将本地可变结构传递给异步 let 绑定

Question

我想创建一个 URL 请求并将其传递到异步 let 绑定中，这对我来说很自然：

func test() async {
    // Force unwraps (!) are just for demo
    var request = URLRequest(url: URL(string:"https://whosebug.com")!)
    request.httpMethod = "GET" // just for example
    // some more tinkering with `request` here.
    
    //  Error on this line: "Reference to captured var 'request' in concurrently-executing code"
    async let responseData = URLSession.shared.data(for: request).0
    
    // It works like this:
    // let immutableRequest = request
    // async let responseData = URLSession.shared.data(for: immutableRequest).0
    
    // other stuff
    print("Response body: \(String(data: try! await responseData, encoding: .utf8))")
}

为什么会出现错误？ URLRequest 是一个结构，所以当我们将它传递给一个函数时，该函数应该得到该结构的副本，所以如果我在异步调用后修改 request，它应该不会影响调用。

我知道调用是异步发生的，但我希望它在调用时捕获参数，然后继续执行，就像调用已经完成一样（因此，request 的副本在调用点已传递到 data(for: request).

此外，有没有一种方便的方法可以在不创建另一个 let 变量并且不使用闭包来初始化 request 的情况下执行此操作，例如：

let request: URLRequest = {
    var result = URLRequest(url: URL(string:"https://whosebug.com")!)
    result.httpMethod = "GET"
    return result
}()

Answer 1

正如SE-0317 - async let bindings所说：

... async let is similar to a let, in that it defines a local constant that is initialized by the expression on the right-hand side of the =. However, it differs in that the initializer expression is evaluated in a separate, concurrently-executing child task.

The child task begins running as soon as the async let is encountered.

...

A async let creates a child-task, which inherits its parent task's priority as well as task-local values. Semantically, this is equivalent to creating a one-off TaskGroup which spawns a single task and returns its result ...

Similarly to the [group.addTask] function, the closure is @Sendable and nonisolated, meaning that it cannot access non-sendable state of the enclosing context. For example, it will result in a compile-time error, preventing a potential race condition, for a async let initializer to attempt mutating a closed-over variable:
var localText: [String] = ...
async let w = localText.removeLast() // error: mutation of captured var 'localText' in concurrently-executing code
The async let initializer may refer to any sendable state, same as any non-isolated sendable closure.

所以，并不是复制data(for:delegate:)的参数然后创建异步任务，而是反过来

通常，如果您使用闭包，您只需将 request 添加到闭包的捕获列表中，但在这种情况下这是不可能的。例如，您可以使用捕获列表自己创建一个 Task，实现类似于 async let 的效果，但具有更大的控制权：

func test() async throws {
    var request = URLRequest(url: URL(string:"https://httpbin.org/get")!)
    request.httpMethod = "GET" // just for example

    let task = Task { [request] in
        try await URLSession.shared.data(for: request).0
    }

    // do some more stuff in parallel

    print("Response body: \(String(data: try await task.value, encoding: .utf8) ?? "Not string")")
}

显然，您可以简单地 await data(for:delegate:)，而不是 async let，问题就消失了：

func test() async throws {
    var request = URLRequest(url: URL(string:"https://httpbin.org/get")!)
    request.httpMethod = "GET" // just for example

    let data = try await URLSession.shared.data(for: request).0

    print("Response body: \(String(data: data, encoding: .utf8) ?? "Not string")")
}

将本地可变结构传递给异步 let 绑定

Passing a local mutable struct into an async let binding

async-await

swift

structured-concurrency