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

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Within the SwiftUI neighborhood, many individuals provide you with their very own model of a conditional view modifier
. It means that you can 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, remodel: (Self) -> M) -> some View {
        if situation {
            remodel(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’s take a look at a pattern.

Within the following code, now we have a single state property myState
. When it adjustments between true
and false
, we need 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 clean in any respect. For those who look intently, 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 lots about how SwiftUI works. In UIKit, views are objects, and objects have inherent id
. Because of this 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 sorts — 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 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 may 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 previous 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 are not any branches for SwiftUI to think about. It might simply interpolate between the previous and new worth and the whole lot 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 non-obligatory coloration, you may add conditional padding through the use of both 0 or a worth, and so forth.

Word that applyIf
(or actually, if/else
) additionally breaks your animations when 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 a substitute.

There’s one more drawback past animations. If you use applyIf
with a view that incorporates a @State
property, all state can 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, think about 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(Shade.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 drawback additionally occurs with @StateObject
.

The tough half about all of that is that you just won’t see any of those points when constructing your view. Your views look superb, however possibly your animations are a bit of funky, otherwise you typically 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 under no circumstances apparent, and I’ve sadly seen a bunch of people that have simply copied this into their code bases and had 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 approach too simple to by chance break animations or state.

For those who’re fascinated with understanding how SwiftUI works, you might learn our guide Considering in SwiftUI
, watch our SwiftUI movies
on Swift Discuss, or attend certainly one of our workshops
.