Cleanup after an effect before executing a new one

[josefdolezal]

Pull Request Type

• Bug fix
• New feature
• Refactoring
• Documentation update
• Chore

Issue for this PR

Link: N/A

Description

Fixes issue where effects with dependencies (eg .preserved(by:)) would cleanup itself after new effect ran. This is different behavior from React's implementation and could cause unexpected results.

Motivation and Context

Running the following code with Hooks v0.6.0, after tapping the button, useHook would first run new effect, then cleanup after previous effect. The timeline is:

1. [Key: 1]
2. Run Effect [1]
3. Increment key [Key: 2]
4. Run Effect [2]
5. Cleanup Effect [1]

Steps 4 and 5 should run in opposite order.

The issue emerges in UseEffectHook.State in willSet property observer. The observation triggers old effect's cleanup by oldValue?() by which time new effect already finished to return new cleanup closure.

SwiftUI Code

func App() -> some View {
    HookScope {
        let state = useState(0)
        let _ = useEffect(.preserved(by: state.wrappedValue)) {
            print("subscribe \(state.wrappedValue)")
            return {
                print("unsubscribe \(state.wrappedValue)")
            }
        }

        Text(state.wrappedValue.description)
        Button("Add") {
            state.wrappedValue += 1
        }
    }
}

React Code

function App() {
  const [state, setState] = React.useState(0);

  useEffect(() => {
    console.log("subscribe " + state);

    return () => console.log("unsubscribe " + state);
  }, [state]);

  return (
    <div>
      <p>{state}</p>
      <button onClick={() => setState((old) => old + 1)}>Add</button>
    </div>
  );
}

Running the sample code in patch-1 branch, the results are same as React app above.

Impact on Existing Code

The change is source-compatible.

Screenshot/Video/Gif

[josefdolezal]

Note: Updating code to willSet is not enough, because effect() was already executed.

[josefdolezal]

Open for naming suggestions 🙏

[ra1028]

@josefdolezal
Good point, it looks pretty good to me! Thanks 🚀

[ra1028]

https://github.com/ra1028/swiftui-hooks/releases/tag/0.0.7