In the last “The Developers Conference Porto Alegre 2017” I presented a talk about common pitfals found while creating a mobile application using the Xamarin.Forms mobile platform.
If you want to check the slides they are in my slideshare, just Click Here.
But right to the point, the idea of this post is to summarize the 8 main points in my presentation that will save your time.
The best REST client for Xamarin mobile is FubarCoder.RestSharp
I’ve tested some other libraries, and there are some good ones, but the best one I’ve found is the FubarCoder.RestSharp.
The library is almost a full port of RestSharp to Xamarin Mobile, of course, if you are writing a .Net Core application use the full blown RestSharp, but for mobile, this port is great and keep the same API
this.client = new RestClient("http://address:port/");
var request = new RestRequest("sessions.json", Method.POST);
request.AddJsonBody(new { session = new { username = Username, password = Password } });
var response = await client.Execute(request);
A REST client will not solve all your problems if you use session
In most Rails applications, you use session to keep track of who is logged in, and if you are creating a front end for one existing application and using the already existing API this can be a problem since Rails use cookies to track the user session and RestClient does not store or send cookies, but that is easy enough to solve, just use this:
var cookieContainer = new System.Net.CookieContainer(); client.CookieContainer = cookieContainer;
Adding a cookie container to the RestClient will teach it to store and send back the cookies, and that will solve all problems with authentication.
Always use async/await
C# has a perfect feature to help handle asynchronous calls, the async and await keywords, the trick here is that await can only be used inside async methods, and to solve that, use this snippet:
Task.Factory.StartNew(async () =>
{
var articles = await clientApi.ListArticles();
})
With the task factory you’ll be able to create async blocks to handle your API calls.
You cannot update the UI from another thread
The problem with async blocks is that they do not run in the main UI thread, and you can only update the UI components from that thread.
To solve that you need to use Device.BeginInvokeOnMainThread, we can update the previous snippet with this new method to update the UI with the article list:
Task.Factory.StartNew(async () =>
{
var articles = await clientApi.ListArticles();
Device.BeginInvokeOnMainThread(() =>
{
this.ArticlesList.ItemsSource = articles;
});
});
Rails tries to prevent simple form submission to prevent form submission attacks and that might be a problem
Rails protect_from_forgery will cause you some trouble, and you have two ways to fix that.
- simply remove protect_from_forgery, of course that might open your application for attacks. If you are creating one API it is not a problem, but be careful with this solution.
- add “skip_before_action :verify_authenticity_token” to the APIs you’ll use from your mobile application
- before sending any “form” do a request and get the authenticity_token from the returned page and send it back on every request (this is the safest)
Show details and navigation
Each mobile platform has it’s own way to display the in app navigation and go back to the previous pages, to help with that, Xamarin.Forms has the NavigationPage class
MainPage = new NavigationPage(new tdc2017poa_xam.MainPage(this.clientApi)); Navigation.PushAsync(new ArticlePage(this.clientApi, (e.SelectedItem as Article).Id));
Saving user preferences in a platform independent way is probably a good idea
In every application you need to save user preferences and similar things, and of course that is platform dependent, but Xamarin.Forms had a Properties helper that will map the calls to each supported platform, the code is really simple, just use these classes:
- Application.Current.Properties
- Application.Current.SavePropertiesAsync
- Application.Current.Properties.ContainsKey
Display notifications for users
Again each platform has it’s own requirements to send async notifications to the users, if you are using Azure to build the backend for your application, just use the Microsoft.Azure.Mobile.Client class/module/library and it will save you loads of times.
Of course you’ll still need to register your application in each platform, but you’ll be able to use only one API for all…
Last but not least
I hope this presentation summary will help you save some time, if you want to check the actual source code for the presentation samples, just go to my github page.