Creation Operators

Creating Observables

This section explains how to create Observables either explicitly or by wrapping an existing data structure.

toObservable() & from()

Make an Observable from an Iterable.

Any object that supports the Iterable<> interface can be converted into a Observable that emits each iterable item in the object, simply by passing the object into the Observable.toObservable( ) method, for example:

myObservable = Observable.toObservable(myIterable);

You can also do this with arrays, for example:

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

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

Observable.just()

Make any object into a Observable.

To convert any object into a Observable that emits that object, pass that object into the Observable.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 is similar to the Observable.toObservable() method, except that Observable.toObservable() will convert an iterable object into a Observable that emits each of the items in the iterable, one at a time, while the Observable.just() method would convert the iterable into a Observable that emits the entire iterable as a single item.

Observable.create( )

Create an explicit Observable

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

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

NOTE: A well-formed Observable must call either the observer’s onCompleted() method exactly once or its onError() method exactly once.