Access single file or folder from IStorageFolder by name

[nickodei]

What does the pull request do?

This PR addresses #17276. It allows to access a single file or folder by name given you have already a folder (IStorageFolder).

What is the current behavior?

You can currently archive this by iteration over the whole folder with GetItemsAsync() and pick the file with the given name. This has performance considerations and therefore, when applicable, the implementations tries to access the file directly.

What is the updated/expected behavior with this PR?

When you get a folder (IStorageFolder) e.g through a file-picker, you can access the child file/folder by name directly, returning IStorageFile or IStorageFolder depending on the called method.

How was the solution implemented (if it's not obvious)?

The interface IStorageFolder was extended as follows:

interface IStorageFolder
{
     Task<IStorageFolder> GetFolderAsync(string name);
     Task<IStorageFile> GetFileAsync(string name);
}

Therefore, an implementation for the following platforms was provided:

• Android (DocumentsContract and the query method)
• Browser (FileSystemDirectoryHandle)
• Desktop (.NET Path.Combine)
• IOS (.NET Path.Combine => NSUrl)

Testing

Tested it on following platforms:

• Desktop
• Android
• Browser
• IOS (dont have an IOS device)

Extended the ControlCatalog for testing:

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

No breaking changes, this only extends the public interface

Fixed issues

Fixes #17276

[nickodei]

I was not able to have an Android-Implementation that does not iterate trough the whole folder to find the first Item with the given name. The other thing is, that I don't have an IOS device to test the IOS-implementation.

[nickodei]

Questions:

• Should I return IStorageFolder or IStorageFolder? ? => When nullable, which Excetions should be catched and which should be thrown (e.g getDirectoryHandle throws 4 different Exceptions. Should I catch all of them and return null?)
• Can I write tests for it or is it enough that I manually tested it?
• How to I resolve the build error?