hitTest

Hit testing is a process used in graphical user interfaces to determine which visual element should receive an input event at a given point, such as a mouse click or a touch. It maps coordinates in a window or canvas to a target element within a hierarchy of views or components. The result is typically the frontmost interactive element under the point.

In many frameworks, each visual element provides a method to test whether a point lies within its bounds, often called containsPoint or pointInside. A complementary method, hitTest, processes a point through the element’s subtree to find the frontmost target that should receive the event. The usual approach starts at the root of the view hierarchy, checks that the point lies within the root’s visible bounds, and then descends into subviews in front-to-back order. The first non-null result found during this traversal is the hit.

Key considerations include interactivity flags (such as enabled or userInteractionEnabled), visibility (hidden or fully transparent), and clipping. Coordinate space conversion is important, since points may originate in screen space and must be translated into local coordinates for each element. Transformed views (rotation, scaling, translation) require applying inverse transforms. Per-pixel hit testing may be used for non-rectangular shapes, typically at higher cost than simple bounds checks.

Variants and related concepts include using hit testing for event dispatch, hit-testing for accessibility, and, in 3D environments, ray casting to determine the first object intersected by a ray. Hit testing is fundamental for routing input to the correct control and ensuring intuitive user interactions.