What are the various GUI models out there?
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EventDriven (GUI engine mostly declarative from app developer's perspective, with optional events that can override or add behavior).
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ImmediateModeGui (contrast with retained mode) is a model frequently used in games. It is also largely used in memory-constrained systems, where the memory overhead of declared object hierarchies is unacceptable (e.g., GEOS/64 allows for only 8 buttons, and text-only menus. If you wish to exceed any of these limits, you necessarily need to resort to something very close to, if not actually using, immediate-mode GUI management).