SwiftUI View re initialized but body property not re computed and view not re rendered

Question

How can a View being re initialised in SwiftUI but the body not recomputed and the view re rendered?

Example

import SwiftUI

struct ContentView: View {
    var body: some View {
        SubView()
            .padding()
    }
}

struct SubView: View {
    
    @State private var items = [Int]()
    
    init() {
        print("SubView init")
    }
    
    var body: some View {
        Text("\(items.count)")
        Button("Add item") { items.append(Int.random(in: 0..<10)) }
        Divider()
        SubSubView()
        SubSubView()
    }
}

struct SubSubView: View {
    init() {
        print("SubSubView init")
    }
    
    var body: some View {
        Text("I'm a subview") (>> _breakpoint here_)
    }
}

If I tap 4 times on the button, the view is the following:

But we have that in the console:

SubView init (>> app launch)
SubSubView init (>> app launch)
SubSubView init (>> app launch)
SubSubView init
SubSubView init
SubSubView init
SubSubView init
SubSubView init
SubSubView init
SubSubView init
SubSubView init

The SubSubView body property is not recomputed (using a breakpoint). Is this SubSubView re rendered only when there is any change in the view dependency, even if it's re initialized again?

This is SwiftUI renderer optimization - it creates view value (as it is struct), but re-renders it (calling body) only if there were some changes since last draw. — Asperi, Nov 14 '20 at 19:37
It would be more helpful if you could tell us why you need to re-initialize a view, since I don't think it's possible like that, or at least not good practice to force re-render in SwiftUI — thisIsTheFoxe, Nov 15 '20 at 08:40
@thisIsTheFoxe actually I was more trying to understand how the life cycles between the struct and the view differ. And also to understand when and why properties from the ```SubView``` (like a ViewModel) were re initialized when the ContentView was re rendered (for example if I provide a custom Init() for a ```View```, then the property is probably re initialized, except if I use the ```@StateObject``` property wrapper). — alpennec, Nov 15 '20 at 09:14

thisIsTheFoxe · Answer 1 · 2020-11-15T09:32:43.817

Now, I'm not an expert and don't have any sources for this, but I could imagine it something like this:

When you change the items variable, SwiftUI has to find out which subviews are depending on that value so that they can be re-rendered with the correct (updated) values. One easy way to find out if an object has changed is via hashes. So, I think SwiftUI (internally) re-initializes the view, computes a hash over the struct and if it differs from the hash of the current view, it gets re-rendered. Otherwise, the change (of items) didn't effect the view so it doesn't need to be re-rendered.

EDIT: for clarity, a computed property (like body) isn't initialzed when the rest of the object is. It is computed on demand. That is why you can also use other variables in them.

SwiftUI View re initialized but body property not re computed and view not re rendered

1 Answers1