How does the Apple iTunes web site launch the iTunes application on my computer when I click the blue “Launch iTunes” button?

Question

This is new to me as a desktop developer.

If I could figure out how this is accomplished, it may be relevant to some research I\'m doing, specifically how to migrate

Answer 1

The easiest way is to register a filetype to your application (also called File Association), for example ".myp" and when the user press "start myapp" on the site it downloads a file "startapp.myp".

Windows will then look at the extention of the file and find that it is registered to your app and start your application with the file as a command-parameter. Your app can then read the file and do stuff depending of it contents.

Here are code to register a filetype to your application done in VB.Net:
(Example is taken from http://www.developerfusion.com/article/36/file-assocation/2/ but copied here for persistent reason, check original site for comments)

'// Registry windows api calls
Private Declare Function RegCreateKey& Lib "advapi32.DLL" Alias "RegCreateKeyA" (ByVal hKey As Long, ByVal lpszSubKey As String, ByVal lphKey As Long)
Private Declare Function RegSetValue& Lib "advapi32.DLL" Alias "RegSetValueA" (ByVal hKey As Long, ByVal lpszSubKey As String, ByVal fdwType As Long, ByVal lpszValue As String, ByVal dwLength As Long)
'// Required constants
Private Const HKEY_CLASSES_ROOT = &H80000000
Private Const MAX_PATH = 256&
Private Const REG_SZ = 1

'// procedure you call to associate the tmg extension with your program.
Private Sub MakeDefault()
    Dim sKeyName As String  '// Holds Key Name in registry.
    Dim sKeyValue As String  '// Holds Key Value in registry.
    Dim ret As Long    '// Holds error status if any from API calls.
    Dim lphKey As Long    '// Holds created key handle from RegCreateKey.

    '// This creates a Root entry called "TextMagic"
    sKeyName = "TextMagic" '// Application Name
    sKeyValue = "TextMagic Document" '// File Description
    ret = RegCreateKey&(HKEY_CLASSES_ROOT, sKeyName, lphKey)
    ret = RegSetValue&(lphKey&, Empty, REG_SZ, sKeyValue, 0&)

    '// This creates a Root entry called .tmg associated with "TextMagic".
    sKeyName = ".tmg" '// File Extension
    sKeyValue = "TextMagic" '// Application Name
    ret = RegCreateKey&(HKEY_CLASSES_ROOT, sKeyName, lphKey)

    ret = RegSetValue&(lphKey, Empty, REG_SZ, sKeyValue, 0&)

    '//This sets the command line for "TextMagic".
    sKeyName = "TextMagic" '// Application Name
    If Right$(App.Path, 1) = "\" Then
        sKeyValue = App.Path & App.EXEName & ".exe %1" '// Application Path
    Else
        sKeyValue = App.Path & "\" & App.EXEName & ".exe %1" '// Application Path
    End If
    ret = RegCreateKey&(HKEY_CLASSES_ROOT, sKeyName, lphKey)
    ret = RegSetValue&(lphKey, "shell\open\command", REG_SZ, sKeyValue, MAX_PATH)
End Sub

Private Sub Form_Load()
    '// ensure we only register once. When debugging etc, remove the SaveSetting line, so your program will
    '// always attempt to register the file extension
    If GetSetting(App.Title, "Settings", "RegisteredFile", 0) = 0 Then
        '// associate tmg extension with this app
        MakeDefault()
        SaveSetting(App.Title, "Settings", "RegisteredFile", 1)
    End If
   '// check command line argument:
    If Command$ <> Empty Then
        '// we have a file to open
        '// Fetch the file name from Command$ and then read the file if needed.
    End If

End Sub