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() — determine whether two Observable sequences are identical
• synchronize() — force a poorly-behaving Observable to be well-behaved
• timestamp() — attach a timestamp to every object emitted by an Observable

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:{ response.getWriter().println(it); },
    onCompleted:{ response.getWriter().println("Sequence complete"); },
    onError:{ response.getWriter().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:{ response.getWriter().println(it); },
    onCompleted:{ response.getWriter().println("Sequence complete"); },
    onError:{ response.getWriter().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 Observable.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(Kind.OnNext == it.kind) response.getWriter().println("Next: " + it.value);
              else if(Kind.OnCompleted == it.kind) response.getWriter().println("Completed");
              else if(Kind.OnError == it.kind) response.getWriter().println("Error: " + it.exception); },
    onCompleted:{ response.getWriter().println("Sequence complete"); },
    onError:{ response.getWriter().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

all()

determine whether all items emitted by an Observable meet some criteria

finallyDo()

register an action to take when an Observable completes

sequenceEqual()

determine whether two Observable sequences are identical

synchronize()

force a poorly-behaving Observable to be well-behaved

timestamp()

attach a timestamp to every object emitted by an Observable