Write device platform specific code in Xamarin.Forms

Question

I have the following Xamarin.Forms.ContentPage class structure

public class MyPage : ContentPage
{
    public MyPage()
    {
        //do work to initialize MyPage 
    }

    public void LogIn(object sender, EventArgs eventArgs)
    {
        bool isAuthenticated = false;
        string accessToken = string.Empty;

        //do work to use authentication API to validate users

        if(isAuthenticated)
        {
            //I would to write device specific code to write to the access token to the device
            //Example of saving the access token to iOS device
            NSUserDefaults.StandardUserDefaults.SetString(accessToken, "AccessToken");

            //Example of saving the access token to Android device
            var prefs = Application.Context.GetSharedPreferences("MySharedPrefs", FileCreationMode.Private);
            var prefsEditor = prefs.Edit();

            prefEditor.PutString("AccessToken", accessToken);
            prefEditor.Commit();
        }
    }
}

I would like to write platform specific code in the MyPage LogIn method to save the access token based on which device OS they are using my application on.

How do I only run device specific code when the user uses my application on their device?

Answer 1

This is a scenario which is easily resolved with dependency injection.

Have a interface with the desired methods on your shared or PCL code, like:

public interface IUserPreferences  {     void SetString(string key, string value);     string GetString(string key); } 

Have a property on your App class of that interface:

public class App  {     public static IUserPreferences UserPreferences { get; private set; }      public static void Init(IUserPreferences userPreferencesImpl)      {         App.UserPreferences = userPreferencesImpl;     }      (...) } 

Create platform-specific implementations on your target projects:

iOS:

public class iOSUserPreferences : IUserPreferences  {     public void SetString(string key, string value)     {         NSUserDefaults.StandardUserDefaults.SetString(key, value);     }      public string GetString(string key)     {         (...)     } } 

Android:

public class AndroidUserPreferences : IUserPreferences {     public void SetString(string key, string value)     {         var prefs = Application.Context.GetSharedPreferences("MySharedPrefs", FileCreationMode.Private);         var prefsEditor = prefs.Edit();          prefEditor.PutString(key, value);         prefEditor.Commit();     }      public string GetString(string key)     {         (...)     } } 

Then on each platform-specific project create an implementation of IUserPreferences and set it using either App.Init(new iOSUserPrefernces()) and App.Init(new AndroidUserPrefernces()) methods.

Finally, you could change your code to:

public class MyPage : ContentPage {     public MyPage()     {         //do work to initialize MyPage      }      public void LogIn(object sender, EventArgs eventArgs)     {         bool isAuthenticated = false;         string accessToken = string.Empty;          //do work to use authentication API to validate users          if(isAuthenticated)         {             App.UserPreferences.SetString("AccessToken", accessToken);         }     } } 

Answer 2

Xamarin.Forms 2.3.4 introduced a new method for this:

if (Device.RuntimePlatform == Device.Android)
{
    // Android specific code
}
else if (Device.RuntimePlatform == Device.iOS)
{
    // iOS specific code
}
else if (Device.RuntimePlatform == Device.UWP)
{
    // UWP specific code
}

There are also other platforms to choose from, you can type in Device. in Visual Studio and it will show you the options.