SwiftUI中为StateObject属性赋值视图函数的警告解决办法

我在SwiftUI视图中尝试将一个常规函数赋值给用@StateObject修饰的SpeechSynthesizer类的属性，代码如下：

视图代码：

var body: some View {
    contentView
        .task {
            navigation.tabBarVisibility = .hidden
            viewModel.fetchWordset()
            synthesizer.onNextUtteranceEnded = onNextUtteranceEnded
        }
}

private func onNextUtteranceEnded(_ utterance: AVSpeechUtterance) {
    guard utterance.voice?.language == "pl-PL" else { return }
    viewModel.next()
}    

SpeechSynthesizer类定义：

public final class SpeechSynthesizer: NSObject, ObservableObject, AVSpeechSynthesizerDelegate {

    @Published public var isSpeaking = false

    public var onNextUtteranceStarted: ((AVSpeechUtterance) -> Void)?
    public var onNextUtteranceEnded: ((AVSpeechUtterance) -> Void)?
}

此时收到警告：

Converting function value of type '@MainActor (AVSpeechUtterance) -> ()' to '(AVSpeechUtterance) -> Void' loses global actor 'MainActor'

问题原因 #

SwiftUI视图的所有属性和方法默认运行在@MainActor上下文（主线程）中，而SpeechSynthesizer的闭包属性未标注任何actor，导致将属于@MainActor的函数赋值给它时，丢失了主Actor上下文信息，触发警告。

解决方法 #

方法1：修改SpeechSynthesizer类，给闭包属性添加@MainActor标注

如果有权限修改SpeechSynthesizer类，直接给闭包属性加上@MainActor，让类型完全匹配：

public final class SpeechSynthesizer: NSObject, ObservableObject, AVSpeechSynthesizerDelegate {

    @Published public var isSpeaking = false

    @MainActor public var onNextUtteranceStarted: ((AVSpeechUtterance) -> Void)?
    @MainActor public var onNextUtteranceEnded: ((AVSpeechUtterance) -> Void)?
}

此时视图中默认@MainActor的函数与闭包属性类型一致，警告自动消失。

方法2：不修改SpeechSynthesizer类，显式指定闭包的MainActor上下文

如果无法修改类，可在赋值时将函数包装为显式的@MainActor闭包：

.task {
    navigation.tabBarVisibility = .hidden
    viewModel.fetchWordset()
    synthesizer.onNextUtteranceEnded = { @MainActor utterance in
        self.onNextUtteranceEnded(utterance)
    }
}

或者在闭包内部通过MainActor.run确保主线程执行：

synthesizer.onNextUtteranceEnded = { utterance in
    MainActor.run {
        self.onNextUtteranceEnded(utterance)
    }
}

方法3：在SpeechSynthesizer内部处理主线程切换

由于AVSpeechSynthesizerDelegate的方法可能在后台线程触发，可在类内部代理方法中强制切换到主线程执行闭包：

public final class SpeechSynthesizer: NSObject, ObservableObject, AVSpeechSynthesizerDelegate {

    @Published public var isSpeaking = false

    public var onNextUtteranceStarted: ((AVSpeechUtterance) -> Void)?
    public var onNextUtteranceEnded: ((AVSpeechUtterance) -> Void)?

    public func speechSynthesizer(_ synthesizer: AVSpeechSynthesizer, didFinish utterance: AVSpeechUtterance) {
        DispatchQueue.main.async {
            self.onNextUtteranceEnded?(utterance)
        }
    }

    // 同理处理onNextUtteranceStarted对应的代理方法
}

这样无论闭包是否标注@MainActor，都会在主线程执行，同时消除警告。

内容的提问来源于stack exchange，提问作者Michał Ziobro