CombineLatest is triggered although sources did not change -- [Question Asked]

Query asked by user

I am using Angular as the frontend

grid$: Observable < any[][] > | null = null
ngOnInit() {
    this.grid$ = combineLatest([this.route.queryParams, source1, source2])
        .pipe(
            map(data => {
                 window.scroll(0,0) // go to top when route params change
                //do something with the sources and route params
                return grid;
            })
        )
}

I am using grid$ with async pipe

<ng-container *ngIf="grid$ | async as grid>
<div *ngFor="let column of columns>
    <div *ngFor="let analyse of grid[column]"> ... </div>
</div>
</ng-container>

which is working fine.

Now I want to change the grid after specific events occured

changeGrid() {
    // I also tried to just use map without assignment and return a new grid but then nothing happens
    this.grid$ = this.grid$.pipe(
        map(grid => {
            // do something with grid
            return grid;
        })
    }
}

which is also working. The only side effect I have is that combineLatest is triggered as well although sources did not change so that you scroll automatically to the top again which is not intended. Could someone explain me why? I tried it with distinctUntilChanged in combineLatest but it is not triggered anymore when using changeGrid(). I used distinctUntilChanged in the following way

distinctUntilChanged((prev,curr) => 
{ 
    return prev[0].get("id") != curr[0].get("id")
}) 

Answer we found from sources

First, by writing this.grid$ = ... you lose the memory reference to its previous value. This means you can’t stop the previous observable, which is in the combineLatest. This might be the cause of your issue.

Second, Since you use the queryParams observable, it means that on every query param change, your observable gets triggered. You didn’t give enough code to show if it’s the case, but this also might be the other cause of your issue.

Finally, distinctUntilChanged compares memory references : when being used with combineLatest, unless you provide a function to it, the operaor is useless (a new array is being created each time the combineLatest emits).

Answered By – temp_user

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