A crude attempt at providing file change notifications for ISE.

IseFileChangeNotifications.ps1

[System.Reflection.Assembly]::LoadWithPartialName("Microsoft.VisualBasic") > $Null
$Watchers = @{}

$TabCollectionChangedEvent = {
    $EventArgs.NewItems | %{
        Register-ObjectEvent -InputObject $_.Files -EventName CollectionChanged -Action $FileCollectionChangedEvent
    }
}

$FileCollectionChangedEvent = {
    Write-Debug "FileCollectionChanged"
    if($EventArgs.Action -eq "Add") {
        $NewFile = $EventArgs.NewItems[0]
        $ParentFolder = Split-Path -Path $NewFile.FullPath -Parent
        if(!$Watchers.ContainsKey($ParentFolder)) {
            Write-Debug "Adding Watcher"
            $Watcher = New-Object System.IO.FileSystemWatcher($ParentFolder)
            $Watcher.NotifyFilter = [System.IO.NotifyFilters]::Size -bor [System.IO.NotifyFilters]::LastWrite
            $Watcher.EnableRaisingEvents = $true
            $Watchers[$ParentFolder] = [PSCustomObject]@{
                Watcher = $Watcher;
                WatchedFiles = (New-Object Collections.ArrayList)
                EventSubscriber = $null;
            }
            Write-Debug $Watchers[$ParentFolder].WatchedFiles.GetType()
            $Watchers[$ParentFolder].WatchedFiles.Add($NewFile.FullPath)
            $Watchers[$ParentFolder].EventSubscriber = Register-ObjectEvent -InputObject $Watcher -EventName Changed -Action $FileChangedEvent
        } else {
            $Watchers[$ParentFolder].WatchedFiles.Add($NewFile.FullPath)
        }
    } elseif($EventArgs.Action -eq "Remove") {
        $OldFile = $EventArgs.OldItems[0]
        Write-Debug "Removing file $($OldFile.FullPath)"
        $ParentFolder = Split-Path -Path $OldFile.FullPath -Parent
        if($Watchers.ContainsKey($ParentFolder)) {
            $Watchers[$ParentFolder].WatchedFiles.Remove($OldFile.FullPath)
            if($Watchers[$ParentFolder].WatchedFiles.Count -eq 0) {
                Unregister-Event -SourceIdentifier $Watchers[$ParentFolder].EventSubscriber.Name
                $Watchers[$ParentFolder].Watcher.EnableRaisingEvents = $false
                $Watchers[$ParentFolder].Watcher.Dispose()
                if((Get-Job -Name $Watchers[$ParentFolder].EventSubscriber.Name).State -ne "Failed") {
                    Remove-Job -Name $Watchers[$ParentFolder].EventSubscriber.Name
                }
                $Watchers.Remove($ParentFolder)
            }
        }
    }
}

$FileChangedEvent = {
    Write-Debug "FileChanged $($EventArgs.Action)"
    $ChangedFile = $EventArgs.FullPath
    $ParentFolder = Split-Path -Path $ChangedFile -Parent
    Write-Debug $psISE.CurrentPowershellTab.Files.SelectedFile.FullPath
    Write-Debug $ChangedFile
    # Don't provide notifications for the current file. This is the only way I could prevent self notifiying when ISE saved a file.
    if($Watchers[$ParentFolder].WatchedFiles -contains $ChangedFile -and $psISE.CurrentPowershellTab.Files.SelectedFile.FullPath -ne $ChangedFile) {
        $psISE.PowerShellTabs | %{
            $Tab = $_
            $FilesToReload = $Tab.Files | ?{$_.FullPath -eq $ChangedFile}
            $FilesToReload | %{
                $ShouldChange = [Microsoft.VisualBasic.Interaction]::MsgBox("File $($_.Fullpath) has changed outside the editor. Would you like to reload?","YesNo,Question","File Changed")
                if($ShouldChange -eq "Yes") {
                    #I don't like the Text= method becuase it marks the file as changed
                    $_.Editor.Text = [System.IO.File]::ReadAllText($_.FullPath)

                    #I don't like the remove/add method because it re-orders the tabs and flashes
                    #$Tab.Files.Remove($_)
                    #$Tab.Files.Add($_.FullPath)
                }
                
            }
        }
    }
}

Register-ObjectEvent -InputObject $psISE.PowerShellTabs -EventName CollectionChanged -Action $TabCollectionChangedEvent > $null
$psise.PowerShellTabs | %{
    Register-ObjectEvent -InputObject $_.Files -EventName CollectionChanged -Action $FileCollectionChangedEvent > $null
}

I'm getting double events so you get 2 popups, and then it "manually" updates the contents of the editor from disk, so ISE marks it as modified :(

And it only works for files other than the current one to prevent the ISE saving from triggering the change event.

Thanks for working this up Chris.

Yeah, I was working with the FileSystemWatcher in .NET in PowerShell and getting double events and there are many who get the same issues in C#. It'd be nice to have a solid watcher to use from PS. We could build tools like grunt and gulp in the JavaScript world.

I think it's something for Microsoft to solve in ISE. Then again, the ISE editor is the Visual Studio editor. Too bad this isn't baked in.