How to Perform Custom Action on a load of SwiftUI view
UIKit offers a convenient viewDidLoad method that gets executed as soon as the ViewController is loaded and is called just once in the view controller lifecycle.
Unfortunately, SwiftUI has no such feature. There is no way to perform the action just once after the view is loaded. Today, we are going to learn how to write our own viewDidLoad method for SwiftUI views that executes a given piece of code just once after the view has been loaded.
Let's go step-by-step through creating a solution,
- Building a
ViewModifier
The first step in building our custom viewDidLoad method is creating a ViewModifier. As SwiftUI dictates, it is going to conform to ViewModifier protocol. We will pass the closure in its initialization which will get called as soon as the view appears for the first time.
In order to keep track of the state and avoid repeat execution, we use the State variable didViewLoad, which gets set to true after the first call. Future calls are ignored since we execute the closure only when didViewLoad flag is false.
struct CustomViewDidLoadModifier: ViewModifier {
@State private var didViewLoad = false
private let action: (() -> Void)?
init(perform action: (() -> Void)? = nil) {
self.action = action
}
func body(content: Content) -> some View {
content.onAppear {
if !didViewLoad {
didViewLoad = true
action?()
}
}
}
}
2. Convenience Method
To make using this view modifier easier, let's wrap it in a nice extension. We will add onLoad method on the View extension which will take the closure as the parameter. This closure will get executed just once after the view appears for the first time.
extension View {
func onLoad(perform action: (() -> Void)? = nil) -> some View {
modifier(CustomViewDidLoadModifier(perform: action))
}
}
3. Demo Time
Now that all parts are ready, let's integrate and see how the final code works.
We will try to load a custom CustomViewDidLoad. We will call onLoad method on it (As provided by the extension above) when it appears for the first time.
//CustomViewDidLoad.swift
struct CustomViewDidLoad: View {
var body: some View {
VStack {
Text("Hello")
}
}
}
//Demo
CustomViewDidLoad().onLoad {
print("YES")
} If you run the app now, you can see YES getting printed just once unless we go back and land on the CustomViewDidLoad view screen again.
Summary
That's it for today's blog post. Hope you liked it. The technique mentioned in this post can be used to create any kind of view modifier and apply a transformation to your view. You could be applying any style, or capturing any action as demonstrated here. Using the same technique, you can also apply custom behavior like ignoring the first load and/or capturing the 2nd, 3rd, or nth load of the view. The possibilities are endless.
Source Code
The full source code for this tutorial is available on GitHub Gist for further reference
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