Allow prepareDependencies to return result. (#332)

Author: mbrandonw

Sources/Dependencies/WithDependencies.swift

@@ -41,11 +41,11 @@ import Foundation
 ///
 /// - Parameter updateValues: A closure for updating the current dependency values for the
 ///   lifetime of your application.
-public func prepareDependencies(
-  _ updateValues: (inout DependencyValues) throws -> Void
-) rethrows {
+public func prepareDependencies<R>(
+  _ updateValues: (inout DependencyValues) throws -> R
+) rethrows -> R {
   var dependencies = DependencyValues._current
-  try DependencyValues.$preparationID.withValue(UUID()) {
+  return try DependencyValues.$preparationID.withValue(UUID()) {
     try updateValues(&dependencies)
   }
 }

Tests/DependenciesTests/DependencyValuesTests.swift

@@ -736,6 +736,13 @@ final class DependencyValuesTests: XCTestCase {
     XCTAssertEqual(now, Date(timeIntervalSinceReferenceDate: 0))
   }
 
+  func testPrepareDependencies_returnResult() {
+    let result = prepareDependencies { _ in
+      42
+    }
+    XCTAssertEqual(result, 42)
+  }
+
   func testPrepareDependencies_setsLiveContext() {
     prepareDependencies {
       $0.context = .live