fix(VirtualScroll): initialize trackByFn reference

[pluscubed]

Short description of what this resolves:

The virtualScroll input setter can be called first, which calls initializes _differ with the default virtualTrackBy function. When the virtualTrackBy input setter is called later, _differ is no longer blank, and so the input virtualTrackBy is not used.

Changes proposed in this pull request:

Do not check whether _differ is undefined in the virtualTrackBy setter.

Ionic Version: 3.x
Fixes: Issue #7531, PR #11492

[masimplo]

Removing isBlank check from the current codebase will have the desired effect. If interested, please updated your pull request to only alter this line.

[manucorporat]

what was the reason to have isBlank(this._differ) in first instance?

[masimplo]

This was from the original code that initialized this._differ only when setting VirtualScroll property as a guard to not reinitialize the differ again. Now we need to initialize it twice. Once when setting VirtualScroll and once when setting TrackBy. If trackBy is not provided, then differ will be initialized with the default comparer.
When DRYing the code in my pull request #11492 I accidently kept the original code that only accounted for differ being set once.

[naveedahmed1]

@manucorporat @masimplo can this issue (https://github.com/driftyco/ionic/issues/11788) be a side effect of this change?

[masimplo]

@naveedahmed1 Can't imagine how this can be related. Are you using virtualTrackBy and removing it solves your issue?

[naveedahmed1]

If I am not wrong, probably this was the only change made to virtual scroll in this release, correct? The issue persists either I add virtualTrackBy or remove it.

[masimplo]

Pretty sure it is not related with virtualTrackBy change then.

[ibnclaudius]

For some reason after the fix for this issue (commit 892e14f) my virtual list no longer updates when changing segments. Below is the relevant part of my code for you understand.

this.historyProvider.getHistory()
      .subscribe((history: Copy[]): Copy[] => this.history = history);

<ion-segment [(ngModel)]="segment" (ionChange)="mergeNewHistory()">...</ion-segment>

<ion-list [virtualScroll]="history" [virtualTrackBy]="virtualTrack">
  <button *virtualItem="let copy" ion-item>...</button>
</ion-list>

virtualTrack(index: number, copy: Copy): Date {
  return copy.date;
}

If I go back to version 3.2.1 it works perfectly! I'm pretty sure it's related to this because it was the only change that occurred in Virtual Scroll.

[masimplo]

@ibnclaudius VirtualTrackBy was not working up to now, so it is possible your function was not called at all previously. Please check if the function is invoked when you run it against 3.2.1 at all.

[ibnclaudius]

@masimplo You're right! The function was not being called! The only issue now is that virtualscroll is not rebuilding the list when I set it empty.

[masimplo]

Was pretty sure that fixing VirtualTrackBy would give a lot of people head scratching moments as people had written all sort of things in trackByFn that had no effect until now and it might not do what they expected.