Understanding UIKit’s Auto Layout System: Calculating and Rendering Views with Constraints 🧙🏻
Introduction
Despite SwiftUI being the buzzword of 2023, I’d like to take a step back and discuss some intriguing UIKit-related topics 🔎
With the release of iOS 6, Auto Layout revolutionised how we view the arrangement of UI elements ✨
Auto Layout is a powerful feature of UIKit, Apple’s user interface framework, that allows developers to create adaptive and flexible user interfaces. It provides a way to define the relationships between views using constraints, which are used to calculate and render views on the screen. This article aims to provide a comprehensive understanding of how UIKit’s Auto Layout system works behind the scenes.
Constraints
Constraints are rules that define the position and size of views relative to each other or the parent view. They specify the distance between views, alignment, aspect ratio, and other layout properties. Auto Layout uses these constraints to calculate the optimal layout for views based on factors such as device orientation, screen size, and content size.
UIKit’s Auto Layout system employs a constraint-based layout engine that automatically calculates the positions and sizes of views based on their constraints. This engine takes into account the available space, priorities assigned to constraints, and content compression and hugging prioritise.
Calculation phase
During the calculation phase, Auto Layout analyzes the defined constraints and uses them to determine the frames (position and size) of each view. The process involves solving a system of linear equations known as the constraint satisfaction problem.
Auto Layout uses an algorithm called Cassowary, which is based on an incremental constraint solver. It starts with an initial estimate for view frames and iteratively adjusts them until all constraints are satisfied. The process continues until a stable solution is reached.
Rendering phase
Once the calculation phase is complete, Auto Layout applies the calculated frames to render views on the screen. The rendering phase involves converting the frames into pixel coordinates appropriate for the device’s screen resolution.
Auto Layout also takes into consideration other factors such as safe area insets (to avoid overlapping with system elements like notches or home indicators), content insets, and margins specified in the constraints.
UIKit’s Auto Layout system is designed to support dynamic layouts that can adapt to changes at runtime. When a change occurs, such as device rotation or changes in the view hierarchy, Auto Layout automatically recalculates and updates the layout based on the existing constraints.
Triggering layout updates manually
Developers can also manually trigger layout updates by calling the `layoutIfNeeded()` method on a view or its superview. This is useful when making changes to constraints programmatically or when explicitly forcing a layout update.
Resources:
Solving Linear Arithmetic Constraints for User Interface Applications (1997) by Alan Borning, Kim Marriott, Peter Stuckey, and Yi Xiao
