Need Help on task complete handler, UI not updating after variable update inside task event

[RohanPhpDev]

I am using NativeScript version 6.0 and after successful upload of an image using nativescript-background-http i have created a task complete handler inside i call a function to update array which updates UI but UI is not updating and variable show new data in the console.
here is sample code:
var that=this;
context
.authorize()
.then(function() {
return context.present();
})
.then(function(selection) {
selection.forEach(function(selected) {
var file = selected.android;
var name = file.substr(file.lastIndexOf("/") + 1);
var fileExt=file.substr(file.lastIndexOf(".") + 1);

          var session = bghttp.session("image-upload");
          var request = {
                  url: *******************",
                  method: "POST",
                  headers: {
                      "Content-Type": "application/octet-stream",
                      "file_name":name,
                  },
                  description: "Uploading " + name,
                  androidNotificationTitle: 'Uploading Image',
                  androidDisplayNotificationProgress:true
              };
          
          var task = session.uploadFile(file, request); 
          task.on("responded", (e) => {
            let res=JSON.parse(e.data);
            console.log(res.status);
          });
         task.on("complete", (e) => {
            that.fetch_img();
          });
      });
  }).catch(function (e) {
      console.log(e);
});

fetch_img(){
this.gallery.push({"name":"image.jpg"});
console.log(this.gallery);
}

I have shown gallery variable in .html file using *ngFor it shows some default image but after task complete, a new object is pushed but it not update UI

[PabloPG]

Try add changeDetectorRef.detectChanges();