ConstraintLayout

ConstraintLayout is a ViewGroup in the Android UI toolkit designed to position and size child views by defining constraints between them and the parent. It promotes flat view hierarchies and reduces the need for nested layouts, making it easier to build responsive interfaces for different screen sizes and orientations. ConstraintLayout is part of the Android Jetpack libraries and is commonly used in modern Android app development.

Core concepts: Each child declares constraints to one or more other views or to the parent, via anchors on the four sides or center. Constraints determine where a view is placed, while margins and bias control spacing and alignment. Widget size can be fixed, wrap_content, or set to match_constraint (the view expands to satisfy constraints).

Key features include horizontal and vertical chains, guidelines for fixed alignment references, barriers that adapt to referenced views, and helper objects that group views or apply visibility constraints. The Flow widget supports wrapping and flowing items like a grid or list. ConstraintSet allows runtime updates to constraints, and MotionLayout provides animated transitions integrated with the same constraint system.

Tooling and usage: The Android Studio Layout Editor visualizes constraints and can generate XML. Constraints can be defined in XML or constructed at runtime via ConstraintSet and ConstraintLayout APIs. Well-constrained layouts avoid ambiguity and improve adaptation across screen sizes.

Performance and adoption: By keeping a flat view hierarchy, ConstraintLayout reduces layout inflation overhead and improves rendering performance on mobile devices. It is widely used in official samples and app code, and continuously updated with new widgets and capabilities.