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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 group, many individuals provide you with their very own model of a conditional view modifier. It lets you take a view, and solely apply a view modifier when the situation holds. It sometimes seems one thing like this:

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

							

There are numerous weblog posts on the market with comparable modifiers. I believe all these weblog posts ought to include an enormous warning signal. Why is the above code problematic? Let us take a look at a pattern.

Within the following code, we’ve a single state property myState. When it adjustments 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, rework: { $0.body(width: 100) })
        }
        
    }
}

							

Curiously, when working this code, the animation doesn’t look clean in any respect. For those who look carefully, you may 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 identification. Which means that two objects are equal if they’re the identical object. UIKit depends on the identification of an object to animate adjustments.

In SwiftUI, views are structs — worth sorts — which implies that they do not have identification. For SwiftUI to animate adjustments, 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 know the distinction in habits between the 2 examples, let us take a look at their sorts. This is the kind of our Rectangle().applyIf(...):

								_ConditionalContent<ModifiedContent<Rectangle, _FrameLayout>, Rectangle>

							

The outermost kind 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 adjustments, SwiftUI can not interpolate between the outdated and the brand new worth, as they’ve differing types. In SwiftUI, when you may have 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>

							

After we animate adjustments to the body properties, there aren’t any branches for SwiftUI to contemplate. It might simply interpolate between the outdated and new worth and every part 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 nearly each view modifier assist. For instance, you may add a conditional foreground coloration through the use of an elective coloration, you may add conditional padding through the use of both 0 or a worth, and so forth.

Be aware that applyIf (or actually, if/else) additionally breaks your animations when you’re doing issues accurately on the “inside”.

								Rectangle()
    .body(width: myState ? 100 : nil)
    .applyIf(situation) { $0.border(Coloration.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 a substitute.

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

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

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

							

After we change flag, the applyIf department adjustments, 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 person’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 high quality, however perhaps your animations are a bit of funky, otherwise you generally lose state. Particularly when the situation would not change all that usually, you won’t even discover.

I’d argue that the entire weblog posts that recommend a modifier like applyIf ought to have a giant 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 have been very pleased with it (till it grew to become a supply of issues weeks later). Actually, I’d argue that no code base ought to have this perform. It simply makes it manner too straightforward to unintentionally break animations or state.

For those who’re excited about understanding how SwiftUI works, you may learn our e book Pondering in SwiftUI, watch our SwiftUI movies on Swift Discuss, or attend certainly one of our workshops.

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