dialogs - Allow multiple selection on Open...

[rschnekenbu]

What it does

fixes #12905 - Allow multiple selection on Open...

• allow multiple selection in doOpen dialog
• handle an array or a single URI or undefined when closing dialog

Contributed on behalf of ST Microelectronics

How to test

When running Theia in browser, select the Open... dialog:

• It is now possible to open multiple files at once.
• It is also possible to select multiple folders. In that case, all folders will be open as folders in a new workspace (as done in Electron).
• In case of heterogeneous selection, all files will be opened in current workspace, and the folders will be opened together as a new workspace.

Electron has 2 different menus to select either Folder or Files:

• This patch updates the Open file... selection dialog to also be able to select multiple items. If the selection has several files and a folder selected in the dialog, the open button of the dialog won't react. It is supposed to browse the folder if there is only one folder selected.
• The open folder command already supports multiple folder selection, creating a new workspace containing all folders. No updates were done to this command.

Follow-ups

No Follow-up was identified during this task development.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

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[tsmaeder]

This could be simpler by doing

if (!Array.isArray(destinationUri)) {
   destinationUri= [ destinationUri] 
}

and omitting the else branch and the added method.

[tsmaeder]

On a general note, I think APIs that return T | T[] don't make sense in many cases: if you care about the "single element" case, you can still check for length === 1 and if you don't care, you don't have to handle the case.

[rschnekenbu]

Thanks for the hint to simplify algorithm! Indeed, that's better to handle all cases at once.
I made explicit in the signature of the doOpen... methods that an array of elements will be returned, getting rid of the MaybeArray. I kept however the signature of the showOpenDialog service with this MaybeArray, to avoid compatibility issues.

[tsmaeder]

Code looks good to me, now