method converts an Observable that emits objects of type T into one that emits objects of type Timestamped<T>, where each such object is stamped with the time at which it was emitted.

def myObservable = Observable.range(1, 1000000).filter({ 0 == (it % 200000) });

myObservable.timestamp().subscribe(
  [ onNext: { myWriter.println(it.toString()); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

Timestamped(timestampMillis = 1369252582698, value = 200000)
Timestamped(timestampMillis = 1369252582740, value = 400000)
Timestamped(timestampMillis = 1369252582782, value = 600000)
Timestamped(timestampMillis = 1369252582823, value = 800000)
Timestamped(timestampMillis = 1369252582864, value = 1000000)
Sequence complete

cache( )

generate the sequence once, and remember it for future subscribers

By default, an Observable will generate its sequence afresh for each subscriber. You can force it to generate its sequence only once and then to serve this identical sequence to every subscriber by using the cache( ) method. Compare the behavior of the following two sets of sample code, the first of which does not use cache( ) and the second of which does:

def myObservable = Observable.range(1, 1000000).filter({ 0 == (it % 400000) }).timestamp();

myObservable.subscribe(
  [ onNext: { myWriter.println(it.toString()); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);
myObservable.subscribe(
  [ onNext: { myWriter.println(it.toString()); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

Timestamped(timestampMillis = 1369252832871, value = 400000)
Timestamped(timestampMillis = 1369252832951, value = 800000)
Sequence complete
Timestamped(timestampMillis = 1369252833074, value = 400000)
Timestamped(timestampMillis = 1369252833154, value = 800000)
Sequence complete

def myObservable = Observable.range(1, 1000000).filter({ 0 == (it % 400000) }).timestamp().cache();

myObservable.subscribe(
  [ onNext: { myWriter.println(it.toString()); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);
myObservable.subscribe(
  [ onNext: { myWriter.println(it.toString()); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

Timestamped(timestampMillis = 1369252924548, value = 400000)
Timestamped(timestampMillis = 1369252924630, value = 800000)
Sequence complete
Timestamped(timestampMillis = 1369252924548, value = 400000)
Timestamped(timestampMillis = 1369252924630, value = 800000)
Sequence complete

Note that in the second example the timestamps are identical for both of the observers, whereas in the first example they differ.

defer( )

observeOn( )

specify on which Scheduler an Observer should observe the Observable

To specify in which Scheduler (thread) the Observable should invoke the Observers' onNext( ), onCompleted( ), and onError( ) closures, call the Observable's observeOn( ) method, passing it the appropriate Scheduler.

subscribeOn( )

specify which Scheduler an Observable should use when its subscription is invoked

To specify that the work done by the Observable should be done on a particular Scheduler (thread), call the Observable's subscribeOn( ) method, passing it the appropriate Scheduler. By default (that is, unless you modify the Observable also with observeOn( )) the Observable will invoke the Observers' onNext( ), onCompleted( ), and onError( ) closures in this same thread.

onErrorResumeNext( )

instructs an Observable to attempt to continue emitting values after it encounters an error

The onErrorResumeNext( ) method returns an Observable that mirrors the behavior of the source Observable, unless that Observable invokes onError( ) in which case, rather than propagating that error to the Observer, onErrorResumeNext( ) will instead begin mirroring a second, backup Observable, as shown in the following sample code:

def myObservable = Observable.create({ anObserver ->
  anObserver.onNext('Three');
  anObserver.onNext('Two');
  anObserver.onNext('One');
  anObserver.onError();
});
def myFallback = Observable.create({ anObserver ->
  anObserver.onNext('0');
  anObserver.onNext('1');
  anObserver.onNext('2');
  anObserver.onCompleted();
});

myObservable.onErrorResumeNext(myFallback).subscribe(
  [ onNext:{ myWriter.println(it); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

Three
Two
One
0
1
2
Sequence complete

onErrorReturn( )

instructs an Observable to emit a particular value to an observer’s onNext closure when it encounters an error

The onErrorReturn( ) method returns an Observable that mirrors the behavior of the source Observable, unless that Observable invokes onError( ) in which case, rather than propagating that error to the Observer, onErrorReturn( ) will instead emit a specified object and call the Observer's onCompleted( ) closure, as shown in the following sample code:

def myObservable = Observable.create({ anObserver ->
  anObserver.onNext('Four');
  anObserver.onNext('Three');
  anObserver.onNext('Two');
  anObserver.onNext('One');
  anObserver.onError();
});

myObservable.onErrorReturn({ return('Blastoff!'); }).subscribe(
  [ onNext:{ myWriter.println(it); },
    onCompleted:{ myWriter.println("Sequence complete"); },
    onError:{ myWriter.println("Error encountered"); } ]
);

Four
Three
Two
One
Blastoff!
Sequence complete

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Observable Utility Operators

toList( )

collect all elements from an Observable and emit as a single List

toSortedList( )

collect all elements emitted by an Observable and emit this as a single sorted List

materialize( )

convert an Observable into a list of Notifications

dematerialize( )

convert a materialized Observable back into its non-materialized form

all( )

determine whether all items emitted by an Observable meet some criteria

finallyDo( )

register an action to take when an Observable completes

sequenceEqual( )

test the equality of pairs of items emitted by two Observables

synchronize( )

force a poorly-behaving Observable to be well-behaved

timestamp( )

attach a timestamp to every object emitted by an Observable

cache( )

generate the sequence once, and remember it for future subscribers

defer( )

observeOn( )

specify on which Scheduler an Observer should observe the Observable

subscribeOn( )

specify which Scheduler an Observable should use when its subscription is invoked

onErrorResumeNext( )

instructs an Observable to attempt to continue emitting values after it encounters an error

onErrorReturn( )

instructs an Observable to emit a particular value to an observer’s onNext closure when it encounters an error

Clone this wiki locally

Observable Utility Operators

toList( )

collect all elements from an Observable and emit as a single List

toSortedList( )

collect all elements emitted by an Observable and emit this as a single sorted List

materialize( )

convert an Observable into a list of Notifications

dematerialize( )

convert a materialized Observable back into its non-materialized form

all( )

determine whether all items emitted by an Observable meet some criteria

finallyDo( )

register an action to take when an Observable completes

sequenceEqual( )

test the equality of pairs of items emitted by two Observables

synchronize( )

force a poorly-behaving Observable to be well-behaved

timestamp( )

attach a timestamp to every object emitted by an Observable

cache( )

generate the sequence once, and remember it for future subscribers

defer( )

observeOn( )

specify on which Scheduler an Observer should observe the Observable

subscribeOn( )

specify which Scheduler an Observable should use when its subscription is invoked

onErrorResumeNext( )

instructs an Observable to attempt to continue emitting values after it encounters an error

onErrorReturn( )

instructs an Observable to emit a particular value to an observer’s onNext closure when it encounters an error

Clone this wiki locally

toList( )

toSortedList( )

materialize( )

dematerialize( )

all( )

finallyDo( )

sequenceEqual( )

synchronize( )

timestamp( )

cache( )

defer( )

observeOn( )

subscribeOn( )

onErrorResumeNext( )

onErrorReturn( )