Notes

SwiftUI View Equality

Main pages: Attribute Graph and Attribute Graph in SwiftUI.

When a view gets marked as potentially dirty in the Attribute Graph, SwiftUI will compare the current and new value. If it can “prove” the view is the same as before, it doesn’t need to reevaluate the view’s body. I’m not 100% sure how this works, this my current mental model:

  • SwiftUI will look at all the properties of the struct and try to compare them. To compare it tries Equatable’s ==, but will also do memcmp.

  • If a view type conforms to Equatable that conformance is used to detect changes. This might not be true, as there is also equatable.

Closures in Swift can’t be compared for equality. This means that storing closures in your view might lead to more view invalidations. See Closures in SwiftUI.

References

John Harper (SwiftUI team) explains this in more detail:

SwiftUI assumes any Equatable.== is a true equality check, so for POD views it compares each field directly instead (via reflection). For non-POD views it prefers the view’s == but falls back to its own field compare if no ==. EqView is a way to force the use of ==.

When it does the per-field comparison the same rules are applied recursively to each field (to choose direct comparison or == if defined). (POD = plain data, see Swift’s _isPOD() function.)

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