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HomeiOS DevelopmentWhy Conditional View Modifiers are a Dangerous Concept · objc.io

Why Conditional View Modifiers are a Dangerous Concept · objc.io


Within the SwiftUI neighborhood, many individuals provide you with their very own model of a conditional view modifier. It permits you to take a view, and solely apply a view modifier when the situation holds. It usually seems one thing like this:

								
extension View {
    @ViewBuilder
    func applyIfM: View>(situation: Bool, remodel: (Self) -> M) -> some View {
        if situation {
            remodel(self)
        } else {
            self
        }
    }
}

							

There are various weblog posts on the market with related modifiers. I feel all these weblog posts ought to include an enormous warning signal. Why is the above code problematic? Let’s take a look at a pattern.

Within the following code, we’ve got a single state property myState. When it modifications between true and false, we wish to conditionally apply a body:

								struct ContentView: View {
    @State var myState = false
    var physique: some View {
        VStack {
            Toggle("Toggle", isOn: $myState.animation())
            Rectangle()
                .applyIf(situation: myState, remodel: { $0.body(width: 100) })
        }
        
    }
}

							

Curiously, when working this code, the animation doesn’t look easy in any respect. In the event you look carefully, you’ll be able to see that it fades between the “earlier than” and “after” state:

This is the identical instance, however written with out applyIf:

								struct ContentView: View {
    @State var myState = false
    var physique: some View {
        VStack {
            Toggle("Toggle", isOn: $myState.animation())
            Rectangle()
                .body(width: myState ? 100 : nil)
        }
        
    }
}

							

And with the code above, our animation works as anticipated:

Why is the applyIf model damaged? The reply teaches us rather a lot about how SwiftUI works. In UIKit, views are objects, and objects have inherent id. Which means that two objects are equal if they’re the identical object. UIKit depends on the id of an object to animate modifications.

In SwiftUI, views are structs — worth sorts — which signifies that they do not have id. For SwiftUI to animate modifications, it wants to check the worth of the view earlier than the animation began and the worth of the view after the animation ends. SwiftUI then interpolates between the 2 values.

To grasp the distinction in conduct between the 2 examples, let’s take a look at their sorts. This is the kind of our Rectangle().applyIf(...):

								_ConditionalContent<ModifiedContent<Rectangle, _FrameLayout>, Rectangle>

							

The outermost sort is a _ConditionalContent. That is an enum that can both comprise the worth from executing the if department, or the worth from executing the else department. When situation modifications, SwiftUI can’t interpolate between the outdated and the brand new worth, as they’ve differing kinds. In SwiftUI, when you have got an if/else with a altering situation, a transition occurs: the view from the one department is eliminated and the view for the opposite department is inserted. By default, the transition is a fade, and that is precisely what we’re seeing within the applyIf instance.

In distinction, that is the kind of Rectangle().body(...):

								ModifiedContent<Rectangle, _FrameLayout>

							

Once we animate modifications to the body properties, there aren’t any branches for SwiftUI to contemplate. It could simply interpolate between the outdated and new worth and all the pieces works as anticipated.

Within the Rectangle().body(...) instance, we made the view modifier conditional by offering a nil worth for the width. That is one thing that just about each view modifier assist. For instance, you’ll be able to add a conditional foreground shade by utilizing an optionally available shade, you’ll be able to add conditional padding by utilizing both 0 or a price, and so forth.

Notice that applyIf (or actually, if/else) additionally breaks your animations if you find yourself doing issues appropriately on the “inside”.

								Rectangle()
    .body(width: myState ? 100 : nil)
    .applyIf(situation) { $0.border(Shade.pink) }

							

If you animate situation, the border is not going to animate, and neither will the body. As a result of SwiftUI considers the if/else branches separate views, a (fade) transition will occur as an alternative.

There’s one more downside past animations. If you use applyIf with a view that accommodates a @State property, all state will probably be misplaced when the situation modifications. The reminiscence of @State properties is managed by SwiftUI, primarily based on the place of the view within the view tree. For instance, think about the next view:

								struct Stateful: View {
    @State var enter: String = ""
    var physique: some View {
        TextField("My Subject", textual content: $enter)
    }
}

struct Pattern: View {
    var flag: Bool
    var physique: some View {
        Stateful().applyIf(situation: flag) {
            $0.background(Shade.pink)
        }
    }
}

							

Once we change flag, the applyIf department modifications, and the Stateful() view has a brand new place (it moved to the opposite department of a _ConditionalContent). This causes the @State property to be reset to its preliminary worth (as a result of so far as SwiftUI is anxious, a brand new view was added to the hierarchy), and the consumer’s textual content is misplaced. The identical downside additionally occurs with @StateObject.

The tough half about all of that is that you simply won’t see any of those points when constructing your view. Your views look wonderful, however possibly your animations are slightly funky, otherwise you generally lose state. Particularly when the situation does not change all that usually, you won’t even discover.

I’d argue that the entire weblog posts that counsel a modifier like applyIf ought to have an enormous warning signal. The downsides of applyIf and its variants are by no means apparent, and I’ve sadly seen a bunch of people that have simply copied this into their code bases and had been very proud of it (till it turned a supply of issues weeks later). In truth, I’d argue that no code base ought to have this perform. It simply makes it means too simple to by chance break animations or state.

In the event you’re involved in understanding how SwiftUI works, you can learn our guide Pondering in SwiftUI, watch our SwiftUI movies on Swift Discuss, or attend certainly one of our workshops.

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