Create README.md

Author: asklar

README.md

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+# react-native-xaml
+A React Native Windows view manager that allows directly using the [Windows XAML](https://docs.microsoft.com/windows/uwp/xaml-platform/xaml-overview) framework.
+
+## Main advantages
+-	Allows developers writing [react-native-windows](https://microsoft.github.io/react-native-windows/) apps (React Native apps targeting Windows 10) to quickly get started, if they are familiar with XAML.
+-	Developers are not limited by the set of controls present today in RNW and community modules that support Windows. 
+-	Best of all, this solution is available *today*!
+-	The set of types/properties/events are the XAML ones, however JS libraries can wrap those to expose the Windows control in a platform-agnostic way (e.g. as a way to implement a cross-plat control for Windows).
+
+react-native-xaml works by leveraging metadata for XAML properties, types and events. This metadata is automatically generated ([code-gen](https://github.com/asklar/react-native-xaml/tree/main/codegen/Codegen)) from the Windows SDK.
+This means we can have a full projection of all of XAML – or any WinRT components written by app devs, that export a [UserControl](https://docs.microsoft.com/uwp/api/Windows.UI.Xaml.Controls.UserControl) – without manual intervention.
+
+## Technical details:
+The CodeGen app in react-native-xaml is a C# net core console app that uses the [Middleweight Reflection](https://github.com/MikeHillberg/middleweight-reflection) library to parse a Windows metadata file from the Windows 10 SDK (windows.winmd).
+The app then uses [T4 runtime templates](https://docs.microsoft.com/visualstudio/modeling/run-time-text-generation-with-t4-text-templates) to produce some [C++/WinRT](https://github.com/microsoft/cppwinrt) code that gets compiled into the react-native-xaml View Manager. The generated code implements the metadata tables for what XAML types exist, their names, how to create them, what properties and events each type supports, etc.
+With this approach developers can write JSX code as shown in the below GIF. `NativeXamlControl` is the base control exposed to TS, and `HyperlinkButton` and `TextBlock` are aliases where they set the type property.
+
+## Implemented features:
+-	All creatable XAML types can be created.
+- All properties of type int, double, float, string, color/brush can be set. Some special casing for properties of type Object (e.g. Content).
+- All events are wired up (see GIF below, clicking on a hyperlink button calls into the JS to execute the onClick handler!).
+- Children are supported for Panel, ContentControl, and Border.
+## Not yet implemented:
+-	Reduce DLL size further. Currently the Release x64 version is about 800 kB, but can likely be trimmed down further.
+- Code-gen the TypeScript information so that apps can use friendly type names and get IntelliSense for properties, instead of using NativeXamlControl directly.
+-	Evaluate code-gen’ing separate view managers with independent properties/events/etc. instead of a single one.
+
+## Demo
+
+![react-native-xaml demo](https://raw.githubusercontent.com/asklar/react-native-xaml/main/example/rnx.gif)