Function inside ngIf is called multiple times, after a single click event- Angular2_Typescript

Question

Hello this is the first time I qm putting question on stackoverflow. I am developing an application in angular2: v2.1.0 (typescript- v2.0.10). I am using "ng2-file-upload" for file upload. The HTML code is as follows:

<div class="container-fluid">
    <h5 class="titleCenter"> File Upload </h5>
    <div class="drop-zone" ng2FileDrop
         [ngClass]="{'nv-file-over': hasBaseDropZoneOver, 'pointerBan': testDisablDropZone}"
         (fileOver)="fileOverBase($event)"
         [uploader]="uploader">
        <span *ngIf="isUploaderEmpty(uploader.queue)">Drop Down box for the CSV file only...!!!</span>
        <span *ngFor="let item of uploader.queue" >
            <p class="row" >
                <i class="fileIcon"></i>
                <strong>{{ item?.file?.name }}</strong>
                <a class="fileUploadRemoveButton fa fa-times-circle" (click)="item.remove()"></a>
            </p>
        </span>
    </div>
    <div  [ngClass]="{'activeCheckButton': testDisablDropZone}" class="CheckboxZone" (click)="disableDropZone($event)" style="margin-top: 10px">
        <span>Click here to disable the dropdown box.</span>
    </div>
</div>
<button type="button" (click)="test($event)">click</button>

Here when I clicked on the div with the class 'CheckboxZone' it calls the function 'disableDropZone($event)' but after that it calls the function 'idUploadEmpty()' too. The same case with the button Click too. The code of the component is as follows:

const URL = 'https://evening-anchorage-3159.herokuapp.com/api/';
@Component({
    selector: 'fileupload',
    templateUrl: './fileupload.component.html',
    styleUrls: ['./fileupload.component.scss']
})
export default class FileuploadComponent {
public uploader:FileUploader = new FileUploader({url: URL, autoUpload:true, allowedMimeType:['text/csv'] });
public hasBaseDropZoneOver:boolean = false;
public hasAnotherDropZoneOver:boolean = false;
private fileType =['json'];
private flagFile = false;
private testDisablDropZone = false;
private fileNotAccepted = false;
public fileOverBase(e:any):void {
    this.hasBaseDropZoneOver = e;
    console.log('EVENT fileOverBase : ', e);
}

public fileOverAnother(e:any):void {
    this.hasAnotherDropZoneOver = e;
    console.log('fileOverAnother : ', e);
}

isUploaderEmpty (uploaderQueue): boolean {
    console.log('Queue pqssed from View : ',this.uploader.queue);
    let qLength = uploaderQueue.length;
    if(uploaderQueue.length==0){
        this.fileNotAccepted = true;
        return true;}

    if (qLength > 1){
        uploaderQueue.pop();
        this.flagFile = true;
        let timer = Observable.timer(3000,10000);
        timer.subscribe(t=>{
            this.flagFile = false
        });
    }
    return false;
}

disableDropZone () {
    console.log('disableDropZone clicked...!!!');
    this.testDisablDropZone =! this.testDisablDropZone;
}

test(event)  {
    console.log('OK')
}
}

Hard to understand why it is calling the function 'isUploaderEmpty()' all the time when an event is triggered.

Thank you.

Answer 1

It's actually quite easy.

When ever you define an ngIf , you put an expression inside it right ?

That means every time there is an update inside that component , Angular needs to make sure that the expression inside the ngIf is evaluated. ( This is what you expect from Angular right ? other wise why would you use ngIf ?)

So every time there is an update in the model , or rather , every time there is something that triggers the changeDetection, Angular evaluates that expression , which in your case is a function ( isUploaderEmpty ).

Generally , events are one of the things that fire a changeDetection( it's more complicated than this ).

So that's why :D.

Answer 2

In angular have change detection Strategy, it will helpful to avoiding multiple calls in ngIf conditional functions.

changeDetection: ChangeDetectionStrategy.OnPush