Creating Observables

This section explains methods that create Observables.

• just( ) — convert an object into an Observable that emits that object
• from( ) — convert an Iterable, a Future, or an Array into an Observable
• repeat( ) — create an Observable that emits a particular item or sequence of items repeatedly
• create( ) — create an Observable from scratch by means of a function
• defer( ) — do not create the Observable until a Subscriber subscribes; create a fresh Observable on each subscription
• range( ) — create an Observable that emits a range of sequential integers
• interval( ) — create an Observable that emits a sequence of integers spaced by a given time interval
• timer( ) — create an Observable that emits a single item after a given delay
• empty( ) — create an Observable that emits nothing and then completes
• error( ) — create an Observable that emits nothing and then signals an error
• never( ) — create an Observable that emits nothing at all

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just( )

convert an object into an Observable that emits that object

To convert any object into an Observable that emits that object and then completes, pass that object into the just( ) method.

// Observable emits "some string" as a single item
def observableThatEmitsAString = Observable.just("some string"); 
// Observable emits the list [1, 2, 3, 4, 5] as a single item
def observableThatEmitsAList = Observable.just([1, 2, 3, 4, 5]); 

This has some similarities to the from( ) method, but note that if you pass an iterable to from( ), it will convert the iterable into an Observable that emits each of the items in the iterable, one at a time, while the just( ) method would convert the iterable into an Observable that emits the entire iterable as a single item.

Note that if you pass null to just( ), the resulting Observable will not merely call onCompleted( ) without calling onNext( ). It will instead call onNext( null ) before calling onCompleted( ).

see also:

• javadoc: just(value) and just(value, scheduler)

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from( )

convert an Iterable, a Future, or an Array into an Observable

You can convert an object that supports Iterable into an Observable that emits each iterable item in the object, or an object that supports Future into an Observable that emits the result of the get call, simply by passing the object into the from( ) methods, for example:

myObservable = Observable.from(myIterable);

You can also do this with arrays, for example:

myArray = [1, 2, 3, 4, 5];
myArrayObservable = Observable.from(myArray);

This converts the sequence of values in the iterable object or array into a sequence of items emitted, one at a time, by an Observable.

You can convert an empty iterable (or array) into an Observable in this way. The resulting Observable will invoke onCompleted() without first invoking onNext().

Note that when the from( ) method transforms a Future into an Observable, such an Observable will be effectively blocking, as its underlying Future blocks.

Note: in the scala language adaptor for RxJava, the version of this method that works with sequences (arrays) is called items( ).

see also:

• javadoc: from(future), from(future, timeout, unit), and from(future, scheduler)
• javadoc: from(iterable), and from(iterable, scheduler)
• javadoc: from(array) and from(array, scheduler)
• javadoc: from(item) (or two, three, four, five, six, seven, eight, nine, or ten items)
• RxJS: fromArray
• RxJS: fromPromise
• Linq: ToObservable

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repeat( )

create an Observable that emits the sequence of items emitted by the source Observable repeatedly

There are also versions of repeat( ) that operate on a scheduler that you specify, and that repeat only a certain number of times before terminating.

see also:

• javadoc: repeat( ) and repeat(scheduler)
• javadoc: repeat(count) and repeat(count, scheduler)
• Linq: Repeat
• RxJS: repeat

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create( )

create an Observable from scratch by means of a function

You can create an Observable from scratch by using the create( ) method. You pass this method a function that accepts as its parameter the Subscriber that is passed to an Observable’s subscribe( ) method (or that is derived from the Observerer that is passed to that method). Write the function you pass to create( ) so that it behaves as an Observable — calling the passed-in Subscriber’s onNext( ), onError( ), and onCompleted( ) methods appropriately. For example:

def myObservable = Observable.create({ aSubscriber ->
  try {
    for (int i = 1; i < 1000000; i++) {
      if (aSubscriber.isUnsubscribed()) {
        return;
      }
      aSubscriber.onNext(i);
    }
    if (!aSubscriber.isUnsubscribed()) {
      aSubscriber.onCompleted();
    }
  } catch(Throwable t) {
    if (!aSubscriber.isUnsubscribed()) {
      aSubscriber.onError(t);
    }
  }
})

NOTE: A well-formed finite Observable must attempt to call either the Subscriber’s onCompleted( ) method exactly once or its onError( ) method exactly once, and must not thereafter attempt to call any of the Subscriber’s other methods. It is good practice to check the Subscriber’s isUnsubscribed( ) state so that your Observable can stop emitting items or doing expensive calculations when there is no longer an interested Subscriber.

Note: in the scala language adaptor for RxJava, this method is called apply( ).

see also:

• javadoc: create(OnSubscribe)
• RxJS: create
• Linq: Create

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defer( )

do not create the Observable until a Subscriber subscribes; create a fresh Observable on each subscription

Pass defer( ) an Observable factory function (a function that generates Observables), and defer( ) will return an Observable that will call this function to generate its Observable sequence afresh each time a new Subscriber subscribes.

see also:

• javadoc: defer(observableFactory)
• RxJS: defer
• Linq: Defer

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range( )

create an Observable that emits a range of sequential integers

To create an Observable that emits a range of sequential integers, pass the starting integer and the number of integers to emit to the range( ) method.

// myObservable emits the integers 5, 6, and 7 before completing:
def myObservable = Observable.range(5, 3);

In calls to range(n,m), a value of zero for m will result in no numbers being emitted (values less than zero will cause an exception). n may be any integer that can be represented as a BigDecimal — posititve, negative, or zero.

see also:

• javadoc: range(start, count) and range(start, count, scheduler)
• RxJS: range
• Linq: Range
• Introduction to Rx: Range

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interval( )

create an Observable that emits a sequence of integers spaced by a given time interval

To create an Observable that emits items spaced by a particular interval of time, pass the time interval and the units of time that interval is measured in (and, optionally, a scheduler) to the interval( ) method.

see also:

• javadoc: interval(interval,unit) and interval(interval,unit,scheduler)
• RxJS: interval
• Linq: Interval
• Introduction to Rx: Interval

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timer( )

create an Observable that emits a single item after a given delay

The timer( ) method returns an Observable that, when subscribed to, waits for a span of time that you have defined, then emits a single zero and completes.

There is also a version of timer( ) that emits a single zero after a specified delay, and then emits incrementally increasing numbers periodically thereafter on a specified periodicity:

For both of these versions of timer( ) you can optionally specify a Scheduler on which the timing will take place.

see also:

• javadoc: timer(delay, unit) and timer(delay, unit, scheduler)
• javadoc: timer(delay, period, unit) and timer(delay, period, unit, scheduler)
• RxJS: timer
• Linq: Timer
• Introduction to Rx: Timer

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empty( ), error( ), and never( )

Observables that can be useful for testing purposes

• empty( ) creates an Observable that does not emit any items but instead immediately calls the Subscriber’s onCompleted( ) method.
  
• error( ) creates an Observable that does not emit any items but instead immediately calls the Subscriber’s onError( ) method.
  
• never( ) creates an Observable that does not emit any items, nor does it call either the Subscriber’s onCompleted( ) or onError( ) methods.
  

import rx.Observable;
import rx.Observer;
import rx.Subscription;
import rx.subscriptions.Subscriptions;
import rx.util.functions.Func1;

println("*** empty() ***");
Observable.empty().subscribe(
  { println("empty: " + it); },             // onNext
  { println("empty: error - " + it.getMessage()); }, // onError
  { println("empty: Sequence complete"); }  // onCompleted
);

println("*** error() ***");
Observable.error(new Throwable("badness")).subscribe(
  { println("error: " + it); },             // onNext
  { println("error: error - " + it.getMessage()); }, // onError
  { println("error: Sequence complete"); }  // onCompleted
);

println("*** never() ***");
Observable.never().subscribe(
  { println("never: " + it); },             // onNext
  { println("never: error - " + it.getMessage()); }, // onError
  { println("never: Sequence complete"); }  // onCompleted
);
println("*** END ***");

*** empty() ***
empty: Sequence complete
*** error() ***
error: error - badness
*** never() ***
*** END ***

see also:

• javadoc: empty() and empty(scheduler)
• javadoc: error(throwable) and error(throwable, scheduler)
• javadoc: never()
• RxJS: empty and never
• Linq: Empty and Never
• Introduction to Rx: Simple factory methods