Setting up a mock for different results on sequential calls

[tahasi]

Is there a recommended way of creating a Pester mock that has a different result for sequential calls?

I currently have a solution that's a bit inelegant that uses a variable to track call counts and returns different results based on that variable.

$script:InvokeCommandCount = 0
Mock Invoke-Command {
    $script:InvokeCommandCount++
    if ($script:InvokeCommandCount -eq 1) {
        return @{
            Count = 4;
            Minimum = Get-Date -Date "2023-06-06 12:00:00Z"
        }
    }
    if ($script:InvokeCommandCount -eq 2) {
        return @{
            Count = 6;
            Minimum = Get-Date -Date "2023-06-06 12:01:00Z"
        }
    }
    throw "Unexpected Invoke-Command call count"
}

By inelegant, I mean that there's too much code here to manage the state of the mock. It'd be great if I provide an array of script blocks and the Mock had internal state to track which sequential result is used.

Mock Invoke-Command -SequentialResults @(
    @{ Count = 4;Minimum = Get-Date -Date "2023-06-06 12:00:00Z" },
    @{ Count = 6; Minimum = Get-Date -Date "2023-06-06 12:01:00Z" }
)

[fflaten]

Hi. Great question! The behavior is likely too specific to become an official API.

If you write a lot of these mocks, I'd try to write a helper function to generate at least the mock scriptblock and counter-variable using the syntax you want.

[fflaten]

I got inspired, so here's two examples 🙂

They leverage .GetNewClosure() to isolate the counter and results-arrays in their own dynamic modules, meaning multiple mocks won't conflict. It also means values provided to -SequentialResults should be constants and not rely on outside variables in the test code that change between calls to the mock.

Describe 'Sequential Results Mock Helper' {
    Context 'Generates scriptblock only' {
        BeforeAll {
            function New-SequentialResultsMockBehavior {
                param(
                    [Parameter(Mandatory)]
                    [ValidateNotNullOrEmpty()]
                    [object[]] $SequentialResults
                )

                $mockCallCounter = 0

                $mockScriptBlock = {
                    $currentIndex = $script:mockCallCounter++
                    if ($mockCallCounter -gt $SequentialResults.Count) {
                        throw 'Unexpected call. No more results are available.'
                    }

                    return $SequentialResults[$currentIndex]
                }.GetNewClosure()

                return $mockScriptBlock
            }
        }

        It 'demo' {
            $sb = New-SequentialResultsMockBehavior -SequentialResults @(
                @{ Count = 4; Minimum = Get-Date -Date '2023-06-06 12:00:00Z' },
                @{ Count = 6; Minimum = Get-Date -Date '2023-06-06 12:01:00Z' }
            )

            Mock -CommandName Invoke-Command -MockWith $sb

            (Invoke-Command {}).Count | Should -Be 4
            (Invoke-Command {}).Count | Should -Be 6
            { Invoke-Command {} } | Should -Throw 'Unexpected call. No more results are available.'
        }
    }

    Context 'Generates full mock' {
        BeforeAll {
            # inline this function (like this) or dot-source it from a script
            # requires modifications to support being inside a module
            function New-SequentialResultsMock {
                param(
                    [Parameter(Mandatory)]
                    [string] $CommandName,
                    [Parameter(Mandatory)]
                    [ValidateNotNullOrEmpty()]
                    [object[]] $SequentialResults,
                    [switch] $Verifiable,
                    [ScriptBlock] $ParameterFilter,
                    [string] $ModuleName,
                    [string[]] $RemoveParameterType,
                    [string[]] $RemoveParameterValidation
                )

                $mockCallCounter = 0

                $scriptBlock = {
                    $currentIndex = $script:mockCallCounter++
                    if ($mockCallCounter -gt $SequentialResults.Count) {
                        throw 'Unexpected call. No more results are available.'
                    }

                    return $SequentialResults[$currentIndex]
                }.GetNewClosure()

                $PSBoundParameters.Remove('SequentialResults')

                Pester\Mock @PSBoundParameters -MockWith $scriptBlock
            }
        }

        It 'demo' {
            New-SequentialResultsMock -CommandName Invoke-Command -SequentialResults @(
                @{ Count = 4; Minimum = Get-Date -Date '2023-06-06 12:00:00Z' },
                @{ Count = 6; Minimum = Get-Date -Date '2023-06-06 12:01:00Z' }
            )

            (Invoke-Command {}).Count | Should -Be 4
            (Invoke-Command {}).Count | Should -Be 6
            { Invoke-Command {} } | Should -Throw 'Unexpected call. No more results are available.'
        }
    }
}

[fflaten]

It actually ended up becoming a blog post if you're interested 🙂
https://frodeflaten.com/posts/generating-pester-mocks/

Spoiler

There will be a follow-up post to modify the second version to work while inside a helper-module.