Describe.ps1

function Describe {
<#
.SYNOPSIS
Creates a logical group of tests.

.DESCRIPTION
Creates a logical group of tests. All Mocks and TestDrive contents

defined within a Describe block are scoped to that Describe; they
will no longer be present when the Describe block exits.  A Describe
block may contain any number of Context and It blocks.

.PARAMETER Name
The name of the test group. This is often an expressive phrase describing
the scenario being tested.

.PARAMETER Fixture
The actual test script. If you are following the AAA pattern (Arrange-Act-Assert),
this typically holds the arrange and act sections. The Asserts will also lie
in this block but are typically nested each in its own It block. Assertions are
typically performed by the Should command within the It blocks.

.PARAMETER Tag
Optional parameter containing an array of strings.  When calling Invoke-Pester,
it is possible to specify a -Tag parameter which will only execute Describe blocks
containing the same Tag.

.EXAMPLE
function Add-Numbers($a, $b) {
    return $a + $b
}

Describe "Add-Numbers" {
    It "adds positive numbers" {
        $sum = Add-Numbers 2 3
        $sum | Should -Be 5
    }

    It "adds negative numbers" {
        $sum = Add-Numbers (-2) (-2)
        $sum | Should -Be (-4)
    }

    It "adds one negative number to positive number" {
        $sum = Add-Numbers (-2) 2
        $sum | Should -Be 0
    }

    It "concatenates strings if given strings" {
        $sum = Add-Numbers two three
        $sum | Should -Be "twothree"
    }
}

.LINK
It
Context
Invoke-Pester
about_Should
about_Mocking
about_TestDrive

#>

    param(
        [Parameter(Mandatory = $true, Position = 0)]
        [string] $Name,

        [Alias('Tags')]
        [string[]] $Tag=@(),

        [Parameter(Position = 1)]
        [ValidateNotNull()]
        [ScriptBlock] $Fixture = $(Throw "No test script block is provided. (Have you put the open curly brace on the next line?)")
    )

    if ($null -eq (& $SafeCommands['Get-Variable'] -Name Pester -ValueOnly -ErrorAction $script:IgnoreErrorPreference))
    {
        # User has executed a test script directly instead of calling Invoke-Pester
        $Pester = New-PesterState -Path (& $SafeCommands['Resolve-Path'] .) -TestNameFilter $null -TagFilter @() -SessionState $PSCmdlet.SessionState
        $script:mockTable = @{}
    }

    DescribeImpl @PSBoundParameters -CommandUsed 'Describe' -Pester $Pester -DescribeOutputBlock ${function:Write-Describe} -TestOutputBlock ${function:Write-PesterResult}
}

function DescribeImpl {
    param(
        [Parameter(Mandatory = $true, Position = 0)]
        [string] $Name,

        [Alias('Tags')]
        $Tag=@(),

        [Parameter(Position = 1)]
        [ValidateNotNull()]
        [ScriptBlock] $Fixture = $(Throw "No test script block is provided. (Have you put the open curly brace on the next line?)"),

        [string] $CommandUsed = 'Describe',

        $Pester,

        [scriptblock] $DescribeOutputBlock,

        [scriptblock] $TestOutputBlock,

        [switch] $NoTestDrive
    )

    Assert-DescribeInProgress -CommandName $CommandUsed

    if ($Pester.TestGroupStack.Count -eq 2)
    {
        if($Pester.TestNameFilter -and -not ($Pester.TestNameFilter | & $SafeCommands['Where-Object'] { $Name -like $_ }))
        {
            #skip this test
            return
        }

        if($Pester.TagFilter -and @(& $SafeCommands['Compare-Object'] $Tag $Pester.TagFilter -IncludeEqual -ExcludeDifferent).count -eq 0) {return}
        if($Pester.ExcludeTagFilter -and @(& $SafeCommands['Compare-Object'] $Tag $Pester.ExcludeTagFilter -IncludeEqual -ExcludeDifferent).count -gt 0) {return}
    }
    else
    {
        if ($PSBoundParameters.ContainsKey('Tag'))
        {
            Write-Warning "${CommandUsed} '$Name': Tags are only effective on the outermost test group, for now."
        }
    }

    $Pester.EnterTestGroup($Name, $CommandUsed)

    if ($null -ne $DescribeOutputBlock)
    {
        & $DescribeOutputBlock $Name $CommandUsed
    }

    $testDriveAdded = $false
    try
    {
        try
        {
            if (-not $NoTestDrive)
            {
                if (-not (Test-Path TestDrive:\))
                {
                    New-TestDrive
                    $testDriveAdded = $true
                }
                else
                {
                    $TestDriveContent = Get-TestDriveChildItem
                }
            }

            Add-SetupAndTeardown -ScriptBlock $Fixture
            Invoke-TestGroupSetupBlocks

            do
            {
                $null = & $Fixture
            } until ($true)
        }
        finally
        {
            Invoke-TestGroupTeardownBlocks
            if (-not $NoTestDrive)
            {
                if ($testDriveAdded)
                {
                    Remove-TestDrive
                }
                else
                {
                    Clear-TestDrive -Exclude ($TestDriveContent | & $SafeCommands['Select-Object'] -ExpandProperty FullName)
                }
            }
        }
    }
    catch
    {
        $firstStackTraceLine = $_.InvocationInfo.PositionMessage.Trim() -split "$([System.Environment]::NewLine)" | & $SafeCommands['Select-Object'] -First 1
        $Pester.AddTestResult("Error occurred in $CommandUsed block", "Failed", $null, $_.Exception.Message, $firstStackTraceLine, $null, $null, $_)
        if ($null -ne $TestOutputBlock)
        {
            & $TestOutputBlock $Pester.TestResult[-1]
        }
    }

    Exit-MockScope

    $Pester.LeaveTestGroup($Name, $CommandUsed)
}

# Name is now misleading; rename later.  (Many files touched to change this.)
function Assert-DescribeInProgress
{
    param ($CommandName)
    if ($null -eq $Pester)
    {
        throw "The $CommandName command may only be used from a Pester test script."
    }
}