Make ValidationChangedObservable pipe pure

[worldbeater]

What kind of change does this PR introduce?

Fixes #19

What is the current behavior?

Currently, _validationSource and _validations won't stay synchronized due to the following code blocks:

ReactiveUI.Validation/src/ReactiveUI.Validation/Contexts/ValidationContext.cs

Line 61 in 0b28a7e

.Bind(out _validations)

ReactiveUI.Validation/src/ReactiveUI.Validation/Contexts/ValidationContext.cs

Lines 83 to 88 in 0b28a7e

	.CountChanged()
	.Count()
	.StartWith(0)
	.Select(_ =>
	_validations
	.Select(v => v.ValidationStatusChange)

Noticed DynamicData misuse here. The _validations collection is empty when validationChangedObservable ticks on a scheduler other than ImmediateScheduler. This happens because the validationConnectable pipe isn't pure — when using a scheduler other than immediate, the _validations read-only observable collection gets updated too late.

What is the new behavior?

Now, the validationConnectable is pure, and IsValid behaves correctly with the default scheduler.

What might this PR break?

All tests pass on my local machine, so probably nothing.

[worldbeater]

Hmm, the CI checks failed, but it doesn't seem the PR caused the failure:

d:/a/1/s/tools/ReactiveUI.Cake.Recipe.1.0.103/parseanalyzeproject.cake(3,16): error CS0246: The type or namespace name 'CustomProjectParserResult' could not be found (are you missing a using directive or an assembly reference?)

BTW out of scope for this PR, but wondering if we support custom delimiters for validation messages:

If not, probably worth opening a new task and challenging it 🥇

[glennawatson]

I think you need to update the cake version. Use Punchcard or Splat as a example.

[glennawatson]

Hopefully those 3 changes get it.

[glennawatson]

Azure Devops has died without access to the internet. So will have to wait I guess.

[codecov]

Codecov Report

Merging #20 into master will increase coverage by 0.18%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #20      +/-   ##
==========================================
+ Coverage   33.09%   33.28%   +0.18%     
==========================================
  Files          16       16              
  Lines         704      706       +2     
  Branches       82       82              
==========================================
+ Hits          233      235       +2     
  Misses        463      463              
  Partials        8        8

Impacted Files	Coverage Δ
...eactiveUI.Validation/Contexts/ValidationContext.cs	80.95% <100%> (+0.62%)	⬆️

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