Keep updating progressbar when view is destroyed

Ionic Framework ionic-v3
1

In my Ionic 2 app, I use this function to download a video file and track its download progress:

download()
{
    this.progressbar=true;
    this.downloadbutton=false;
    this.fileTransfer.download('https://...videoURL....mp4', this.file.dataDirectory + 'path/to/downloads/test.mp4').then((entry) => {
        console.log('download complete: ' + entry.toURL());
        this.progressbar=false;
        this.startbutton=true;
    }, (error) => {
        console.log('error');
        console.log(error);
    });

    this.fileTransfer.onProgress(progressEvent => {
        if (progressEvent.lengthComputable) {
            console.log("### Download percentage ###: "+Math.round((progressEvent.loaded / progressEvent.total)*100));
            this.setpercentage((progressEvent.loaded / progressEvent.total) * 100);
        } else {
        }
    });

}

Using setpercentage, I update a progressbar on the page

setpercentage(perc) {
        this.loadProgress = Math.round(perc);
        this.ref.detectChanges();
    }

This all works fine. When the user navigates away (back), the view is destroyed. The video keeps downloading (as it should), but when I navigate back to the page (while the video is still downloading), the progressbar isn’t updated: the value stays at 0%.

How could I keep the progressbar updating, even when the view is destroyed?

Basically, I want to have this page to have the same functionality as a tab. When users navigate between different tabs, everything of the previous tab is remembered.

Any help is much appreciated!