@Binding – data between views

In C world, if we want to pass a parameter down into a functional call, and allow the receiving function to change it’s value, we’d pass a pointer to the variable. Something like this:

#include <stdio.h>
#include <stdlib.h>

void increment(int* b) {
    *b=*b+1;
}

int main() {
    int a = 5;
    increment(&a);
    printf("%d", a);
    return 0;
}

// prints '6'

For youngsters, what’s happening is that we’ve set the value of a to 5, then passed the memory address of a into the increment() function. That’s what the @a means.

In the increment function we’re expecting an *int – ie the address of an int. Then we dereference it with the asterisks to operate on the value stored in the address.

The reason I mention all this ancient lore is because it’s what I imagine is happening with the @Binding in a subview – although I’m also sure the story underneath is more complex than that.

	import SwiftUI
	struct ContentView: View {
	@State var someInt = 5
	var body: some View {
	VStack {
	SubView(someValue: $someInt)
	Text("Content view \(someInt)")
	}
	}
	}
	struct SubView: View {
	@Binding var someValue: Int
	var body: some View {
	HStack {
	Text("Subview \(someValue)")
	Button("Inc") {
	someValue += 1
	}
	}.padding()
	}
	}

view raw Binding.swift hosted with ❤ by GitHub

There’s a couple of things to note here. In the SubView struct, we’ve used the @Binding property wrapper, and in the ContentView where we’ve passed the state variable from, we’ve marked it with the $ so indicate we know its a binding variable that can be mutated by whatever it’s being passed to.

If we leave the $ off, the compiler will point out our error because it knows it’s bound in the other view.