SwiftUI中onSubmit导致视图实例无法释放的问题排查

在SwiftUI中，若在onSubmit修饰符的闭包内直接访问视图自身的@State或@Binding属性，会导致视图实例无法被正常释放，始终有一个实例驻留在内存中。

验证结果 #

通过切换显示/隐藏带有onSubmit修饰符的Input视图，多次操作后的控制台输出如下：

[Input] constructed: (1)
[Input] constructed: (2)
[Input] deallocating: (1)
[Input] constructed: (2)
[Input] deallocating: (1)
[Input] constructed: (2)
[Input] deallocating: (1)

可见每次创建新实例后，仅释放一个旧实例，始终有一个实例存活，存在内存泄漏。

若注释掉onSubmit代码行，或者将其替换为onTapGesture，视图实例则能正常释放，控制台输出恢复正常：

[Input] constructed: (1)
[Input] deallocating: (0)
[Input] constructed: (1)
[Input] deallocating: (0)
[Input] constructed: (1)
[Input] deallocating: (0)
[Input] constructed: (1)
[Input] deallocating: (0)

完整代码示例 #

import SwiftUI

struct ContentView : View {
    
    @State private var state: Bool = false
    
    var body: some View {
        VStack {
            Button("push") { state.toggle() }
            if state { Input() } else { Color.red }
        }
    }
    
}

struct Input : View {
    
    @State private var text: String
    @State private var whatever: Bool
    private let tracing: InstanceTracer
    
    init() {
        self.text = "text"
        self.whatever = true
        self.tracing = InstanceTracer("Input")
    }
    
    var body: some View {
        TextField("placeholder", text: $text)
            .onSubmit { whatever.toggle() } // <- 此处导致实例无法释放
    }
    
}

final class InstanceTracer {
    
    private static var instances: [String : Int] = [ : ]
    
    private let name: String
    
    init(_ name: String) {
        self.name = name
        print("[\(name)] constructed: (\(increment(numberOf: name)))")
    }
    
    deinit { print("[\(name)] deallocating: (\(decrement(numberOf: name)))") }
    
    private func increment(numberOf name: String) -> Int {
        var number: Int = Self.instances[name] ?? 0
        number += 1
        Self.instances[name] = number
        return number
    }
    
    private func decrement(numberOf name: String) -> Int {
        var number: Int = Self.instances[name] ?? 0
        guard number > 0 else { return 0 }
        number -= 1
        Self.instances[name] = number
        return number
    }
    
}

内容的提问来源于stack exchange，提问作者sugar baron