The Basic Animation Mechanism in Egui

As an immediate-mode UI framework, egui handles animations in a fundamentally different way from traditional retained-mode UI frameworks. Instead of maintaining an explicit animation object, egui provides helper functions such as ui.animate_bool_with_time() -> f32 to track animation progress internally.
The method takes three arguments: an Id (a unique identifier for the animation state), a Boolean flag, and a duration specified as a 32-bit floating-point value. Internally, the egui runtime caches a floating-point state associated with the given Id. This value smoothly transitions from 0.0 to 1.0, or reverses from 1.0 back to 0.0, depending on the state change of the Boolean flag.
During each repaint cycle, egui retrieves the cached animation state using the corresponding Id and updates its progress. The Boolean flag determines the animation direction, while the returned floating-point value represents the current animation progress. Developers can then use this value to control rendering parameters, such as opacity, position, or size.
Insight: Although egui's built-in animation functions are convenient, they are not always sufficient for more sophisticated animation requirements. In such cases, developers should manually manage animation progress using ui.time(). These helper animation functions implicitly maintain internal state, such as a floating-point progress variable, behind the scenes. Understanding this underlying mechanism is essential for designing more advanced animations and for gaining a deeper understanding of egui's immediate-mode paradigm.