The subscribe method registers a function or observer with an Observable<T>, allowing code to react when values arrive instead of repeatedly checking whether asynchronous work has finished. An observable can produce a series of values, unlike a Promise, which represents a single result. section 10.5.1
Choose the subscription form according to the sequence events that need handling:
- Pass a function when only emitted values matter; the function receives each new value. section 10.5.1
- Pass an observer object when the subscription must handle next values, errors, and sequence completion. section 10.5.1
- Subscribe during ngAfterContentInit to a QueryList changes observable when projected content can change. section 15.4.2
- Account for asynchronous notifications: Angular change detection may need to run afterward through ChangeDetectorRef.detectChanges(). section 15.4.2
Displaying values from a ticker
A Ticker exposes an Observable<number> created with interval(500). A component subscribes with `this.ticker.value.subscribe(newValue => this.tickerValue = newValue);`, and its template displays `tickerValue`. Each emitted number updates the component without polling. section 10.5.1
Testing an output property
An EventEmitter output is an Observable object, so a unit test subscribes to the output before triggering mouseenter and mouseleave events. The received event is then used to verify the expected result. section 29.4.5
Table 10.5. The Observable<T> subscribe argument properties
|
Name
|
Description
|
|
next
|
This function is invoked when a new value is produced.
|
|
error
|
This function is invoked when an error occurs.
|
|
complete
|
This function is invoked when the sequence of values ends.
|
Use the callback-property table to distinguish value, error, and completion notifications accepted by subscribe. section 10.5.1
