How do I update a value in an AngularJS (with TypeScript) controller when a value on an injected service updates

Question

At the moment I am trying to learn Typescript and AngularJS coming from a background of Actionscript. I have been playing around with some code experiments and van generally achieve what I want to but not always in the way that I would like!

At the moment I have this code:

export class CountingService
{
    countScope = { count : 0 };

    public increment()
    {
        this.countScope.count++;
    }
}

export class PageController
{
    constructor( private service : CountingService )
    {
        this.countScope = service.countScope;
    }

    public localCount : number = 0;

    countScope;

    public increment()
    {
        this.service.increment();

        this.localCount++;
    }

}

and I have a few different views that have this code in:

<div ng-controller="PageController as page">

    <Button ng-click="page.increment()">Count {{page.localCount}}/{{page.countScope.count}}</Button>

</div>

When I switch between the views the localCount always resets to zero but the count in the countScope from the service does not reset and is incremented by each different controller.

This works but it's a bit messy. I have to have the untyped localScope floating around and the views need to know about the internal structure of the countScope object and bind to page.countScope.count rather than something like page.globalCount.

In Actionscript I would have a read only getting on the controller. The service would dispatch an event each time the value changed and the controller would listen for this and then update it's own property which would then update the view.

My question is what is the best practice way to achieve what I am doing here that does not require the view to have knowledge of the internals of an untyped object. I am pretty sure that public getters do not exist but can I dispatch and listen for an event?

Many Thanks

Answer 1

Finally answer three. I didn't expect this to work but it does and IMHO this is by far the nicest solution.

It enforces encapsulation with getters, it doesn't need $scope injected and there are no concerns about memory leaks.

I welcome any criticisms that point out issues that I am not aware of.

The only thing that I AM aware of is that you need to enable ECMAScript 5 which means that IE 7 and 8 are not supported. That's fine for me.

export class CountingService
{
    private _serviceCount : number = 100;

    public increment()
    {
        this._serviceCount++;
    }

    get serviceCount() : number
    {
        return this._serviceCount;
    }
}

export class PageController
{
    constructor( private countingService : CountingService )
    {}

    private _localCount : number = 0;

    get localCount() : number
    {
        return this._localCount;
    }

    get serviceCount() : number
    {
        return this.countingService.serviceCount;
    }

    public increment()
    {
        this.countingService.increment();

        this._localCount++;
    }
}

Answer 2

This seems like the perfect fit for a reactive library and is exactly what we do. I highly recommend you check out RxJs which has some awesome features. https://github.com/Reactive-Extensions/RxJS/tree/master/doc

https://www.youtube.com/watch?v=XRYN2xt11Ek

You code using Rx

export class CountingService {

    //Create a stream of changes.  When caller subscribe always give them the most recent value. 
    private rawCountStream = new Rx.Subject<number>();
    public countStream : Rx.Observable<number>;
    private count = 0;

    constructor() {
        this.countStream = this.rawCountStream.replay(1);
    }

    public increment() {
        this.count++;
        //Pump the value to callers. 
        this.rawCountStream.onNext(this.count);
    }
}

export class PageController {

    constructor(private service: CountingService) {
        //Listen to the stream of changes. 
        service.countStream.subscribe(value => {
            //do something
            //IMPORTANT If you want angular to update the ui you will need to call $apply on a scope. 
            //RXJS has a scheduler for this so you look at that here. 
            //https://github.com/Reactive-Extensions/rx.angular.js
            }
        );
    }

    public localCount: number = 0;

    public increment() {
        this.service.increment();
        this.localCount++;
    }
}

Answer 3

This is answer number 1 and I think is the approach that John Kurlak was talking about:

export class CountingService
{
    serviceCount : number = 100;

    public increment()
    {
        this.serviceCount++;
    }
}

export class PageController
{
    constructor( $scope, private service : CountingService )
    {
        this.serviceCount = service.serviceCount;

        $scope.$watch( () => this.service.serviceCount, ( newValue : number, oldValue : number ) => this.updateLocal( newValue, oldValue ) );
    }

    localCount : number = 0;

    serviceCount : number;

    public increment()
    {
        this.service.increment();

        this.localCount++;
    }

    private updateLocal( newValue : number, oldValue : number )
    {
        this.serviceCount = newValue;
    }

}

This works and I think is the sort of solution that I was after. There are things that I don't like about it though.

I don't like having to inject the $scope into my service. It seems to be an extra dependency on something that I shouldn't need. I also really don't like the public members that can be updated by anything and break encapsulation. I want to fix this with getters but I can't figure out how to update to ECMAScript 5 (question here: Targeting ES5 with TypeScript in IntelliJ IDEA 14).

John - is this what you were proposing? If anyone comments with Pros and Cons to this answer I'd be very grateful. I will mark this as the answer as I think this is what I was after to start with.

Answer 4

This is answer number 2 and I think this is what PSL was getting at (or something similar)

interface CountingCallBack
{
    parent : any;
    callback : ( value : number ) => void;
}

export class CountingService
{
    private _observerCallBacks : Array<CountingCallBack> = [];

    private _serviceCount : number = 100;

    public increment()
    {
        this._serviceCount++;

        this.notifyObservers();
    }

    public registerObserverCallback( callbackParent : any, callbackFunction : ( value : number ) => void )
    {
        var callback : CountingCallBack = { parent : callbackParent, callback : callbackFunction };

        this._observerCallBacks.push( callback );
        this.updateObserver( callback );
    }

    private notifyObservers()
    {
        angular.forEach( this._observerCallBacks, this.updateObserver, this );
    }

    private updateObserver( callback : CountingCallBack )
    {
        callback.callback.apply( callback.parent, [ this._serviceCount ] );
    }

}

export class PageController
{
    constructor( private countingService : CountingService )
    {
        countingService.registerObserverCallback( this, this.updateLocal );
    }

    localCount : number = 0;

    serviceCount : number;

    public increment()
    {
        this.countingService.increment();

        this.localCount++;
    }

    private updateLocal( newValue : number )
    {
        this.serviceCount = newValue;
    }
}

I'm quite pleased that I managed to implement this as I now understand how the function interface syntax works (and I quite like it) and I managed to fix the issue with the 'this' scope issue in the updateLocal function on the controller.

This solution doesn't require and $scope to be injected which is nice but I find the callback implementation quite messy. I don't like having to pass the controller and the function on the controller to the service to add a callback. I suppose I could create an interface, ICountListener or something, and just pass that to the service and then call updateCount on the controller. That would be a bit neater but still not that good. Is there a neater way around setting up the callback than this?

The biggest problem with this code (and a reason that it should not be used) is that it creates a memory leak. If you keep switching views the controllers never get cleaned up so you have many controllers left in memory all responding to the updated count. It would be relatively easy to clean up the callback and de-register a callback but that would presumably involve injecting a $scope and then listening for a destroy event (I don't know how to do this but I think this was under discussion in the comments above).

Answer 5

Short answer: Services are singletons i.e. the constructor is only called once. If you want it reset you can do that from the constructor of your controller.

this.service.countScope.count = 0

Notes

I see a lot of things I don't like about that code sample. But I am sure its just a sample so don't take it as a personal offence ;)

• don't have a service called service. Call it countService.
• don't have a member countScope on the service. Just use count. And don't copy countScope to the current controller. Just use this.countService.count

Update

The root of this question is how do I update a displayed value in html when a value in a service has changed

Since you have the controller as page and page.service is the service simply {{page.service.countScope.count}} would display teh count.