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

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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 usually appears to be like 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 lots of weblog posts on the market with comparable 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 now have 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) })
        }
        
    }
}

							

Apparently, when working this code, the animation doesn’t look easy in any respect. Should you look carefully, you possibly can see that it fades between the “earlier than” and “after” state:

Here 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 lots 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 adjustments.

In SwiftUI, views are structs — worth varieties — which signifies that they do not have id. 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’s take a look at their varieties. Here 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 include the worth from executing the if department, or the worth from executing the else department. When situation adjustments, SwiftUI can’t interpolate between the outdated and the brand new worth, as they’ve differing types. In SwiftUI, when you’ve gotten 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 adjustments to the body properties, there are not any branches for SwiftUI to think about. It may 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 just about each view modifier assist. For instance, you possibly can add a conditional foreground coloration through the use of an optionally available coloration, you possibly can 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 if you end up doing issues appropriately on the “inside”.

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

							

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 incorporates a @State property, all state shall be misplaced when the situation adjustments. The reminiscence of @State properties is managed by SwiftUI, primarily based on the place of the view within the view tree. For instance, take into account the next view:

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

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

							

Once 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 worried, 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 just may not see any of those points when constructing your view. Your views look wonderful, however possibly your animations are just a little funky, otherwise you generally lose state. Particularly when the situation would not change all that always, you may not even discover.

I’d argue that the entire weblog posts that recommend a modifier like applyIf ought to have an enormous warning signal. The downsides of applyIf and its variants are in no way 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). Actually, I’d argue that no code base ought to have this operate. It simply makes it manner too simple to by chance break animations or state.

Should you’re concerned with understanding how SwiftUI works, you can learn our ebook Considering in SwiftUI, watch our SwiftUI movies on Swift Speak, or attend certainly one of our workshops.

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