How to convert Xamarin iOS project to Xamarin Forms - SimpleBackgroundTransfer

Question

It has been recommended to me that I should try to implement this Xamarin iOS app:

https://github.com/xamarin/ios-samples/tree/main/SimpleBackgroundTransfer

in Xamarin Forms for doing the iOS part of a background download. I have done lots of Xamarin Forms - just not much in Xamarin iOS and request some guidance as how to proceed.

@Wenyan Zhang (Shanghai Wicresoft Co,.Ltd.) What would your recommendation be?

Thanks!
Will

Answer 1

Hello,

Welcome to our Microsoft Q&A platform!

I have converted this Xamarin iOS demo to Xamarin Forms, it relys on DependencyService, and I use Action to callback the image path, you could refer to the following code:

XML(MainPage)

<?xml version="1.0" encoding="utf-8" ?>  
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"  
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"  
             xmlns:ios="clr-namespace:Xamarin.Forms.PlatformConfiguration.iOSSpecific;assembly=Xamarin.Forms.Core"  
             ios:Page.UseSafeArea="true"  
             x:Class="TestSimpleBackgroundTransferDemo.MainPage">  
    <StackLayout BackgroundColor="Red">  
        <Button Text="start download " Clicked="Button_Clicked"></Button>  
        <Image x:Name="MyImage"></Image>  
    </StackLayout>  
</ContentPage>  

CodeBehind(MainPage.cs)

private void Button_Clicked(object sender, EventArgs e)  
        {  
            DependencyService.Get<IDownloadService>().StartDownload();  
        }  
        protected override void OnAppearing()  
        {  
            base.OnAppearing();  
            DependencyService.Get<IDownloadService>().SetSession((p) =>  
            {  
                Device.BeginInvokeOnMainThread(() =>  
                {  
                    MyImage.Source = p;  
                });  
 });  
        }  

Interface

public interface IDownloadService  
    {  
        void StartDownload();  
        void SetSession(Action<string>pacthCallBack);  
    }  

Add method to Appdelegate

        public Action BackgroundSessionCompletionHandler { get; set; }  
        public override void HandleEventsForBackgroundUrl(UIApplication application, string sessionIdentifier, Action completionHandler)  
        {  
            Console.WriteLine("HandleEventsForBackgroundUrl");  
            BackgroundSessionCompletionHandler = completionHandler;  
        }  

DownloadService.cs in iOS Project

using Foundation;  
using System;  
using System.Collections.Generic;  
using System.Linq;  
using System.Text;  
using TestSimpleBackgroundTransferDemo.iOS;  
using UIKit;  
using Xamarin.Forms;  
  
  
[assembly: Dependency(typeof(DownloadService))]  

namespace TestSimpleBackgroundTransferDemo.iOS  
{  
    public class DownloadService : IDownloadService  
    {  
        const string Identifier = "TestSimpleBackgroundTransferDemo.iOS";  
        const string DownloadUrlString = "https://upload.wikimedia.org/wikipedia/commons/9/97/The_Earth_seen_from_Apollo_17.jpg";  
        public NSUrlSessionDownloadTask downloadTask;  
        public NSUrlSession session;  
          
    public void StartDownload()  
    {  
        if (downloadTask != null)  
            return;  
        using (var url = NSUrl.FromString(DownloadUrlString))  
        using (var request = NSUrlRequest.FromUrl(url))  
        {  
            downloadTask = session.CreateDownloadTask(request);  
            downloadTask.Resume();  
        }  
    }  
    public void SetSession(Action<string>pathCallBack)  
    {  
        if (session == null)  
            PathCallBackAction = pathCallBack;  
            session = InitBackgroundSession();  
    }  

    public Action<string> PathCallBackAction;  


    public NSUrlSession InitBackgroundSession()  
    {  
        Console.WriteLine("InitBackgroundSession");  
        //var config = NSUrlSessionConfiguration.CreateBackgroundSessionConfiguration(Identifier);  
        //config.SharedContainerIdentifier = "XXXXX";  

        using (var configuration = NSUrlSessionConfiguration.CreateBackgroundSessionConfiguration(Identifier))  
        {  
            return NSUrlSession.FromConfiguration(configuration, new UrlSessionDelegate(this), null);  
        }  
    }  


    public class UrlSessionDelegate : NSObject, INSUrlSessionDownloadDelegate  
    {  
        public DownloadService downloadService;  

        public UrlSessionDelegate(DownloadService downloadService)  
        {  
            this.downloadService = downloadService;  
        }  

        [Export("URLSession:downloadTask:didWriteData:totalBytesWritten:totalBytesExpectedToWrite:")]  
        public void DidWriteData(NSUrlSession session, NSUrlSessionDownloadTask downloadTask, long bytesWritten, long totalBytesWritten, long totalBytesExpectedToWrite)  
        {  
            Console.WriteLine("Set Progress");  
            if (downloadTask == downloadService.downloadTask)  
            {  
                float progress = totalBytesWritten / (float)totalBytesExpectedToWrite;  
                Console.WriteLine(string.Format("DownloadTask: {0}  progress: {1}", downloadTask, progress));  
            }  
        }  


        public void DidFinishDownloading(NSUrlSession session, NSUrlSessionDownloadTask downloadTask, NSUrl location)  
        {  
            Console.WriteLine("Finished");  
            Console.WriteLine("File downloaded in : {0}", location);  
            NSFileManager fileManager = NSFileManager.DefaultManager;  

            var URLs = fileManager.GetUrls(NSSearchPathDirectory.DocumentDirectory, NSSearchPathDomain.User);  
            NSUrl documentsDictionry = URLs[0];  
            NSUrl originalURL = downloadTask.OriginalRequest.Url;  
            NSUrl destinationURL = documentsDictionry.Append("image1.png", false);  
            NSError removeCopy;  
            NSError errorCopy;  

            fileManager.Remove(destinationURL, out removeCopy);  
            bool success = fileManager.Copy(location, destinationURL, out errorCopy);  


            if (success)  
            {  
                // callback the path  
                //UIImage image = UIImage.FromFile(destinationURL.Path);  
                downloadService?.PathCallBackAction?.Invoke(destinationURL.Path);  
            }  
            else  
            {  
                Console.WriteLine("Error during the copy: {0}", errorCopy.LocalizedDescription);  
            }  
        }  


        [Export("URLSession:task:didCompleteWithError:")]  
        public void DidCompleteWithError(NSUrlSession session, NSUrlSessionTask task, NSError error)  
        {  
            Console.WriteLine("DidComplete");  
            if (error == null)  
                Console.WriteLine("Task: {0} completed successfully", task);  
            else  
                Console.WriteLine("Task: {0} completed with error: {1}", task, error.LocalizedDescription);  

            float progress = task.BytesReceived / (float)task.BytesExpectedToReceive;  
            downloadService.downloadTask = null;  
        }  

        [Export("URLSession:downloadTask:didResumeAtOffset:expectedTotalBytes:")]  
        public void DidResume(NSUrlSession session, NSUrlSessionDownloadTask downloadTask, long resumeFileOffset, long expectedTotalBytes)  
        {  
            Console.WriteLine("DidResume");  
        }  

        [Export("URLSessionDidFinishEventsForBackgroundURLSession:")]  
        public void DidFinishEventsForBackgroundSession(NSUrlSession session)  
        {  
            using (AppDelegate appDelegate = UIApplication.SharedApplication.Delegate as AppDelegate)  
            {  
                var handler = appDelegate.BackgroundSessionCompletionHandler;  
                if (handler != null)  
                {  
                    appDelegate.BackgroundSessionCompletionHandler = null;  
                    handler();  
                }  
            }  


            Console.WriteLine("All tasks are finished");  
        }  
    }  
}  
}  

Best Regards,
Wenyan Zhang

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