- Episode 38 with Immo Landwerth
- .NET Code Contracts
- C# Source Generators
- Kotlin/Native as an Apple Framework
- Swift on Android
I’ve updated my previous post “The many flavors of HttpClient” with current information on everything you need to know about HttpClient on Xamarin.
I co-authored a blog post with my co-worker Robin Wiegand on bringing Xamarin and Cordova together. Enjoy!
When using Xamarin, you can use the standard .NET
HttpClient. By default, HttpClient is Mono’s complete reimplementation of the entire HTTP stack. It is sufficient for many use cases but there are other alternatives out there that can be defined by selecting an alternative
HttpClientHandler. For my Evolve talk, I put together an overview of the different HttpClientHandlers you can use:
CFNetworkHandler (iOS 6+) and the new
NSUrlSessionHandler (iOS 7+, starting with Xamarin.iOS 9.8) are the handlers that utilize Apple’s native APIs instead of the Mono implementation. You can define which handler the
HttpClient default constructor will use either in the IDE or by providing an argument to
For Android, there is now
AndroidClientHandler (starting with Xamarin.Android 6.1). There is no IDE option for defining the default handler yet but you can define it using the
@(AndroidEnvironment) build action on a text file in your Android project to define an environment variable
XA_HTTP_CLIENT_HANDLER_TYPE to the value
Alternatively, you can use ModernHttpClient by handing a
NativeMessageHandler to the HttpClient constructor which will also use native implementations for making HTTP calls.
The default Mono implementation does not support the newest (and most secure) TLS standard 1.2 while the native handlers do. To use TLS 1.2 with the Mono implementation, Xamarin.iOS 9.8 introduced the option to swap the TLS implementation with P/Invoke calls into the Apple’s TLS implementation. This can be selected either in the IDE or by adding the
--tls-provider=appletls option to
For Android, there is no such option but it is expected that
BoringSSL support will be added soon.
Here’s the summary slide I showed in my talk:
Xamarin have actually gone through the trouble of reimplementing the TLS code to support TLS 1.1 and 1.2. However, it is expected that it will be abandoned because of security considerations in favor of the native platform implementations, just as Microsoft has done for Windows.
Here’s an update on the current state of
AndroidClientHandleryour Android project’s properties page, just as you already could for iOS.
BoringSSLinto their codebase. For Android, this option is also selectable in your project’s properties page.
BoringSSLalso brings TLS 1.2 to the Unix/Linux implementations of Mono.
ServicePointManager. Thomas Bandt wrote an excellent blog post on how to get certificate pinning working with ModernHttpClient and even
And here’s the updated matrix:
The Xamarin platform allows developing iOS and Android applications entirely in C#. Sometimes, however, some legacy code may be too large or complex to make porting it to C# for your mobile app worthwhile. Most examples found online show you how to use existing Objective-C libraries in Xamarin.iOS and existing Java libraries in Xamarin.Android. However, it is entirely possible to call C code from a Xamarin app. Microsoft’s recent support for C/C++ on iOS and Android in Visual Studio can make this a simple task.
You will need the sources for the code you want to call to compile it for iOS and Android. The code cannot access any libraries which you only have binaries for that have been compiled for other platforms.
To get started, create a new C++ cross-platform project using Visual Studio 2015.
You will need to have the corresponding Visual Studio package installed to have this option available.
This will create three projects: one for Android, one for iOS, and a shared project.
It is important to understand the shared project concept for this. The code in the shared project does not result in a DLL or library being generated. A shared project can hold any type of file. Projects referencing the shared project can access code from any file in the shared project. The code will then be compiled in the context of the referencing project. If code is not referenced, it is not compiled. For this reason, it is necessary to have calls into all code needed contained in the platform-specific projects. This means that the platform-specific C/C++ projects need to contain code, too. This code can be identical in the Android and iOS projects and reside in the same files. Unlike in .NET projects, file linking is not needed for C++ projects; source code files do not have to reside inside a project’s folder or subfolder.
In the example, I’ve created a simple function
clib_add_internal() that add two integers and resides in the shared project.
This code is called from a function in the platform-specific projects.
For the example code, this doesn’t make any sense. For a real project, you will put any call that needs to be directly invoked from the .NET code (your interface) into the platform-specific projects while underlying code can reside in the shared project.
The calls from .NET will be using P/Invoke since C++/CLI is not available on iOS or Android. So the basic P/Invoke rules apply for your interface’s code: Either the code is plain C or, if you are using C++, the calls are either plain functions or static methods and contain only POC parameters and return values.
For the iOS project, the output format needs to be changed to a static library (.a file) since iOS does not allow dynamically loading code. For Android, a dynamic library (.so) should be built.
While the Android library can be compiled within Visual Studio, compiling the iOS library needs a connection to a build agent on the Mac. This is not the regular Xamarin.iOS installation on a Mac but a Microsoft tool that is installed on OS X using npm. Visual Studio will communicate with this tool through TCP/IP.
To call the native code, create a Xamarin project for iOS and Android each. Include the native libraries in the respective Xamarin projects.
On iOS, if you’re targeting multiple processor architectures (either supporting pre-iPhone 5s or supporting the simulator), you can build the iOS project for multiple platforms using the dropdown in the toolbar. Then, combine the libraries into one using
lipo according the Xamarin docs. On Android, you’ll need to modify the
.csproj according to the Xamarin documentation to specify the
.so files for each platform.
The calls into the native code are regular P/Invoke calls (although DLLs don’t play a role here). On Android, you specify the name of the shared library.
On iOS, use the special Mono name
__Internal to call into the static library.
And that’s it! You can find the example code at https://github.com/lothrop/XamarinNative. In the example, I put the C# marshaling code into another shared project that is referenced by the Xamarin.iOS and Xamarin.Android projects.
When using MvvmCross in a cross-platform mobile project you have the choice of four different approaches for creating the iOS user interface. Xamarin.Forms is one of those approaches that can be a good choice for many use cases (even if there is not graphical designer at this point). However, if your view has very specific visual requirements, it may be necessary to create a native view. There are three different ways you can do this:
MvvmCross supports all three native approaches.
This approach obviously produces the most code. It may be challenging for people new to the platform but it saves you the hassle of battling with the UI designers.
If you’re already on Xamarin 4 and prefer a visual editor, this is likely the best choice for you. Create a new XIB View Controller file using Xamarin Studio or Visual Studio. The ViewController code will also be created for you. All you need to do for MvvmCross is to change the base class from
MvxViewController. In the example, three UI elements were created using the designer and were given the (not so creative) names
UiButton. These elements are then referenced in the data binding code that is common to all three approaches.
The big advantage of storyboards, the ability to design your screenflow in the designer, does not fit well with MvvmCross’s cross-platform navigation approach. If you define a transition from view A to view B in the designer it will only apply to the iOS platform. It is typically a better idea to move this navigation logic down into the cross-platform code inside your ViewModels. That is why it’s best to use a one-ViewController-per-storyboard approach when using storyboards in MvvmCross.
In the editor, you need to set the storyboard ID to the name of your ViewController. As with the XIB approach, change the base class from
MvxViewController. Create a constructor taking an
IntPtr and passing that on to the base class’s constructor. The last step is to add the
[MvxFromStoryboard] attribute to the class.
The good thing is: You don’t have to make this decision at project start. MvvmCross supports implementing your view with the technology that is best for each view. You only need to ensure you don’t have multiple views for the same ViewModel and the same platform.
I’ve updated the example in the MvvmCross Starter Pack Nuget (starting with 4.0.0-beta8) to use a XIB file instead of the previous hand-coded approach with absolute coordinates (from the auto layout days). That should make it easier for people to get started.
If you’re using iOS auto layout (you should) and you’ve ever tried to add a scroll view in Interface Builder or the Xamarin iOS Designer you might have noticed that it is not an easy task. If you’re interested, here are some instructions. This is especially true if you only realize that you need a scroll view after designing your view (after you’ve realized that the keyboard covers up parts of the view).
Here’s code to inject a
UIScrollView at runtime to solve all your worries. Add this code to your base
UIViewController for all controllers that should be vertically scrollable.
In Interface Builder or Xamarin iOS Designer you’ll need to make sure you’ve not only defined constraints for the vertical position of your elements but also a constraint to define the distance from the bottom of the bottom element to the bottom of the your view controller’s view. That way, you’ll have defined the vertical size of the scroll view’s content view.
There’s a deadline coming up for all iOS developers. Starting February 1, 2015, all apps and app updates submitted to the iOS app store must be built against the iOS 8 SDK and must include 64-bit support. While this is more of a routine update procedure for Objective-C developers, it has deeper implications for Xamarin.iOS developers.
The original design of the MonoTouch API chose to use some of the builtin .NET types, such as
System.Single (float) and
RectangleF. Unfortunately, these types are all either 32-bit types or consist of 32-bit types. What is needed now, though, is a type that is 32-bit on 32-bit platforms and 64-bit on 64-bit platforms.
For this reason, Xamarin introduced a breaking change that will affect all Xamarin.iOS developers. The good news is that they are using the opportunity to fix a few issues with the old API. If you’re interested in all the changes, there is an Evolve session by Miguel de Icaza available.
Projects using the new Unified API need to use a new project type.
From within the new project types, you can’t reference Classic API projects.
The word “Unified” in Unified API comes from Xamarin’s attempt to facilitate code sharing for iOS and Mac projects. This is also the reason the namespace
MonoTouch was dropped entirely, making it possible to share some code between the two platforms that needed to have platform-specific implementations previously, e.g, through Shared Projects.
Updating your existing projects from Classic API to Unified API can be done by hand or semi-automated. Xamarin has published instructions for the manual steps as well as the automatic update script or update feature integrated in Xamarin Studio.
I’ve updated the iOS code in MvvmCross to the Unified API. Here are some of the things I stumbled across while porting the code.
const, you can’t use
System.Math.Max()on them, etc.).
MonoTouch.Foundation.NSActionseems to have been dropped in favor of
Xamarin’s current stable channel contains a preview for the Unified API. The final version is expected to hit the alpha and beta channels by early December 2014. Every existing Xamarin.iOS project will need to be updated if submitted to the App Store after February 1, 2015. It’s time to start looking at updating your projects.
Update January 18, 2015:
System.NMathfor arithmetic operations on the new types
.csprojfile for Visual Studio users.