Reactive-Extensions/RxJS

# This is RxJS v 4. [Find the latest version here](https://github.com/reactivex/rxjs)
# Observer object #
 
The Observer object provides support for push-style iteration over an observable sequence.
 
The Observer and Objects interfaces provide a generalized mechanism for push-based notification, also known as the observer design pattern. The Observable object represents the object that sends notifications (the provider); the Observer object represents the class that receives them (the observer).
 
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## `Observer Methods`
- [`create`](#rxobservercreateonnext-onerror-oncompleted)
- [`fromNotifier`](#rxobserverfromotifierhandler)
 
## `Observer Instance Methods`
- [`asObserver`](#rxobserverprototypeasobserver)
- [`checked`](#rxobserverprototypechecked)
- [`notifyOn`](#rxobserverprototypenotifyonscheduler)
- [`onCompleted`](#rxobserverprototypeoncompleted)
- [`onError`](#rxobserverprototypeonerrorerror)
- [`onNext`](#rxobserverprototypeonnextvalue)
- [`toNotifier`](#rxobserverprototypetonotifier)
 
## _Observer Methods_ ##
 
### <a id="rxobservercreateonnext-onerror-oncompleted"></a>`Rx.Observer.create([onNext], [onError], [onCompleted])`
<a href="#rxobservercreateonnext-onerror-oncompleted">#</a> [&#x24C8;](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js#L1828-L1833 "View in source") [&#x24C9;][1]
 
Creates an observer from the specified `onNext`, `onError`, and `onCompleted` actions.
 
#### Arguments
1. `[onNext]` *(Function)*: Observer's onNext action implementation.
2. `[onError]` *(Function)*: Observer's onError action implementation.
3. `[onCompleted]` *(Function)*: Observer's onCompleted action implementation.
 
#### Returns
*(Observer)*: The observer object implemented using the given actions.
 
#### Example
```js
var source = Rx.Observable.return(42);
 
var observer = Rx.Observer.create(
    function (x) {
        console.log('Next: ' + x);
    },
    function (err) {
        console.log('Error: ' + err);
    },
    function () {
        console.log('Completed');
    }
);
 
var subscription = source.subscribe(observer);
 
// => Next: 42
// => Completed
```
 
### Location
 
- rx.js
 
* * *
 
### <a id="rxobserverfromotifierhandler"></a>`Rx.Observer.fromNotifier(handler, [thisArg])`
<a href="#rxobserverfromotifierhandler">#</a> [&#x24C8;](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js#L1843-L1851 "View in source") [&#x24C9;][1]
 
Creates an observer from a notification callback.
 
#### Arguments
1. `handler` *(Function)*: Function that handles a notification.
2. `[thisArg]` *(`Any`)*: Object to use as `this` when executing `handler`.
 
#### Returns
*(Observer)*: The observer object that invokes the specified handler using a notification corresponding to each message it receives.
 
#### Example
```js
function handler(n) {
    // Handle next calls
    if (n.kind === 'N') {
        console.log('Next: ' + n.value);
    }
 
    // Handle error calls
    if (n.kind === 'E') {
        console.log('Error: ' + n.exception);
    }
 
    // Handle completed
    if (n.kind === 'C') {
        console.log('Completed')
    }
}
 
Rx.Observer.fromNotifier(handler).onNext(42);
// => Next: 42
 
Rx.Observer.fromNotifier(handler).onError(new Error('error!!!'));
// => Error: Error: error!!!
 
Rx.Observer.fromNotifier(handler).onCompleted();
// => false
```
 
### Location
 
- rx.js
 
* * *
 
## _Observer Instance Methods_ ##
 
### <a id="rxobserverprototypeasobserver"></a>`Rx.Observer.prototype.asObserver()`
<a href="#rxobserverprototypeasobserver">#</a> [&#x24C8;](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js#L1810-L1812 "View in source") [&#x24C9;][1]
 
Hides the identity of an observer.
 
#### Returns
*(Observer)*: An observer that hides the identity of the specified observer.
 
#### Example
```js
function SampleObserver () {
    Rx.Observer.call(this);
    this.isStopped = false;
}
 
SampleObserver.prototype = Object.create(Rx.Observer.prototype);
SampleObserver.prototype.constructor = SampleObserver;
 
Object.defineProperties(SampleObserver.prototype, {
    onNext: {
        value: function (x) {
            if (!this.isStopped) {
                console.log('Next: ' + x);
            }
        }
    },
    onError: {
        value: function (err) {
            if (!this.isStopped) {
                this.isStopped = true;
                console.log('Error: ' + err);
            }
        }
    },
    onCompleted: {
        value: function () {
            if (!this.isStopped) {
                this.isStopped = true;
                console.log('Completed');
            }
        }
    }
});
 
var sampleObserver = new SampleObserver();
 
var source = sampleObserver.asObserver();
 
console.log(source === sampleObserver);
// => false
```
 
### Location
 
- rx.js
 
* * *
 
### <a id="rxobserverprototypechecked"></a>`Rx.Observer.prototype.checked()`
<a href="#rxobserverprototypechecked">#</a> [&#x24C8;](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js#L1819 "View in source") [&#x24C9;][1]
 
Checks access to the observer for grammar violations. This includes checking for multiple `onError` or `onCompleted` calls, as well as reentrancy in any of the observer methods.
 
If a violation is detected, an Error is thrown from the offending observer method call.
 
#### Returns
*(Observer)*: An observer that checks callbacks invocations against the observer grammar and, if the checks pass, forwards those to the specified observer.
 
#### Example
```js
var observer = Rx.Observer.create(
    function (x) {
        console.log('Next: ' + x)
    },
    function (err) {
        console.log('Error: ' + err);
    },
    function () {
        console.log('Completed');
    }
);
 
var checked = observer.checked();
 
checked.onNext(42);
// => Next: 42
 
checked.onCompleted();
// => Completed
 
// Throws Error('Observer completed')
checked.onNext(42);
```
 
### Location
 
- rx.js
 
* * *
 
### <a id="rxobserverprototypenotifyonscheduler"></a>`Rx.Observer.prototype.notifyOn(scheduler)`
<a href="#rxobserverprototypenotifyonscheduler">#</a> [&#x24C8;](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js#L1858-1860 "View in source") [&#x24C9;][1]
 
Schedules the invocation of observer methods on the given scheduler.
 
#### Arguments
1. `scheduler` *(Scheduler)*: Scheduler to schedule observer messages on.
 
#### Returns
*(Observer)*: Observer whose messages are scheduled on the given scheduler.
 
#### Example
```js
var observer = Rx.Observer.create(
    function (x) {
        console.log('Next: ' + x)
    },
    function (err) {
        console.log('Error: ' + err);
    },
    function () {
        console.log('Completed');
    }
);
 
// Notify on timeout scheduler
var timeoutObserver = observer.notifyOn(Rx.Scheduler.timeout);
 
timeoutObserver.onNext(42);
// => Next: 42
```
 
### Location
 
- rx.js
 
* * *
 
### <a id="rxobserverprototypeoncompleted"></a>`Rx.Observer.prototype.onCompleted()`
<a href="#rxobserverprototypeoncompleted">#</a> [&#x24C8;](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js#L1899-L1904 "View in source") [&#x24C9;][1]
 
Notifies the observer of the end of the sequence.
 
#### Example
```js
var observer = Rx.Observer.create(
    function (x) {
        console.log('Next: ' + x)
    },
    function (err) {
        console.log('Error: ' + err);
    },
    function () {
        console.log('Completed');
    }
);
 
observer.onCompleted();
// => Completed
```
 
### Location
 
- rx.js
 
* * *
 
### <a id="rxobserverprototypeonerrorerror"></a>`Rx.Observer.prototype.onError(error)`
<a href="#rxobserverprototypeonerrorerror">#</a> [&#x24C8;](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js#L1889-L1894 "View in source") [&#x24C9;][1]
 
Notifies the observer that an exception has occurred.
 
#### Arguments
1. `error` *(Any)*: The error that has occurred.
 
#### Example
```js
var observer = Rx.Observer.create(
    function (x) {
        console.log('Next: ' + x)
    },
    function (err) {
        console.log('Error: ' + err);
    },
    function () {
        console.log('Completed');
    }
);
 
observer.onError(new Error('error!!'));
// => Error: Error: error!!
```
 
### Location
 
- rx.js
 
* * *
 
### <a id="rxobserverprototypeonnextvalue"></a>`Rx.Observer.prototype.onNext(value)`
<a href="#rxobserverprototypeonnextvalue">#</a> [&#x24C8;](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js#L1881-L1883 "View in source") [&#x24C9;][1]
 
Notifies the observer of a new element in the sequence.
 
#### Arguments
1. `value` *(Any)*: Next element in the sequence.
 
#### Example
```js
var observer = Rx.Observer.create(
    function (x) {
        console.log('Next: ' + x)
    },
    function (err) {
        console.log('Error: ' + err);
    },
    function () {
        console.log('Completed');
    }
);
 
observer.onNext(42);
// => Next: 42
```
 
### Location
 
- rx.js
 
* * *
 
### <a id="rxobserverprototypetonotifier"></a>`Rx.Observer.prototype.toNotifier()`
<a href="#rxobserverprototypetonotifier">#</a> [&#x24C8;](https://github.com/Reactive-Extensions/RxJS/blob/master/dist/rx.js#L1801-L1804 "View in source") [&#x24C9;][1]
 
Creates a notification callback from an observer.
 
#### Returns
*(Function)*: The function that forwards its input notification to the underlying observer.
 
#### Example
```js
var observer = Rx.Observer.create(
    function (x) {
        console.log('Next: ' + x)
    },
    function (err) {
        console.log('Error: ' + err);
    },
    function () {
        console.log('Completed');
    }
);
 
var notifier = observer.toNotifier();
 
// Invoke with onNext
notifier(Rx.Notification.createOnNext(42));
// => Next: 42
 
// Invoke with onCompleted
notifier(Rx.Notification.createOnCompleted());
// => Completed
```
 
### Location
 
- rx.js
 
* * *