wx.lib.agw.thumbnailctrl should be able to preload thumbnails

Operating system:all wxPython version & source: master Python version & source: python3

Description of the problem: https://github.com/wxWidgets/Phoenix/blob/fd1c26d2e14fc0cee3e03a794c49446f9f45e2af/wx/lib/agw/thumbnailctrl.py#L11 The thumbnail view (wx.lib.agw.thumbnailctrl) cannot preload images which sucks for user experience with large images, as the thumbnails are loaded only after the thumbnail grid is moved into view. It would be better if in addition to loading the currently shown area some upcoming thumbnails were loaded, and some were retained to speed up scrolling backwards.

For the purposes of this feature let a page be a float defined as page = scroll_y/viewport_height

Ideally there should be two parameters which dictates how many "pages" of thumbnails should be preloaded both before and after the current page.

Calculate the entire width of the scrollable view and divide it into N pages. If at any time part of a page is visible it should stay loaded and its preload rules should apply.

Which pages are in view should be checked every frame the scrollbar is being dragged, not only when it is release.

e.g.

Suppose we set the preload to 1 page

At the top of the scroll view, 1 full scroll ahead would be loaded
Scrolling down one page would move the cursor from page 0 to page 1, so page 0 would be retained and page 2 would be loaded.
Scrolling to page 2 would unload page 0 and load page 3
Scrolling back up to page 1.5 would mean both page 1 and 2 are loaded, so page 0 would be loaded and page 3 would stay loaded, because page 2 is still visible.

Suppose we set the preload to 2 pages

At the top of the scroll view, 2 full scrolls ahead would be loaded
Scrolling down one page would move the cursor from page 0 to page 1, so page 0 would be retained and page 3 would be loaded.
Scrolling to page 3 would cause page 0 to unload and page 4 and 5 to load
Scrolling back to page 2.5 would mean page 2 and page 3 are in view so page 5 will not be unloaded, and page 0 must be loaded.

Example code

import os

import wx
import wx.lib.agw.thumbnailctrl as TC

class MyFrame(wx.Frame):

    def __init__(self, parent):

        wx.Frame.__init__(self, parent, -1, "ThumbnailCtrl Demo")

        panel = wx.Panel(self)

        sizer = wx.BoxSizer(wx.VERTICAL)

        thumbnail = TC.ThumbnailCtrl(panel, imagehandler=TC.NativeImageHandler)
        sizer.Add(thumbnail, 1, wx.EXPAND | wx.ALL, 10)

        thumbnail.ShowDir(os.getcwd())
        panel.SetSizer(sizer)

       panel.SetPreload(2) #Load 2 full scrolls ahead and behind including the current view (5 total)

Preload and Retain

You might also consider splitting preload and retain. But this may needlessly complicate the implementation for not much more flexibility.

Define a preload window, if a page within the preload window is not loaded then load it.
Define an retain window, if a page is loaded, but is not inside the retain window then unload it.

wxWidgets / Phoenix