Layout is harder than you think..

2026-06-24 · Source: ThePrimeagen · Field: Technology & Digital — Software Development & Engineering, Gaming & Interactive Media · Depth: Advanced, extended

Summary

An immediate mode UI layout system has been developed in Odin, offering an alternative to traditional retained mode UIs like the DOM. This system regenerates the entire UI each frame, handling up to 50-100 elements with minimal overhead. Its layout algorithm employs a post-order traversal to calculate element sizes, starting from leaf nodes (fixed, text, or sprite-defined sizes) and propagating upwards, considering padding and gaps. Mouse interactions are managed by using the previous frame's layout for hit testing, resolving the challenge of dynamic element states. This approach simplifies UI state management and animation integration compared to retained mode. Developer experience is enhanced through "start"/"end" functions and a temporary allocator that automatically frees memory each frame.

Key takeaway

For software engineers building performance-sensitive UIs or game interfaces, adopting an immediate mode UI paradigm can significantly simplify state management and interaction logic. By regenerating the UI each frame and leveraging techniques like post-order layout traversal and temporary allocators, you can achieve a more declarative and less stateful UI codebase. Consider implementing "start"/"end" functions for cleaner UI construction and snapshot testing to ensure visual consistency across iterations.

Key insights

Immediate mode UI simplifies state management and interaction logic by regenerating the UI each frame based on current state.

Principles

• Immediate mode UIs regenerate the entire UI per frame.
• Post-order traversal calculates UI element sizes.
• Temporary allocators simplify transient UI memory.

Method

Implement layout via post-order traversal: size leaf nodes (fixed, text, sprite), then parent boxes based on children, padding, and gaps. Position elements from calculated sizes. Handle mouse interactions using the previous frame's layout state.

In practice

• Use "start"/"end" functions for declarative UI.
• Employ temporary allocators for per-frame UI memory.
• Implement snapshot testing for UI consistency.

Topics

• Immediate Mode UI
• UI Layout Algorithms
• Odin Programming Language
• Temporary Allocators
• Game Development
• Snapshot Testing

Best for: Software Engineer, Research Scientist

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Editorial summary, takeaway, and curation by AIssential. Original article published by ThePrimeagen.