Understanding SwiftUI view lifecycles – Ole Begemann

I wrote an app referred to as SwiftUI View Lifecycle. The app permits you to observe how completely different SwiftUI constructs and containers have an effect on a view’s lifecycle, together with the lifetime of its state and when onAppear will get referred to as. The code for the app is on GitHub. It may be constructed for iOS and macOS.

Once we write SwiftUI code, we assemble a view tree that consists of nested view values. Situations of the view tree are ephemeral: SwiftUI continuously destroys and recreates (elements of) the view tree because it processes state modifications.

The view tree serves as a blueprint from which SwiftUI creates a second tree, which represents the precise view “objects” which can be “on display screen” at any given time (the “objects” could possibly be precise UIView or NSView objects, but additionally different representations; the precise which means of “on display screen” can fluctuate relying on context). Chris Eidhof likes to name this second tree the render tree (the hyperlink factors to a 3 minute video the place Chris demonstrates this duality, extremely beneficial).

The render tree persists throughout state modifications and is utilized by SwiftUI to determine view id. When a state change causes a change in a view’s worth, SwiftUI will discover the corresponding view object within the render tree and replace it in place, relatively than recreating a brand new view object from scratch. That is in fact key to creating SwiftUI environment friendly, however the render tree has one other necessary perform: it controls the lifetimes of views and their state.

We will outline a view’s lifetime because the timespan it exists within the render tree. The lifetime begins with the insertion into the render tree and ends with the removing. Importantly, the lifetime extends to view state outlined with @State and @StateObject: when a view will get faraway from the render tree, its state is misplaced; when the view will get inserted once more later, the state will likely be recreated with its preliminary worth.

The SwiftUI View Lifecycle app tracks three lifecycle occasions for a view and shows them as timestamps:

• @State = when the view’s state was created (equal to the beginning of the view’s lifetime)
• onAppear = when onAppear was final referred to as
• onDisappear = when onDisappear was final referred to as

The app permits you to observe these occasions in numerous contexts. As you click on your means by way of the examples, you’ll discover that the timing of those occasions modifications relying on the context a view is embedded in. For instance:

• An if/else assertion creates and destroys its baby views each time the situation modifications; state will not be preserved.
• A ScrollView eagerly inserts all of its youngsters into the render tree, no matter whether or not they’re contained in the viewport or not. All youngsters seem immediately and by no means disappear.
• A Checklist with dynamic content material (utilizing ForEach) lazily inserts solely the kid views which can be at the moment seen. However as soon as a toddler view’s lifetime has began, the listing will preserve its state alive even when it will get scrolled offscreen once more. onAppear and onDisappear get referred to as repeatedly as views are scrolled into and out of the viewport.
• A NavigationStack calls onAppear and onDisappear as views are pushed and popped. State for mum or dad ranges within the stack is preserved when a toddler view is pushed.
• A TabView begins the lifetime of all baby views immediately, even the non-visible tabs. onAppear and onDisappear get referred to as repeatedly because the consumer switches tabs, however the tab view retains the state alive for all tabs.

Listed below are a couple of classes to remove from this:

• Totally different container views could have completely different efficiency and reminiscence utilization behaviors, relying on how lengthy they preserve baby views alive.
• onAppear isn’t essentially referred to as when the state is created. It will possibly occur later (however by no means earlier).
• onAppear could be referred to as a number of occasions in some container views. When you want a facet impact to occur precisely as soon as in a view’s lifetime, think about writing your self an onFirstAppear helper, as proven by Ian Eager and Jordan Morgan in Working Code Solely As soon as in SwiftUI (2022-11-01).

I’m positive you’ll discover extra fascinating tidbits once you play with the app. Suggestions is welcome!