SwiftUI/Examples/React Meets SwiftUI/ComponentDidMount.md

ComponentDidMount

With React it's common to perform one-time operations (such as async fetches to a server) in the componentDidMount function or the useEffect hook. You can replicate this behaviour using the onAppear and onDisappear callbacks on any View. This means that you can actually have multiple onAppear callbacks attached to multiple views contained within your body function. Your parent view can also attach these callbacks to custom child views.

The below example mounts and unmounts the counter readout when it reaches a certain value. We log when the mount/unmount happens.

React

import React from "react";

export function Counter() {
  const [count, setCount] = React.useState(0);

  function increment() {
    setCount(count + 1);
  }

  return (
    <div>
      <button onClick={increment}>Increment</button>
      {(count < 10 || count > 12) && <ChildCounter count={count} />}
    </div>
  );
}

export function ChildCounter({ count }) {
  React.useEffect(() => {
    console.log("mounted");

    return () => {
      console.log("unmounting");
    };
  }, []);

  return <div>{count}</div>;
}

Swift

Note that this only approximates the React behaviour. I'm not sure what the appear / mount distinction implies, but it seems to provide similar utility.

import SwiftUI

struct Counter : View {
    @State var count = 0

    func increment () {
        count += 1
    }

    var body: some View {

        VStack {
            Button(action: increment) {
                Text("Increment")
            }
            if count < 10 || count > 12 {
                ChildCounter(count: count)
            }
        }

    }
}

struct ChildCounter : View {
    var count: Int

    func onMount () {
        print("on mount")
    }

    func onUnmount () {
        print("on unmount")
    }

    var body: some View {
        Text("\(count)")
            .onAppear(perform: onMount)
            .onDisappear(perform: onUnmount)
    }
}

You can attach multiple onAppear and onDisappear callbacks in your body function:

struct Counter : View {
    @State var count = 0

    func increment () {
        count += 1
    }

    func mount () {
        print("Child mount")
    }

    func unmount () {
        print("Child unmount")
    }

    func parentMount () {
        print("parent mount")
    }


    var body: some View {
        VStack {
            Button(action: increment) {
                Text("Increment")
            }
            if count < 10 || count > 12 {
                ChildCounter(count: count)
                    .onAppear(perform: mount)
                    .onDisappear(perform: unmount)
            }
        }
        .onAppear(perform: parentMount)
    }
}