Understanding SwiftUI view lifecycles – Ole Begemann

I wrote an app known as SwiftUI View Lifecycle. The app means that you can 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 known as. The code for the app is on GitHub. It may be constructed for iOS and macOS.

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

The view tree serves as a blueprint from which SwiftUI creates a second tree, which represents the precise view “objects” which are “on display” at any given time (the “objects” could possibly be precise UIView or NSView objects, but in addition different representations; the precise that means of “on display” can range 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 really useful).

The render tree persists throughout state adjustments 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, slightly than recreating a brand new view object from scratch. That is after all key to creating SwiftUI environment friendly, however the render tree has one other vital operate: 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 elimination. 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 shall 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 known as
• onDisappear = when onDisappear was final known as

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

• An if/else assertion creates and destroys its little one views each time the situation adjustments; state isn’t preserved.
• A ScrollView eagerly inserts all of its kids into the render tree, no matter whether or not they’re contained in the viewport or not. All kids seem immediately and by no means disappear.
• A Checklist with dynamic content material (utilizing ForEach) lazily inserts solely the kid views which are presently seen. However as soon as a toddler view’s lifetime has began, the record will hold its state alive even when it will get scrolled offscreen once more. onAppear and onDisappear get known 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 dad or mum ranges within the stack is preserved when a toddler view is pushed.
• A TabView begins the lifetime of all little one views immediately, even the non-visible tabs. onAppear and onDisappear get known as repeatedly because the person switches tabs, however the tab view retains the state alive for all tabs.

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

• Totally different container views might have completely different efficiency and reminiscence utilization behaviors, relying on how lengthy they hold little one views alive.
• onAppear isn’t essentially known as when the state is created. It could actually occur later (however by no means earlier).
• onAppear may be known as a number of instances in some container views. In case you want a aspect impact to occur precisely as soon as in a view’s lifetime, contemplate writing your self an onFirstAppear helper, as proven by Ian Eager and Jordan Morgan in Operating Code Solely As soon as in SwiftUI (2022-11-01).

I’m positive you’ll discover extra attention-grabbing tidbits while you play with the app. Suggestions is welcome!