Within the SwiftUI group, many individuals give you 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 to be one thing like this:
extension View {
@ViewBuilder
func applyIf<M: View>(situation: Bool, rework: (Self) -> M) -> some View {
if situation {
rework(self)
} else {
self
}
}
}
There are lots of weblog posts on the market with related 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 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, rework: { $0.body(width: 100) })
}
}
}
Curiously, when operating this code, the animation doesn’t look clean in any respect. For those who look intently, you’ll be able to 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 so much 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 modifications.
In SwiftUI, views are structs — worth sorts — which implies that they do not have identification. For SwiftUI to animate modifications, it wants to match 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 conduct 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 kind 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 modifications, SwiftUI can not interpolate between the previous and the brand new worth, as they’ve differing kinds. 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 modifications to the body properties, there aren’t any branches for SwiftUI to think about. It could actually 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 just about each view modifier help. For instance, you’ll be able to add a conditional foreground coloration through the use of an elective coloration, you’ll be able to add conditional padding through the use of both 0 or a worth, and so forth.
Observe 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(Coloration.purple) }
Whenever 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 an alternative.
There may be yet one more downside past animations. Whenever you use applyIf with a view that comprises a @State property, all state will likely be misplaced when the situation modifications. 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 Discipline", textual content: $enter)
}
}
struct Pattern: View {
var flag: Bool
var physique: some View {
Stateful().applyIf(situation: flag) {
$0.background(Coloration.purple)
}
}
}
After 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 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 won’t see any of those points when constructing your view. Your views look high quality, however possibly your animations are a little bit funky, otherwise you typically lose state. Particularly when the situation would not change all that always, you won’t even discover.
I might 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). Actually, I might argue that no code base ought to have this operate. It simply makes it manner too straightforward to by accident break animations or state.
For those who’re excited about understanding how SwiftUI works, you may learn our e-book Considering in SwiftUI, watch our SwiftUI movies on Swift Speak, or attend considered one of our workshops.
