Demonstrates a memory leak bug SwiftUI's toolbar APIs exhibit on macOS

Matt Curtis

Last update: Aug 6, 2022

Related tags

Debugging mac-swiftui-toolbar-memory-leak-demo

Overview

Mac SwiftUI Toolbar Memory Leak Demo

On Big Sur (macOS 11), SwiftUI's APIs for adding toolbar items to a window have (at least) two bugs around view recreation that lead to memory leaks and increased CPU pressure over time. Monterey (macOS 12) appears to not have these bugs.

Issue #1: `ToolbarItem` always creates an additional, "ghost" copy of the view it contains

For example,

Color.clear
    .toolbar {
        ToolbarItem {
            Color.red
                .onAppear { print("This will be printed twice.") }
        }
    }

will print "This will be printed twice" twice. ToolbarItem creates an additional copy of Color.red. This ghost copy is fully functional, reacts to state changes (assuming it hosts something within itself that changes state), and is not destroyed until its parent .toolbar() leaves the view hierarchy. This is true even if your view hierarchy is complete static, with nothing to trigger a state change.

Issue #2: `ToolbarItem` creates an additional ghost copy of the view it contains every single time state changes

Pretend you have some state:

@State var counter = 0

...and a toolbar item in your view, that updates that state whenever every so often:

Color.clear
    .toolbar {
        ToolbarItem {
            Text("\(counter)")
                .onAppear { print("This will be printed every change of counter.") }
        }
    }
    .onAppear {
        //  Start a timer that updates counter += 1 every so often
    }

that onAppear() will be printed every single time counter changes. The same issues outlined in the previous issue are true here.

Closing

Both of these issues are demonstrated in the sample project in this repo.

I've filed this bug under FB9453009.

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Comments

I noticed there is no leak if you use a binding in the ToolbarItem

struct ViewDemonstratingViewRecreation: View {
    
    @State var someState = 0
    
    var body: some View {
        Color.blue
            .toolbar {
                ToolbarItem {
                    ViewDemonstratingViewRecreation2(someState:$someState)
                        .onAppear {
                            print("You should see this message printed many times.")
                        }
                }
            }
            .onAppear {
                Timer.scheduledTimer(withTimeInterval: 0.01, repeats: true) {
                    _ in
                    someState += 100
                }
            }
    }
    
}

struct ViewDemonstratingViewRecreation2: View {
    
    @Binding var someState: Int
    
    var body: some View {
        Text("\(someState)")
            .onAppear {
                print("You should see this message printed many times.")
            }
    }
}

opened by malhal 2