[Xamarin.Android.Build.Tasks] prevent _BuildLibraryProjectImportsCache always running

[jonathanpeppers]

When testing a build with no changes, I started noticing:

Building target "_BuildLibraryImportsCache" completely.
Input file "App7\bin\Debug\netstandard2.0\App7.dll" is newer than output file "obj\Debug\90\libraryimports.cache".

What was weird, is that earlier in the build log:

Did not copy from file "bin\Debug\netstandard2.0\App7.dll" to file "bin\Debug\App7.dll" because the "SkipUnchangedFiles" parameter was set to "true" in the project and the files' sizes and timestamps match.

So the NetStandard assembly didn't change.

When I looked at these files:

8/31/2018  11:02 AM App7.dll
8/31/2018  10:09 AM resourcepaths.cache

Weird? It looks like resourcepaths.cache's timestamp needs to be
updated?

Then I looked and noticed that the timestamp of resourcepaths.cache
would only be updated if the XML changed.

My first attempt to fix this:

• I made XDocumentExtensions.SaveIfChanged return a bool
  indicating if the file changed.
• I changed the GetImportedLibraries task so that it would update
  the timestamp in cases where the file didn't change.

Unfortunately, that made _UpdateAndroidResgen run all the time,
which would be even worse!

So my second attempt was much better:

• Add a $(_AndroidLibraryImportsCache).stamp file, to use as the
  Outputs of _BuildLibraryImportsCache.
• This file's timestamp can operate independently of
  $(_AndroidLibraryImportsCache)

_BuildLibraryImportsCache now builds incrementally and gets skipped
properly:

Skipping target "_BuildLibraryImportsCache" because all output files are up-to-date with respect to the input files.

It seems to me this problem was always occurring on builds with no
changes. So this will likely help any incremental build.

Before this change:

146 ms  _BuildLibraryImportsCache                  1 calls
Total time: 2.824s

After this change:

  4 ms  _BuildLibraryImportsCache                  1 calls
Total time: 2.698s

I also updated an existing test to make sure the
_BuildLibraryImportsCache target isn't running all the time.

[jonathanpeppers]

Need to think about this one more next week:

 Building target "_UpdateAndroidResgen" completely.
 Input file "obj\Debug\90\libraryimports.cache" is newer than output file "obj\Debug\90\R.cs.flag".

This is happening and shouldn't ^^

So it's as if the timestamp needs to be set to the date of the oldest input to _BuildLibraryImportsCache. I'm not sure if we currently have anything that does this right now.

[jonathanpeppers]

I have another idea that seems like it will work, hoping it's green this time.

Build logs: BuildLibraryProjectImports.zip

[dellis1972]

@jonathanpeppers this good to go?

[jonathanpeppers]

@dellis1972 yes, the second approach doesn’t seem break anything. Should be good to go.