Add -Previous switch to retrieve last shown tip

src/tiPS/Private/TipsAlreadyShownFunctions.Tests.ps1

@@ -144,4 +144,49 @@ InModuleScope -ModuleName tiPS { # Must use InModuleScope to call private functi
 			}
 		}
 	}
+
+	Describe 'Calling GetLastShownTipId' {
+		BeforeEach {
+			# Use a temp configuration data directory instead of reading/overwriting the current user's configuration.
+			Mock -CommandName Get-TiPSDataDirectoryPath -MockWith {
+				[string] $directoryPath = "$TestDrive/tiPS" # Use $TestDrive variable so .NET methods can resolve the path.
+				if (-not (Test-Path -Path $directoryPath -PathType Container))
+				{
+					New-Item -Path $directoryPath -ItemType Directory -Force > $null
+				}
+				return $directoryPath
+			}
+
+			ClearTipIdsAlreadyShown
+		}
+
+		Context 'When no tips have been shown' {
+			It 'Should return null' {
+				$lastShownTipId = GetLastShownTipId
+				$lastShownTipId | Should -BeNullOrEmpty
+			}
+		}
+
+		Context 'When tips have been shown' {
+			It 'Should return the last shown tip ID' {
+				AppendTipIdToTipIdsAlreadyShown -TipId 'Tip1'
+				AppendTipIdToTipIdsAlreadyShown -TipId 'Tip2'
+				AppendTipIdToTipIdsAlreadyShown -TipId 'Tip3'
+
+				$lastShownTipId = GetLastShownTipId
+
+				$lastShownTipId | Should -Be 'Tip3'
+			}
+		}
+
+		Context 'When only one tip has been shown' {
+			It 'Should return that tip ID' {
+				AppendTipIdToTipIdsAlreadyShown -TipId 'OnlyTip'
+
+				$lastShownTipId = GetLastShownTipId
+
+				$lastShownTipId | Should -Be 'OnlyTip'
+			}
+		}
+	}
 }

src/tiPS/Private/TipsAlreadyShownFunctions.ps1

@@ -64,6 +64,27 @@ function ClearTipIdsAlreadyShown
 	[System.IO.File]::WriteAllText($tipIdsAlreadyShownFilePath, [string]::Empty)
 }
 
+function GetLastShownTipId
+{
+	[CmdletBinding()]
+	[OutputType([string])]
+	Param()
+
+	[string[]] $tipIdsAlreadyShown = ReadTipIdsAlreadyShownOrDefault
+	if ($tipIdsAlreadyShown.Count -eq 0)
+	{
+		return $null
+	}
+
+	[string] $lastShownTipId = $tipIdsAlreadyShown | Select-Object -Last 1
+	if ([string]::IsNullOrWhiteSpace($lastShownTipId))
+	{
+		return $null
+	}
+
+	return $lastShownTipId
+}
+
 function GetTipIdsAlreadyShownFilePath
 {
 	[CmdletBinding()]

src/tiPS/Public/Get-PowerShellTip.Tests.ps1

@@ -140,6 +140,60 @@ Describe 'Get-PowerShellTip' {
 			$tip.CreatedDate | Should -Be $oldestTipDate
 		}
 	}
+
+	Context 'Given the Previous switch' {
+		BeforeEach {
+			# Use a temp configuration data directory instead of reading/overwriting the current user's configuration.
+			Mock -ModuleName $ModuleName -CommandName Get-TiPSDataDirectoryPath -MockWith {
+				[string] $directoryPath = "$TestDrive/tiPS" # Use $TestDrive variable so .NET methods can resolve the path.
+				if (-not (Test-Path -Path $directoryPath -PathType Container))
+				{
+					New-Item -Path $directoryPath -ItemType Directory -Force > $null
+				}
+				return $directoryPath
+			}
+		}
+
+		It 'Should return the last shown tip' {
+			InModuleScope -ModuleName $ModuleName {
+				# Show a tip to populate the last shown tip ID
+				AppendTipIdToTipIdsAlreadyShown -TipId '2023-07-16-powershell-is-open-source'
+			}
+
+			$tip = Get-PowerShellTip -Previous
+
+			$tip.Id | Should -Be '2023-07-16-powershell-is-open-source'
+		}
+
+		It 'Should write an error when no tips have been shown yet' {
+			InModuleScope -ModuleName $ModuleName {
+				# Clear all shown tips
+				ClearTipIdsAlreadyShown
+			}
+
+			Get-PowerShellTip -Previous -ErrorVariable error -ErrorAction SilentlyContinue > $null
+			$error | Should -Not -BeNullOrEmpty
+		}
+
+		It 'Should not mark the tip as shown again' {
+			InModuleScope -ModuleName $ModuleName {
+				# Show a tip to populate the last shown tip ID
+				AppendTipIdToTipIdsAlreadyShown -TipId '2023-07-16-powershell-is-open-source'
+
+				# Get count of shown tips before calling -Previous
+				[string[]] $tipIdsBeforePrevious = ReadTipIdsAlreadyShownOrDefault
+
+				# Get the previous tip
+				Get-PowerShellTip -Previous > $null
+
+				# Get count of shown tips after calling -Previous
+				[string[]] $tipIdsAfterPrevious = ReadTipIdsAlreadyShownOrDefault
+
+				# The count should be the same
+				$tipIdsAfterPrevious.Count | Should -Be $tipIdsBeforePrevious.Count
+			}
+		}
+	}
 }
 
 InModuleScope -ModuleName tiPS { # Must use InModuleScope to access script-level variables of the module.

src/tiPS/Public/Get-PowerShellTip.ps1

@@ -20,6 +20,11 @@ function Get-PowerShellTip
 
 	When this parameter is used, the list of tips shown is not updated.
 
+	.PARAMETER Previous
+	Return the last tip that was shown.
+
+	When this parameter is used, the list of tips shown is not updated.
+
 	.INPUTS
 	You can pipe a [string] of the ID of the tip to retrieve, or a PSCustomObject with a [string] 'Id' property.
 
@@ -43,6 +48,11 @@ function Get-PowerShellTip
 
 	Get all tips.
 
+	.EXAMPLE
+	Get-PowerShellTip -Previous
+
+	Get the last tip that was shown.
+
 	.EXAMPLE
 	'2023-07-16-powershell-is-open-source' | Get-PowerShellTip
 
@@ -56,6 +66,7 @@ function Get-PowerShellTip
 
 	[CmdletBinding(DefaultParameterSetName = 'Default')]
 	[OutputType([tiPS.PowerShellTip], ParameterSetName = 'Default')]
+	[OutputType([tiPS.PowerShellTip], ParameterSetName = 'Previous')]
 	[OutputType([System.Collections.Specialized.OrderedDictionary], ParameterSetName = 'AllTips')]
 	Param
 	(
@@ -64,8 +75,11 @@ function Get-PowerShellTip
 			HelpMessage = 'The ID of the tip to retrieve. If not supplied, a random tip will be returned.')]
 		[string] $Id,
 
-		[Parameter(ParameterSetName = 'AllTips', Mandatory = $false, HelpMessage = 'Return all tips.')]
-		[switch] $AllTips
+		[Parameter(ParameterSetName = 'AllTips', HelpMessage = 'Return all tips.')]
+		[switch] $AllTips,
+
+		[Parameter(ParameterSetName = 'Previous', HelpMessage = 'Return the last tip that was shown.')]
+		[switch] $Previous
 	)
 
 	Process
@@ -75,6 +89,27 @@ function Get-PowerShellTip
 			return ReadAllPowerShellTipsFromJsonFile
 		}
 
+		if ($Previous)
+		{
+			$lastShownTipId = GetLastShownTipId
+			if ($null -eq $lastShownTipId)
+			{
+				Write-Error "No tips have been shown yet."
+				return
+			}
+
+			[hashtable] $allTips = ReadAllPowerShellTipsFromJsonFile
+			[bool] $tipIdDoesNotExist = (-not $allTips.Contains($lastShownTipId))
+			if ($tipIdDoesNotExist)
+			{
+				Write-Error "The last shown tip with ID '$lastShownTipId' no longer exists."
+				return
+			}
+
+			[tiPS.PowerShellTip] $tip = $allTips[$lastShownTipId]
+			return $tip
+		}
+
 		[bool] $allTipsHaveBeenShown = $script:UnshownTips.Count -eq 0
 		if ($allTipsHaveBeenShown)
 		{

src/tiPS/Public/Write-PowerShellTip.Tests.ps1

@@ -56,4 +56,40 @@ Describe 'Write-PowerShellTip' {
 			Should -InvokeVerifiable # Verify that the Write-Host mock was called.
 		}
 	}
+
+	Context 'Given the Previous switch' {
+		BeforeEach {
+			# Use a temp configuration data directory instead of reading/overwriting the current user's configuration.
+			Mock -ModuleName $ModuleName -CommandName Get-TiPSDataDirectoryPath -MockWith {
+				[string] $directoryPath = "$TestDrive/tiPS" # Use $TestDrive variable so .NET methods can resolve the path.
+				if (-not (Test-Path -Path $directoryPath -PathType Container))
+				{
+					New-Item -Path $directoryPath -ItemType Directory -Force > $null
+				}
+				return $directoryPath
+			}
+		}
+
+		It 'Should write the last shown tip without error' {
+			InModuleScope -ModuleName $ModuleName {
+				# Show a tip to populate the last shown tip ID
+				AppendTipIdToTipIdsAlreadyShown -TipId '2023-07-16-powershell-is-open-source'
+			}
+
+			$Error.Clear()
+			{ Write-PowerShellTip -Previous } | Should -Not -Throw
+			Should -InvokeVerifiable # Verify that the Write-Host mock was called.
+			$Error[0] | Should -BeNullOrEmpty
+		}
+
+		It 'Should write an error when no tips have been shown yet' {
+			InModuleScope -ModuleName $ModuleName {
+				# Clear all shown tips
+				ClearTipIdsAlreadyShown
+			}
+
+			Write-PowerShellTip -Previous -ErrorVariable tipError -ErrorAction SilentlyContinue > $null
+			$tipError | Should -Not -BeNullOrEmpty
+		}
+	}
 }

src/tiPS/Public/Write-PowerShellTip.ps1

@@ -14,6 +14,10 @@ function Write-PowerShellTip
 	The ID of the tip to write. If not supplied, a random tip will be written.
 	If no tip with the specified ID exists, an error is written.
 
+	.PARAMETER Previous
+	Write the last tip that was shown instead of a new tip.
+	The tip is not marked as shown again when using this parameter.
+
 	.INPUTS
 	You can pipe a [string] of the ID of the tip to write, or a PSCustomObject with a [string] 'Id' property.
 
@@ -29,20 +33,36 @@ function Write-PowerShellTip
 	Write-PowerShellTip -Id '2023-07-16-powershell-is-open-source'
 
 	Write the tip with the specified ID.
+
+	.EXAMPLE
+	Write-PowerShellTip -Previous
+
+	Write the last tip that was shown.
 #>
-	[CmdletBinding()]
+	[CmdletBinding(DefaultParameterSetName = 'Default')]
 	[Alias('Tips')]
 	[OutputType([void])]
 	Param
 	(
-		[Parameter(Mandatory = $false, ValueFromPipeline = $true, ValueFromPipelineByPropertyName = $true,
+		[Parameter(ParameterSetName = 'Default', Mandatory = $false, ValueFromPipeline = $true, ValueFromPipelineByPropertyName = $true,
 			HelpMessage = 'The ID of the tip to write. If not supplied, a random tip will be written.')]
-		[string] $Id
+		[string] $Id,
+
+		[Parameter(ParameterSetName = 'Previous', HelpMessage = 'Write the last tip that was shown.')]
+		[switch] $Previous
 	)
 
 	Process
 	{
-		[tiPS.PowerShellTip] $tip = Get-PowerShellTip -Id $Id
+		if ($Previous)
+		{
+			[tiPS.PowerShellTip] $tip = Get-PowerShellTip -Previous
+		}
+		else
+		{
+			[tiPS.PowerShellTip] $tip = Get-PowerShellTip -Id $Id
+		}
+
 		if ($null -ne $tip)
 		{
 			WritePowerShellTipToTerminal -Tip $tip