[Lazarus] LCL and Lazarus Accessibility - restarting the discussion

David Jenkins david at scootersoftware.com
Fri Mar 5 18:04:37 CET 2021

On 3/5/21 10:44 AM, Marco Zehe via lazarus wrote:
> Hi David!
> Wow, that sounds exciting! Look forward to your patch.
> I did some digging, and found that on Windows, TTreeView does not create any handles at all yet. It would require at least some basics there, or even accessibility properties, but I am still hoping we can avoid what I had to do for TVirtualStringTree in 2007, implementing literally two IAccessible interfaces, one for tree, one for the focused tree item. That all was managed by a singleton class instance, and juggled quite a bit. Pretty unreliable, so I hope we can make it better in TTreeView for the LCL.
> Right now, however, I don’t even know where to start yet to implement this. There is a unit in the Win32 folder, but nothing in there besides the comments. And from what it sounds like, it makes sense to wait for your patch so I, or we, can then build on that and make the Windows implementations better.
> Marco

For any tree type objects that I have dealt with (both LCL and custom) I 
have created Ax objects for the tree and Ax objects for the items as 
well.  And in cocoa I had to create an additional layer: Tree/Row/Item 
in Row.

A couple of deficiencies that I have run across with the LCL 
accessibility layer are

1) No notification capability

both Cocoa and Qt include functionality to send notifications to Ax 
Clients to let them know of changes to the UI

2) TLazAccessibleObject handle creation doesn't sync well with 
TLazAccessibleObject.Ownercontrol handle creation.

Both Cocoa and Qt include accessibilty support in their base widgets 
that can be leveraged as TlazAccessibileObject handles.  But 
TLazAccessibleObject will request a handle before the associated 
TWinControl has handleallocated and doesn't know to retry for a handle 
after the TWinControl get's its handle.

It would be good to include a path where TLazAccessibleObject handle is 
requested after the TWinControl.Handle is created.

I will note that reading the comments surrounding the 
TLazAccessibleObject code in Controls and control.inc are important.  I 
made a couple of bad assumptions about how the code worked before I read 
all the comments.

The first was that TLazAccessibleObject.Children objects are not meant 
for TControl or TWinControl objects.  They are only for visual items 
that don't have a TControl or TWinControl object.  E.g. row items in a 
tree view.

And the difference in getting children by either using the iterator or 
doing First / Next.  The iterator returns the objects in the child tree 
only.  First / Next will get the objects from the child tree but also 
any TControls that are in the ownercontrol.Controls() array.  The latter 
allows code to find ALL the children - visual only and TControl objects.

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