Ionic VirtualScroll 无法读取 null 的属性“长度" [英] Ionic VirtualScroll Cannot read property 'length' of null

问题描述

I´m trying to use virtual scroll in Ionic 3 but it is not working.

I have this function on my provider:

getActiveAds(){
    return this.afDb.list<AngularFireList<any>>('/ads-active', ref => ref.orderByChild('adPlanPriority').startAt(1).endAt(3))
  }

On my list page, i have this:

constructor(public navCtrl: NavController, public navParams: NavParams, public loadingCtrl: LoadingController, public adProvider: AdProvider) {
    this.loading = this.loadingCtrl.create();
    this.loading.present();

    this.ads = this.adProvider.getActiveAds().valueChanges()
    this.ads.subscribe((cat)=> {
      this.loading.dismiss()
    })
  }

and my list.html this:

<ion-list no-lines [virtualScroll]="ads | async">
        <button ion-item *virtualItem="let ad" (click)="onAdSelect(ad)" class="aero-item ">
            <ion-thumbnail item-start>
                <img src="assets/images/noimage.jpg" />
            </ion-thumbnail>
            <h2>{{ ad.model}}</h2>

        </button>
    </ion-list>

With this code, I´m getting this error:

Cannot read property 'length' of null
TypeError: Cannot read property 'length' of null
    at VirtualScroll._stepDOMWrite (http://localhost:8100/build/vendor.js:92118:60)
    at http://localhost:8100/build/vendor.js:92078:23
    at dispatch (http://localhost:8100/build/vendor.js:20601:9)
    at DomController._flush (http://localhost:8100/build/vendor.js:20545:13)
    at rafCallback (http://localhost:8100/build/vendor.js:20534:22)

What am I doing wrong?

解决方案

I got the same issue and found a workaround, although it's not optimal.

You see from the documentation [virtualScroll]expect an Array. While your this.ads return an Observable. That's why you got Cannot read property 'length' of null.

Workaround: Subscribe to it first then display on view (without async pipe), also keep in mind to unsubsribe when done.

ads: any[] = [];
constructor(public adProvider: AdProvider) {
    this.adProvider.getActiveAds().valueChanges().subscribe(results => this.ads = results);  
  }

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