Add a Future.async(upon:flags:execute:) convenience

Author: zwaldowski

Deferred.xcodeproj/project.pbxproj

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@@ -384,6 +393,7 @@
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 				DB524C9A1D85200C00DDF16D /* Future.swift */,
 				DBA01B032071E68F00083CD0 /* FutureAndThen.swift */,
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Sources/Deferred/FutureAsync.swift

@@ -0,0 +1,28 @@
+//
+//  FutureAsync.swift
+//  Deferred
+//
+//  Created by Zachary Waldowski on 6/3/18.
+//  Copyright © 2018 Big Nerd Ranch. Licensed under MIT.
+//
+
+import Dispatch
+
+extension Future {
+    /// Captures the value of asynchronously executing `work` on `queue`.
+    ///
+    /// - parameter queue: A dispatch queue to perform the `work` on.
+    /// - parameter flags: Options controlling how the `work` is executed with
+    ///   respect to system resources.
+    /// - parameter work: A function body that calculates and returns the
+    ///   fulfilled value for the future.
+    public static func async(upon queue: DispatchQueue = .any(), flags: DispatchWorkItemFlags = [], execute work: @escaping() -> Value) -> Future {
+        let deferred = Deferred<Value>()
+
+        queue.async(flags: flags) {
+            deferred.fill(with: work())
+        }
+
+        return Future(deferred)
+    }
+}

Tests/DeferredTests/FutureAsyncTests.swift

@@ -0,0 +1,48 @@
+//
+//  FutureAsyncTests.swift
+//  Deferred
+//
+//  Created by Zachary Waldowski on 6/3/18.
+//  Copyright © 2018 Big Nerd Ranch. Licensed under MIT.
+//
+
+import XCTest
+import Dispatch
+import Deferred
+
+class FutureAsyncTests: XCTestCase {
+    static let allTests: [(String, (FutureAsyncTests) -> () throws -> Void)] = [
+        ("testThatPeekingBeforeStartingReturnsNil", testThatPeekingBeforeStartingReturnsNil)
+    ]
+
+    private var queue: DispatchQueue!
+
+    override func setUp() {
+        super.setUp()
+
+        queue = DispatchQueue(label: "FutureAsyncTests")
+        queue.suspend()
+    }
+
+    override func tearDown() {
+        queue = nil
+
+        super.tearDown()
+    }
+
+    func testThatPeekingBeforeStartingReturnsNil() {
+        let future = Future<Int>.async(upon: queue) { 1 }
+
+        XCTAssertNil(future.peek())
+
+        queue.resume()
+
+        let expect = expectation(description: "future fulfils")
+        future.upon(queue) { (result) in
+            XCTAssertEqual(result, 1)
+            expect.fulfill()
+        }
+
+        shortWait(for: [ expect ])
+    }
+}