List changes on a mono repository to build conditionally

call-find-changes-example.yml

    - task: PowerShell@2
      name: Changes
      inputs:
        targetType: "filePath"
        filePath: "list-changes.ps1"
        arguments: -sourceBranch $(Build.SourceBranch) -sourceCommit $(Build.SourceVersion) -targetBranch $(TargetBranch) -rules @(@("Web", "src/web/**"),@("Mobile", "src/mobile/**"),@("Shared", "src/shared/**"))

list-changes.ps1

param ([string]$sourceBranch, [string]$sourceCommit, [string]$targetBranch, [array]$rules)
# -sourceBranch $(Build.SourceBranch)
# -sourceCommit $(Build.SourceVersion) 
# -targetBranch $(System.PullRequest.TargetBranch)
# -rules @(@("Rule1", "Proj1/*.cs", "Proj1/*.ts"), ("Rule2", "Proj2/*.cs"))
if ($sourceBranch -like 'refs/pull/*') {
    Write-Host "Comparing $sourceBranch with origin/$targetBranch"
    $files = $(git diff --name-only -r origin/$targetBranch) -split ' '
}
else {
    Write-Host "Recursive comparing $sourceBranch with $sourceCommit"
    $files = $(git diff-tree --no-commit-id --name-only -r $sourceCommit^1 $sourceCommit) -split ' '
}
$result = New-Object Collections.Generic.List[string]
Write-Host "Total changed $($files.Length) files"
if ($files.Length -eq 0) {
    Write-Error "No changes, this is a bug"
    Exit 1
}

foreach ($rule in $rules) {
    $ruleName = $rule[0]
    foreach ($pattern in ($rule | Select-Object -skip 1)) {
        foreach ($file in $files) {
            if ($file -like $pattern) {
                Write-Debug $file
                $result.Add($ruleName)
            }
        }
    }
}
if (![string]::IsNullOrWhiteSpace($env:FORCE_BUILD)) {
    $result.Add($env:FORCE_BUILD)
}

$result = $result | Get-Unique
Write-Host "Matched changes: $($result -join ' ')"
foreach ($r in $result) {
    Write-Host "##vso[task.setvariable variable=$r;isoutput=True]True"
}