Using Operator_Lookup

General
1

I’ve been using an example application (based on this blog post: https://blog.xojo.com/2014/01/27/saving-preferences/) to create preferences in JSON format. It’s working fine, but I’d like to understand the code in a little more detail (for reasons below [1]). The part that’s tripping me up is the use of Operator_Lookup. In the example it seems to be used to overload the accessor methods Get and Set.

        Function Operator_Lookup(name As String) As Variant
	  // Allow lookup of preference using this syntax:
	  // Dim top As Integer = Preferences.MainWindowTop
	  Return Get(name)
	End Function

	Sub Operator_Lookup(name As String, Assigns value As Variant)
	  // Set a preference using this syntax:
	  // Preferences.MainWindowTop = 345
	  Set(name) = value
	End Sub

However, I can’t see anything that actually uses these overloaded methods.

I’ve looked up Operator_Lookup in the documentation, where it says “If you implement Operator_Lookup on a class, you will no longer get compilation errors if you, for example, inadvertently misspell a property name. Instead, the Operator_Lookup method is called at runtime and is passed the name of your incorrectly spelled property.” I’m afraid I can’t understand this; it seems to be there to catch potential misspelling of property names.

Can someone enlighten me as to how and why this code is used? Can I actually just remove it to simplify the code?

2

BTW this is the “why” that I forgot to put in my post. The example code creates a JSON preferences file on just one level. I want to have a second level of hierarchy. My preferences file needs to look like this:

{
  "WinMainLeft":567,
  "WinMainTop":108,
  "WinPreferencesLeft":675,
  "WinPreferencesTop":254,
  "SelectedPPInstance":0,
  "ppInstances":[
    {
      "UserID":"myuserid1",
      "ServerFriendlyName":"Descriptive name 1",
      "ServerURL":"https:\\/\\/foo.com\\/foldername",
      "ServerDescription":"Description of server connection"
    },
    {
      "UserID":"admin",
      "ServerFriendlyName":"Descriptive name 2",
      "ServerURL":"https:\\/\\/bar.com\\/foldername",
      "ServerDescription":"This is another server connection"
    }
  ]
}

So I am going to need to elaborate my version to allow for more hierarchy. Hence the need to understand exactly how it works!

3

The actual work is done in the .Get and .Set methods which reads from/writes to the .mPreferences JSONItem

4

What Operator_Lookup does is makes it possible to have access to undefined property names. For example, if you define:

Function Operator_Convert(name as String) as String Return name + “_1” End Function

You could do something like this:

dim s as String = myClass.Xyz

“s” would be set to “Xyz_1”

If Foobar is not already defined in the class as a method or property, this method gets called with the undefined name as a string.

NOTE: Be.very careful with this method. Using it can prevent the compiler from showing you error messages for undefined properties.

5

This should be explained here:

http://developer.xojo.com/userguide/advanced-oop-features$operator-lookup