Xamarin 8.13.x Releases
The Xamarin.iOS 8.13.0 series upgrades the Mono stack from the Mono 4.0.xx series to the Mono 4.2.xx series. One of the major themes in Mono 4.2.xx series is the continued adoption of Microsoft's open sourced .NET code to improve the conformance, speed and memory usage of our stack.
Visual Studio: New Mac Build Agent
We have rewritten the Mac Build agent that Visual Studio uses to connect to.
The user experience is vastly superior. Not only because it is now easier, more convenient and more reliable to link your Windows and Mac computers, but because we have addressed many of the fundamental design limitations, shortcomings and bugs from our previous implementation. This means more reliable builds, debugging and runs.
We have replaced our pairing and authentication system with one powered by SSH, which now encrypts all data transfers and uses the MacOS provided SSH server as the mechanism to connect your Windows and Mac computers.
Support for Embedded Frameworks
It is now possible to use iOS Frameworks with your iOS projects. This is Apple's way of packaging shared libraries, they are useful for sharing code between multiple components of your app (your main application and your extensions) or to easily add third party frameworks without doing native linking .
Xamarin.iOS will automatically switch to using Mono.framework instead of statically linking the Mono framework into your executable when you use extensions.
Embedded frameworks only work on iOS 8.0 and higher.
This behavior can be controlled with
--mono= command line option to mtouch, you can
--mono=static to force static linking
--mono=framework to force the use of a shared --
--Mono.framework in your project.
Some examples showing how this can be used for linking with native frameworks and invoking manually, using it for C# bindings to Objective-C APIs and how this is used with many extensions can be found in embedded-frameworks github module.
 While frameworks are convenient units of deployment, they do not participate in the linking process, so if you are merely using a limited set of features from a framework, you wont get the beneifts that come from statically linking, which would only include the pieces that you actually use.
Build Time Improvements: Partial Static Registrar
This is an optimization we made to the runtime to improve the development speed for iOS simulator builds. It uses the static registration system for the system, and uses dynamic registration for user code, which does not require any new native code to be compiled on each iteration.
Code Generation Optimizations
When compiling with LLVM, Xamarin.iOS will no longer generate intermediate files, and instead will generate object code directly. This allowed us to generate more efficient code (in some large application scenarios, LLVM produced binaries are 3% smaller) and slightly improved build times.
The following document contains a complete list of the API changes from 8.10 to 8.13.0.