I've used Unity3D's immediate mode a lot, both the editor API and the runtime API (and I've also implemented and used a lot of different retained mode GUIs), and I'm much happier with the newer "retained mode" GUI in Unity3D.
The problem with immediate mode that is you have to come up with somewhere to store and retrieve any values or state required on a case-by-case basis (including the edited value itself, and other view state like scrollbar state for text editors, etc), and that tightly couples your component function with whatever's using it, so they're not very reusable or simple. With OOP, the object has its own place to store that stuff, which is cleanly decoupled from whatever's using the component.
Then there's the issue of event handlers. Some user interface components just aren't so simple that they only have one true/false return value like buttons. Text editors can notify on value change, end edit, select, deselect, etc. And Unity's retained mode GUI supports persistent event handlers that let designers hook up events with methods of objects with parameters, without writing or recompiling any code.
And there's also a lot more to layout that you can easily express with an immediate mode GUI. Sometimes you have to measure a bunch of things and align them in various ways, like grids or flex layouts, and adapt to the screen size and other constraints (i.e. responsive design), and that's really hard to do with immediate mode, while retain mode has a rich set of layout components you can use and extend. And there's nothing like XCode's layout constraints for immediate mode.
A perfect example is TextMesh Pro's text editor. It has so many different (and useful) features and parameters and callbacks and ways to configure it, and it keeps so much state in order to redisplay efficiently with the fewest number of draw calls and reformatting, that it would be extremely impractical and inefficient and unwieldy for it to use an immediate mode API. Especially with C# positional arguments instead of Pythonic or Lisp-like optional unordered keyword arguments.
When you encapsulate your immediate mode layout in a class, or write an engine that interprets (i.e. JSON) data to drive the immediate mode API, or use reflection in your wrapper functions to dispatch event callbacks and bind your widget data to your model data, you're just rolling your own informally-specified, bug-ridden, slow implementation of half of retained mode. (See Greenspun's tenth rule.)
The problem with immediate mode that is you have to come up with somewhere to store and retrieve any values or state required on a case-by-case basis (including the edited value itself, and other view state like scrollbar state for text editors, etc), and that tightly couples your component function with whatever's using it, so they're not very reusable or simple. With OOP, the object has its own place to store that stuff, which is cleanly decoupled from whatever's using the component.
Then there's the issue of event handlers. Some user interface components just aren't so simple that they only have one true/false return value like buttons. Text editors can notify on value change, end edit, select, deselect, etc. And Unity's retained mode GUI supports persistent event handlers that let designers hook up events with methods of objects with parameters, without writing or recompiling any code.
And there's also a lot more to layout that you can easily express with an immediate mode GUI. Sometimes you have to measure a bunch of things and align them in various ways, like grids or flex layouts, and adapt to the screen size and other constraints (i.e. responsive design), and that's really hard to do with immediate mode, while retain mode has a rich set of layout components you can use and extend. And there's nothing like XCode's layout constraints for immediate mode.
A perfect example is TextMesh Pro's text editor. It has so many different (and useful) features and parameters and callbacks and ways to configure it, and it keeps so much state in order to redisplay efficiently with the fewest number of draw calls and reformatting, that it would be extremely impractical and inefficient and unwieldy for it to use an immediate mode API. Especially with C# positional arguments instead of Pythonic or Lisp-like optional unordered keyword arguments.
When you encapsulate your immediate mode layout in a class, or write an engine that interprets (i.e. JSON) data to drive the immediate mode API, or use reflection in your wrapper functions to dispatch event callbacks and bind your widget data to your model data, you're just rolling your own informally-specified, bug-ridden, slow implementation of half of retained mode. (See Greenspun's tenth rule.)
https://en.wikipedia.org/wiki/Greenspun%27s_tenth_rule