iOS Startup Performance in Practice, Part 1: Understanding Launch Types and Entry Points
Learn how iOS handles cold and warm launches, resume, scenes, deep links, and notifications - and where startup performance measurement should begin and end.
The process of launching an iOS app can vary significantly based on how the user interacts with the application. Apple distinguishes between four types of launch scenarios: cold launch, warm launch, resume, and prewarmed launch.
A cold launch is the most resource-intensive scenario. It occurs when the application process does not exist in memory and needs to be created entirely from scratch. This includes loading the executable and its dependencies from storage, performing runtime initialization, and entering the application at the `main` point. Typical cold launches happen after a device reboot, after an app update, or after a long period of inactivity.
A warm launch, on the other hand, also creates a new process but has the advantage of possibly having recent application state in memory or system caches. This means that during a warm launch, the process can be created again, `main` can execute, the app delegate can be created, and the scene can be connected. However, the first UI still needs to be constructed, and the timings differ slightly from a cold launch.
A resume happens when the user returns to an existing scene, usually from the Home Screen or app switcher. The process may have been suspended, meaning it was kept in memory but not executing application code. When the user resumes the app, iOS resumes the process and moves the scene toward the active state. Unlike a warm launch, the executable is not loaded again, `main` does not execute, and the app delegate is not recreated.
A prewarmed launch is an optimization where some aspects of the launch process are started before the user explicitly opens the app. This allows the system to perform part of the process initialization in advance, potentially shortening the time it takes for the user to see the app after tapping on it. However, in this scenario, your startup implementation still needs to function correctly even if the system decides not to prewarm the process.
Written by urgent.news from HackerNoon's reporting — not their text. Machine-written — may contain errors; check the original before relying on it.
