Cocoa Internals/Widgetset

From Lazarus wiki


THANDLE - an opaque reference value that's being generated during .CreateHandle() call of a particular Widgetset class. THANDLE is the value that's used to interface with a control. Using the value a widgetset recognizes a particular control.

Most of Widget interface methods are accepting TWinControl as a parameter. It's assumed that Handle property of the control would be used to back reference the actual control. The property stores the value received from CreateHandle() call made earlier. HandleAllocated property can be checked in order to know, if handle has been previously created or not. If handle was not created it's expected that the method can fail (returning the proper failure information. Exceptions are not expected?).

Overall LCL Handles refer to a complex controls. For example HANDLE can refer to a Memo control, that's capable of editing AND scrolling text as needed.

Cocoa's composite engine operates controls on a lower level scope. With MEMO example, there's not a single control that does both: text editing and scrolling.

Instead there are two separate controls: NSTextView and NSScrollView. (Both are descendant of NSView). But LCL expects only 1 value to be returned as a handle.

The common rule is the following: the outter bounds (aka container) control is returned as the handle. Thus in case of Memo, two NSViews are allocated: NSTextView and NSScrollView. But the outter NSScrollView is returned as HANDLE. All TCustomMemoWSControl methods are aware of that and treat handle as such. For example:

  • if paste action is required, the handle would be used as NSScrollView and then it's documentview would be used to get NSTextView to call for paste.
  • if bounds changes are required (moving, resizing) then the sizes are changed directly for the NSView referenced by HANDLE. (It's assumed that the view would automatically resize its controls. In case of TMemo, NSScrollView would adjust size of NSTextView, if wordwrapping is off).
  • Forms (TForm), that generally correspond to NSWindow, also return its handle as NSView. That view represents the root NSView of a form. However, CocoaWS API's know that and are calling necessary NSWindow methods where applicable.

HandleView are stored within Callback object that's being created for each CreateHandle() method.

Handles Reference

  • Window handle (HWND) is always NSView.
    • TCustomWSForm it is content NSView.
    • Any control that has scroll bars (i.e. TCustomWSList) the it its the embedding NSScrollView (TCocoaScrollView)
LCL Control WS Part Cocoa Class (NS class) / Handle Notes
TScrollBar Std TCocoaScrollBar (NSScrollBar)
TGroupBox Std TCocoaGroupBox (NSBox) There's also a content view created to hold all the child controls within the box
TComboBox Std TCocoaComboBox (NSComboBox)


TCocoaReadOnlyComboBox (NSPopUpButton)

The actual type is defined by the style of LCL combobox (readonly vs editable)
TListBox Std TCocoaTableView (NSTableView) / NSScrollView The behavior of TListbox is defined by the callback object TLCLListBoxCallback assigned to TCocoaTableView.
TEdit Std TCocoaTextField (NSTextField) TCocoaTextField itself is readonly static field. The editing capabilities are implemented by TCocoaFieldEditor, which is driven by AppKit "editing" schema
TMemo Std TCocoaTextView (NSTextView) / NSScrollView Lines property is implemented via TCocoaMemoStrings
TButton Std TCocoaButton (NSButton)
TCheckBox Std TCocoaButton (NSButton) Checkbox is still a button but with a different Bezel
TToggleBox Std TCocoaButton (NSButton)
TRadioButton Std TCocoaButton (NSButton) Radiobutton is still a button but with a different Bezel
TStaticText Std TCocoaTextField (NSTextField) This field doesn't allow editor to be enabled
TTrackBar Com TCocoaSlider (NSSlider)


That has important impact of FOCUS-ing. Cocoa doesn't operate the term "focused" instead it's using terms "firstRepsonder" and "key" view/window. Generally focused control corresponds to Cocoa's "firstResponder". The NSResponder object (typically NSView) is the first object that would handle key or mouse events. However, this is not the case for composed controls. TMemo is a combination of NSScrollView and NSTextField. NSScrollView is used as a handle. If, NSTextField would be the firstRepsonder, CocoaWS needs to return NSScrollView as focused HANDLE.

The same approach is applicable in reverse. If SetFocus is made for the HANDLE that represents NSScrollView with NSTextField inside. NSTextField should actually become Cocoa's firstResponder.

The expected order of events is:


Note that unlike WinAPI WM_KILLFOCUS, LM_KILLFOCUS doesn't provide the information about "the next suggested" control to be focused. For controls that have some sort of editors (i.e. TTreeView with a node editor (TEdit) active) such approach is causing to switch focus to TForm (the hosting form), for the next focus control selection.

The focus switch notification is handled in TCocoaWindow makeFirstResponder call. Due to problems with the recursive calls of changing the focus, the current order of events is:


(for Cocoa WS)

Need of Window

The "firstResponder" is a property of a window. Thus a view can become a "firstResponder" only if it's within NSWindow's view hierarchy. This is important for complex controls, such as a TabControl. It manages multiple child views (each view for a tab). Only one view is visible at a time. All over views are actually DETACHED from NSWindow hierarchy. (while from LCL perspective they are still children of TForm). When switching between tabs a notification about a successful switch should only be delivered when a new view is in the Window hierarchy.

Ignoring System Setting

LCL basic logic is to focus every control that is a tab stoppable (which is every TWinControl by default) user. Where in macOS there's a system setting of Full Keyboard Access (which is off by default). If the setting is off then macOS acts similar to what LCL does. If it's on only "keyboard input" controls are "tab-reachable" text fields (including editable combo-boxes) and lists. Buttons, scrollers are skipped from tabbing. LCL doesn't currently support limited tabbing.


Widgetset cursors are based on NSCursor class. Which very nice and convenient to use. Cocoa however, doesn't provide some of the cursors that do exist in windows/LCL. For example: diagonal resize. Diagonal cursors are generated by rotation of the horizontal cursors.

User Prompts and Modal Dialogs

Most of the dialogs are based on NSModal class.

With the switch of rendering engine to layered drawing (effective macOS 10.14), an attempt to show a modal dialog during rendering (or any other stage of animation. I.e. "Activation") would cause an exception:

Application Specific Information:
*** Terminating app due to uncaught exception 'NSGenericException', reason: '-[NSAlert runModal]
may not be  invoked inside of transaction begin/commit pair, or inside of transaction commit 
(usually this means it was invoked inside of a view's -drawRect: method.)'

However, if the error dialog is shown as a "sheet" then the error doesn't occur.

See Also