[ui] Refactor the access to the list of recent project files

[cbentejac]

Description

This pull request addresses some performance issues that were detected when interacting with the list of recent project files.

It mostly focuses on refactoring the access to the list of recent project files, which involved reading the QSettings every single time the content of the list was to be accessed, and accessing it far too many times.

It also decouples the need for thumbnails between the homepage and the application: while the homepage needs access to thumbnails (if available) to set up the list of recent project files, the application has no use for them. When the list of project files is edited, there is no need to retrieve the corresponding thumbnail (if it exists) unless the current view is the homepage's.

Finally, it prevents the homepage from performing any such request for thumbnails unless it is actually being displayed.

Performance Summary

QSettings used to be read every single time recentProjectFiles was called:

• When getting the list of project files to add a project file.
  • This includes "open file" and "save as" operations.
• When getting the list of project files to remove a project file.
• When getting the list of project files to set the root folder of any file dialog.
• When getting the list of project files to fill the "Open Recent" menu.
• When getting the list of project files to set the model for the homepage.
  • For this specific case, the model for the homepage was updated for every single update to the list (either adding or removing a project file) as many times as there were entries in the list. For example, if there were 30 project files in the list, QSettings were read 30 times just to setup the homepage at launch, and then 30 times again when a file was added (+ 1 time) or removed (-1 time).

=> QSettings are now only read once throughout the lifetime of the Meshroom instance.

Features list

• Reduce the reading of the QSettings to a single occurrence (upon Meshroom launch)
• Refactor the accesses and interactions (addition/deletion) with the list of recent project files
• Separate the update of the project files' path and thumbnail
• Prevent the homepage from requesting updates when it is not the active view

Implementation remarks

Prior to this PR, a call to the recentProjectFiles property necessarily involved a full reading of the QSettings, which provides for each entry both a path to the project file itself and a path to its thumbnail (if it exists). Upon the launch of Meshroom alone, if there were 40 project files in the QSettings, these QSettings were fully accessed and read 40 times just to set the homepage properly.

recentProjectFiles is now only an accessor to a list that is filled and edited in other calls, and which is initialized with a full reading of the QSettings when Meshroom is started, and never again during the lifetime of the instance. QSettings are thus only read once.

From there, any project file that is added or deleted leads to an update of the QSettings, but these settings are not read again to update the content of recentProjectFiles: recentProjectFiles is updated directly within these methods, ensuring it is always accurately reflecting the content of the QSettings.

When a project file is added, only the path to the project is added to the QSettings; there is no attempt to retrieve the thumbnail. However, a flag indicating that there may be some thumbnail paths to retrieve will be set.

When the homepage will become the current view again, it will request an update of the thumbnails. If the flag is set (meaning at least a file has been added to the list), all the project files in recentProjectFiles will be parsed in an attempt to retrieve their thumbnails. Otherwise (meaning that files have either been deleted or nothing has happened), nothing will happen.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.93%. Comparing base (0586336) to head (60029f4).

Additional details and impacted files

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  Coverage    69.93%   69.93%           
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  Files          121      121           
  Lines         7088     7088           
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  Hits          4957     4957           
  Misses        2131     2131           

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