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Recursion: Emptying a Folder

Do you need code to delete the files and folders from the selected folder? In that case, there is a technique I propose to you and that is based on recursion. That is, the ability of a function to call itself repeatedly until the task is complete; in this case, the function will call itself every time it detects a new folder/directory inside the designated original folder/directory.

But, beware! The following implementation will delete all the files and original containing folder every time it finds an alias… not just the alias. Anyway, it is easy to change this behavior in case you just want to delete the alias and not the original it points to.

As you probably already know, when we work with files or folders in Xojo we are actually using the FolderItem class, so we are able to access to all its properties and the methods acting on every instance: changing the file/folder/directory name, getting the creation date, create a copy, to move the file and/or folder to a new destination … and also for deleting the file or folder.

In addition, recursion is a technique that is not always appropriate because the function adds a new frame of data to the execution stack every time it calls itself, something that can cause a stack overflow error if it calls itself a large amount amount of times. This may occur, for example, if a folder or directory has many nested folders. To alleviate this, we can resort to the mechanism provided by the Xojo language itself and gracefully catch this situation on runtime.

Having said this, what is the best way to implement our recursive function so it is available to all the FolderItem instances? You may create a new FolderItem subclass, but that would mean that the method would only be available from the instances created from the subclass, and not for those created from the parent FolderItem class. The answer in this case is to apply it as a Class Extension technique. This technique allows us to add additional functionality to a existing class without needing to subclass it previously.

The way to add a Class Extension in Xojo is through a Module, so it gives us the option to set the method as globally available from all the app code. This way, the Xojo compiler will know that we want to extend a particular class because we use the Extends keyword in the method definition, followed by the variable whose data type designates the class we want to extend.

So, our method (or function) signature will be:

DeleteAllFilesInside(extends f as FolderItem)

Once we’ve added that method to a Module it’s important to set the Scope to the value Global in the Inspector. Now we can put the following code inside the associated Code Editor for the method, so this is the responsible of deleting the files and call itself every time it detects a new folder/directory:

  // We keep the initial (designated) FolderItem
  // So it doesn't get deleted.

  Static originFolder As String = f.NativePath

  // Is the pointed FolderItem a Directory?
  // then proceed to iterate the containing
  // files and (probably) folders.

  If f.Directory Then

    For n As Integer = f.Count DownTo 1

      If f.Item(n) <> Nil Then

        // Are we detecting a new folder
        // while iterating the current one?
        // then we call the function again!

        If f.item(n).Directory And f.Count <> 0 Then

          Dim f1 As FolderItem = f.item(n)


        End If

        // Otherwhise, we proceed to delete the file/folder
        // always it is not the initial FolderItem
        // that is, the initial folder/directory

        If f.item(n) <> Nil And f.item(n).NativePath <> originfolder Then f.item(n).Delete

      End If
  // we are outside a folder, so let's check
  // that the pointing FolderItem is not the initial one
  // if it doens't, then we can delete the file/folder

  If f <> Nil And f.NativePath <> originFolder Then f.Delete

Javier Rodri­guez has been the Xojo Spanish Evangelist since 2008, he’s also a Developer, Consultant and Trainer who has be using Xojo since 1998. He manages and is the developer behind the GuancheMOS plug-in for Xojo Developers, Markdown Parser for Xojo, HTMLColorizer for Xojo and the Snippery app, among others.

*Watch a video of this Tutorial in Spanish

*Read this post in Spanish