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Within the SwiftUI neighborhood, 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 to be 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 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 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 operating this code, the animation doesn’t look clean in any respect. When you look intently, you may 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 loads about how SwiftUI works. In UIKit, views are objects, and objects have inherent identification. Because of this 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 signifies 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’s take a look at their sorts. Here 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 outdated and the brand new worth, as they’ve differing kinds. In SwiftUI, when you will 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>
Once we animate adjustments to the body properties, there are not any branches for SwiftUI to contemplate. It may simply interpolate between the outdated and new worth and every thing 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 shade by utilizing an non-obligatory shade, you may add conditional padding by utilizing both 0 or a worth, and so forth.
Observe 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(Coloration.purple) }
If you animate situation, the border won’t 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 yet one more downside past animations. If you use applyIf with a view that accommodates a @State property, all state will likely 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, take into account 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(Coloration.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 consumer’s textual content is misplaced. The identical downside additionally occurs with @StateObject.
The difficult half about all of that is that you simply won’t see any of those points when constructing your view. Your views look nice, however perhaps your animations are a bit of funky, otherwise you generally lose state. Particularly when the situation does not change all that always, you won’t even discover.
I’d argue that all the weblog posts that counsel 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 had been very proud of it (till it grew to become a supply of issues weeks later). In actual fact, I’d argue that no code base ought to have this operate. It simply makes it manner too straightforward to by chance break animations or state.
When you’re serious about understanding how SwiftUI works, you can learn our e book Pondering in SwiftUI, watch our SwiftUI movies on Swift Speak, or attend one in every of our workshops.
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