formdownloadasyncwithworker.ps1

Add-Type -Assembly System.Windows.Forms

[System.Windows.Forms.Application]::EnableVisualStyles()

$MainForm = New-Object System.Windows.Forms.Form
$MainForm.Width = 420
$MainForm.Height = 200
$MainForm.FormBorderStyle = "Fixed3d"
$MainForm.MaximizeBox = $false
$MainForm.StartPosition = "CenterScreen"

$Button1 = New-Object System.Windows.Forms.Button
$Button1.Location = New-Object System.Drawing.Size(120,50)
$Button1.Size = New-Object System.Drawing.Size(160,50)
$Button1.Text = "Download selected"
$Button1.Name = "Button1"
$Button1.Add_Click({
    # from this event handler we just enqueue downloads, there is no need to disable
    # this button because the current implementation will allow as many downloads as you like
    # and the download is limited by a `SemaphoreSlim` to handle throttling.
    if ($CheckBox1.Checked) {
        $queue.Enqueue(@{
            Link = "https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf"
            Path = Join-Path $pwd -ChildPath ('chrome' + [guid]::NewGuid() + '.exe')
        })
    }
    if ($CheckBox2.Checked) {
        $queue.Enqueue(@{
            Link = "https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf"
            Path = Join-Path $pwd -ChildPath ('firefox' + [guid]::NewGuid() + '.exe')
        })
    }
})
$MainForm.Controls.Add($Button1)

$CheckBox1 = New-Object System.Windows.Forms.CheckBox
$CheckBox1.Location = New-Object System.Drawing.Size(10,10)
$CheckBox1.Size = New-Object System.Drawing.Size(100,20)
$CheckBox1.Text = "Chrome"
$MainForm.Controls.Add($CheckBox1)

$CheckBox2 = New-Object System.Windows.Forms.CheckBox
$CheckBox2.Location = New-Object System.Drawing.Size(10,30)
$CheckBox2.Size = New-Object System.Drawing.Size(100,20)
$CheckBox2.Text = "Firefox"
$MainForm.Controls.Add($CheckBox2)

$ProgressBar1 = New-Object System.Windows.Forms.ProgressBar
$ProgressBar1.Location = New-Object System.Drawing.Size(5,120)
$ProgressBar1.Size = New-Object System.Drawing.Size(390,30)
$ProgressBar1.Name = "ProgressBar1"
$MainForm.Controls.Add($ProgressBar1)

$queue = [System.Collections.Concurrent.ConcurrentQueue[hashtable]]::new()

$rs = [runspacefactory]::CreateRunspace($Host)
$rs.Open()
# the 2 objects that we need available in the worker thread are the queue and the progressbar,
# if you want more objects available from the form itslef it might be better to pass in `$MainForm`
# then use `.Controls.Find(...)` as shown in the previous answer
$rs.SessionStateProxy.PSVariable.Set('queue', $queue)
$rs.SessionStateProxy.PSVariable.Set('progress', $ProgressBar1)

$ps = [powershell]::Create().AddScript({
    $client = [System.Net.WebClient]::new()
    $task = $null
    # allow only 1 download at a time
    $throttleLimit = [System.Threading.SemaphoreSlim]::new(1, 1)
    # NOTE: A single instance of `WebClient` can handle more than one download at the same
    #       but you need additional logic, i.e. check for `$client.IsBusy` before downloading async.

    $registerObjectEventSplat = @{
        InputObject = $client
        EventName   = 'DownloadProgressChanged'
        Action      = {
            $progress.Value = $eventArgs.ProgressPercentage
            Write-Progress -Activity 'Downloading...' -PercentComplete $eventArgs.ProgressPercentage
        }
    }

    Register-ObjectEvent @registerObjectEventSplat

    $registerObjectEventSplat = @{
        InputObject = $client
        EventName   = 'DownloadFileCompleted'
        Action      = {
            # Release the Semaphore handle here once the download is completed
            $throttleLimit.Release()
            "A download was completed...", "Items in Queue: $($queue.Count)" | Out-Host
            Write-Progress -Activity 'Downloading...' -Completed
        }
    }

    Register-ObjectEvent @registerObjectEventSplat

    while ($true) {
        # if there is nothing in queue
        if (-not $queue.TryDequeue([ref] $task)) {
            # sleep for a bit
            Start-Sleep -Milliseconds 200
            # and go to the next iteration
            continue
        }

        # else, we know there is a download here but we want to allow only 1 at a time
        # so, this inner `while` will unblock only when the SemaphoreSlim allows it
        while (-not $throttleLimit.Wait(200)) { }
        # once we have the handle of the Semaphore we can start the download here
        $client.DownloadFileAsync($task['Link'], $task['Path'])
        "A download has started...", "Items in Queue: $($queue.Count)" | Out-Host
        $task | Out-Host
    }
}, $false)

$ps.Runspace = $rs
$task = $ps.BeginInvoke()

$MainForm.ShowDialog()

# this should be a must in your code,
# always dispose the resources when done
$ps.Stop()
$ps.Dispose()
$rs.Dispose()