PromiseProtocol

public protocol PromiseProtocol

A promise models writing the result of some asynchronous operation.

Promises should generally only be determined, or filled, once. This allows an implementing type to clear a queue of subscribers, for instance, and provides consistent sharing of the determined value.

An implementing type should discourage race conditions around filling. However, certain use cases inherently race (such as cancellation). Any attempts to check for programmer error should be active by default.

  • A type that represents the result of some asynchronous operation.

    Declaration

    Swift

    associatedtype Value

  • Creates an instance in a default, unfilled state.

    Declaration

    Swift

    init()
  • isFilled Default implementation

    Check whether or not the receiver is filled.

    Default Implementation

    By default, checks for a fulfilled future value.

    Declaration

    Swift

    var isFilled: Bool { get }

  • Determines the promise with value.

    Filling a deferred value should usually be attempted only once.

    Declaration

    Swift

    @discardableResult
func fill(with value: Value) -> Bool

    Return Value

    Whether the promise was fulfilled with value.

  • mustFill(with:file:line:) Extension method

    Determines the deferred value.

    Filling a deferred value should usually be attempted only once. A user may choose to enforce this.

    • In playgrounds and unoptimized builds (the default for a Debug configuration) where the deferred value is already filled, program execution will be stopped in a debuggable state.

    • In optimized builds (the default for a Release configuration) where the deferred value is already filled, stop program execution.

    • In unchecked builds, filling a deferred value that is already filled is a serious programming error. The optimizer may assume that it is not possible.

    Declaration

    Swift

    @inlinable
public func mustFill(with value: Value, file: StaticString = #file, line: UInt = #line)