task(id:priority:_:)

Return Value 戻り値

A view that runs the specified action asynchronously when the view appears, or restarts the task with the id value changes.

Discussion 議論

This method behaves like task(priority:_:), except that it also cancels and recreates the task when a specified value changes. To detect a change, the modifier tests whether a new value for the id parameter equals the previous value. For this to work, the value’s type must conform to the Equatable protocol.

For example, if you define an equatable Server type that posts custom notifications whenever its state changes — for example, from signed out to signed in — you can use the task modifier to update the contents of a Text view to reflect the state of the currently selected server:

Text(status ?? "Signed Out")
    .task(id: server) {
        let sequence = NotificationCenter.default.notifications(
            named: .didChangeStatus,
            object: server)
        for try await notification in sequence {
            status = notification.userInfo["status"] as? String
        }
    }

This example uses the notifications(named:object:) method to wait indefinitely for an asynchronous sequence of notifications, given by an AsyncSequence instance.

Elsewhere, the server defines a custom didUpdateStatus notification:

extension NSNotification.Name {
    static var didUpdateStatus: NSNotification.Name {
        NSNotification.Name("didUpdateStatus")
    }
}

The server then posts a notification of this type whenever its status changes, like after the user signs in:

let notification = Notification(
    name: .didUpdateStatus,
    object: self,
    userInfo: ["status": "Signed In"])
NotificationCenter.default.post(notification)

The task attached to the Text view gets and displays the status value from the notification’s user information dictionary. When the user chooses a different server, SwiftUI cancels the task and creates a new one, which then starts waiting for notifications from the new server.