Commits on Sep 12, 2019

1. [xamlc] new <XFXamlCValidateOnly> MSBuild property for Debug builds …

   XamlC is currently enabled for most Xamarin.Forms projects in `Debug`
   and `Release` configurations. It enables faster startup/runtime
   performance, XAML-syntax checking at build time--both quiet useful!
   
   However, there is a build-time cost to using XamlC: each assembly is
   loaded via Mono.Cecil, IL generated, and saved back to disk as an
   additional step after Rosyln has compiled the assembly.
   
   The proposal would be to add a new experimental MSBuild property, that
   can be enabled for `Debug` mode such as:
   
       <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
         <!-- ... -->
         <XFXamlCValidateOnly>True</XFXamlCValidateOnly>
       </PropertyGroup>
   
   This would do the following:
   
   * `OptimizeIL` is skipped.
   * Assemblies are analyzed, but no changes written to disk.
   * No symbols are loaded via Mono.Cecil, we don't need them if we
     aren't writing back to disk!
   
   This will improve build performance, and we don't lose the build-time
   error checking for XAML.
   
   Down the road, we could consider moving the MSBuild property to be the
   default for `Debug` builds or change the Xamarin templates. I think it
   would be wise to require developers to opt-in to try this out.
   
   ~~ Results ~~
   
   I tested this change by building the ControlGallery after changing one
   line of XAML:
   
       msbuild Xamarin.Forms.ControlGallery.Android\Xamarin.Forms.ControlGallery.Android.csproj /clp:performancesummary /p:XFXamlCValidateOnly=True
   
       Before:
       1979 ms  XamlCTask                                  1 calls
       After:
        923 ms  XamlCTask                                  1 calls
   
   I *only* tested `Debug` builds.
   
   Right, so it's faster. But let's keep the entire developer loop in
   mind, how much slower is startup?
   
       Before:
       09-05 14:37:32.274  1169  1192 I ActivityManager: Displayed AndroidControlGallery.AndroidControlGallery/md546303760447087909496d02dc7b17ae8.Activity1: +3s890ms
       09-05 14:38:30.178  1169  1192 I ActivityManager: Displayed AndroidControlGallery.AndroidControlGallery/md546303760447087909496d02dc7b17ae8.Activity1: +3s848ms
       09-05 14:38:40.300  1169  1192 I ActivityManager: Displayed AndroidControlGallery.AndroidControlGallery/md546303760447087909496d02dc7b17ae8.Activity1: +3s848ms
       After:
       09-05 14:40:38.512  1169  1192 I ActivityManager: Displayed AndroidControlGallery.AndroidControlGallery/md546303760447087909496d02dc7b17ae8.Activity1: +3s894ms
       09-05 14:40:55.497  1169  1192 I ActivityManager: Displayed AndroidControlGallery.AndroidControlGallery/md546303760447087909496d02dc7b17ae8.Activity1: +3s856ms
       09-05 14:41:03.754  1169  1192 I ActivityManager: Displayed AndroidControlGallery.AndroidControlGallery/md546303760447087909496d02dc7b17ae8.Activity1: +3s897ms
   
   After three runs, it seems this app suffers 25-50ms slowdown to
   startup, and gains 1 second of build time improvement. A good net-win!
   
   Other apps that have significantly more XAML will have different
   results. I suspect the build time improvement will be even better, but
   the hit to startup could be worse. With this setting opt-in, we can
   experiment and find out.

   

   jonathanpeppers committed Sep 12, 2019
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