Revise Boxes 📦

We have had two `Box` variants for a very long time: `Box` and `VarBox`, containing an immutable and mutable value respectively. However, from our experience the immutable variant is not used often if at all, which means its immutability guarantee is not something that people want or need. Furthermore, following the latest language developments a `Box` is a very good candidate for a property wrapper, and more so now that they can be defined as local properties. ## Changes - Deprecate `VarBox` in favor of `Box`, making it wrap a mutable value. - Make `Box` a `@propertyWrapper`, and `@dynamicMemberLookup`. The latter is useful if we use `Box<T>` as a regular type (i.e. not a PW).
Mindera · Jan 17, 2022 · 538c294 · 538c294
1 parent a1b58a2
commit 538c294
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Showing 6 changed files with 65 additions and 40 deletions.
diff --git a/Sources/Network/Network+URLSessionNetworkStack.swift b/Sources/Network/Network+URLSessionNetworkStack.swift
@@ -114,20 +114,20 @@ extension Network {
                 fatalError("🔥 `session` is `nil`! Forgot to 💉?")
             }
 
-            let taskIdentifierBox = VarBox<Int>(.max)
+            @Box var taskIdentifierBox = Int.max
             let cancelableBag = CancelableBag()
 
             let completionHandler = makeTaskCompletionHandler(
                 request: request,
                 resource: resource,
-                taskIdentifierBox: taskIdentifierBox,
+                taskIdentifierBox: _taskIdentifierBox,
                 cancelableBag: cancelableBag,
                 completion: completion
             )
 
             let task = session.dataTask(with: request, completionHandler: completionHandler)
 
-            taskIdentifierBox.value = task.taskIdentifier
+            taskIdentifierBox = task.taskIdentifier
             cancelableBag.add(cancelable: WeakCancelable(task))
 
             resource.interceptScheduledTask(withIdentifier: task.taskIdentifier, request: request)
@@ -141,7 +141,7 @@ extension Network {
         private func makeTaskCompletionHandler(
             request: URLRequest,
             resource: Resource,
-            taskIdentifierBox: VarBox<Int>,
+            taskIdentifierBox: Box<Int>,
             cancelableBag: CancelableBag,
             completion: @escaping FetchCompletionClosure
         ) -> URLSessionDataTaskClosure {

diff --git a/Sources/Utils/Box.swift b/Sources/Utils/Box.swift
@@ -1,33 +1,39 @@
-/// An arbitrary container which stores a **constant** value of type `T`.
+/// An arbitrary container (and property wrapper) which stores a **mutable** value of type `T`.
 ///
-/// This main purpose of this object is to encapsulate value types so that they can be used like a reference type
-/// (e.g. pass value types around without copying them, "share" value types between closures, etc)
+/// The main purpose of this object is to encapsulate value types so that they can be used like a reference type
+/// (e.g. pass value types around without copying them, memoize value types inside closures via capture list, etc)
+@propertyWrapper
+@dynamicMemberLookup
 public final class Box<T> {
 
-    /// The encapsulated value.
-    public let value: T
+    /// The wrapped value.
+    public var wrappedValue: T
 
-    /// Instantiate a new constant value box with the given value.
+    /// Instantiate a new mutable value box with the given value.
     ///
-    /// - parameter value: the value to encapsulate.
+    /// - parameter wrappedValue: the value to encapsulate.
     ///
     /// - returns: a newly instantiated box with the encapsulated value.
-    public init(_ value: T) { self.value = value }
+    public init(wrappedValue: T) { self.wrappedValue = wrappedValue }
+
+    public subscript<U>(dynamicMember keyPath: KeyPath<T, U>) -> U { wrappedValue[keyPath: keyPath] }
 }
 
-/// An arbitrary container which stores a **variable** value of type `T`.
-///
-/// This main purpose of this object is to encapsulate value types so that they can be used like a reference type
-/// (e.g. pass value types around without copying them, "share" value types between closures, etc)
-public final class VarBox<T> {
+extension Box {
 
-    /// The encapsulated value.
-    public var value: T
+    /// The wrapped value (compact).
+    public var value: T {
+        get { wrappedValue }
+        set { wrappedValue = newValue }
+    }
 
-    /// Instantiate a new variable value box with the given value.
+    /// Instantiate a new mutable value box with the given value (compact).
     ///
-    /// - parameter value: the value to encapsulate.
+    /// - parameter wrappedValue: the value to encapsulate.
     ///
     /// - returns: a newly instantiated box with the encapsulated value.
-    public init(_ value: T) { self.value = value }
+    public convenience init(_ value: T) { self.init(wrappedValue: value) }
 }
+
+@available(*, unavailable, renamed: "Box")
+public typealias VarBox<T> = Box<T>
diff --git a/Tests/AlicerceTests/Persistence/DiskMemoryPersistenceTestCase.swift b/Tests/AlicerceTests/Persistence/DiskMemoryPersistenceTestCase.swift
@@ -266,7 +266,7 @@ final class DiskMemoryPersistenceTestCase: XCTestCase {
         let measureExpectation = expectation(description: "measure")
         measureExpectation.expectedFulfillmentCount = 2
 
-        performanceMetrics.measureInvokedClosure = { [count = VarBox(0)] identifier, metadata in
+        performanceMetrics.measureInvokedClosure = { [count = Box(0)] identifier, metadata in
             if count.value == 0 {
                 XCTAssertEqual(identifier, performanceMetrics.memoryWriteIdentifier)
                 XCTAssertDumpsEqual(metadata, [performanceMetrics.blobSizeMetadataKey : mrMinderSize,
@@ -313,7 +313,7 @@ final class DiskMemoryPersistenceTestCase: XCTestCase {
         let measure = self.expectation(description: "measure")
         measure.expectedFulfillmentCount = 2
 
-        performanceMetrics.measureInvokedClosure = { [count = VarBox(0)] identifier, metadata in
+        performanceMetrics.measureInvokedClosure = { [count = Box(0)] identifier, metadata in
             if count.value == 0 {
                 XCTAssertEqual(identifier, performanceMetrics.memoryWriteIdentifier)
                 XCTAssertDumpsEqual(metadata, [performanceMetrics.blobSizeMetadataKey : mrMinderSize,
@@ -371,7 +371,7 @@ final class DiskMemoryPersistenceTestCase: XCTestCase {
         let measureExpectation = expectation(description: "measure")
         measureExpectation.expectedFulfillmentCount = 2
 
-        performanceMetrics.measureInvokedClosure = { [count = VarBox(0)] identifier, metadata in
+        performanceMetrics.measureInvokedClosure = { [count = Box(0)] identifier, metadata in
             if count.value == 0 {
                 XCTAssertEqual(identifier, performanceMetrics.memoryWriteIdentifier)
                 XCTAssertDumpsEqual(metadata, [performanceMetrics.blobSizeMetadataKey : mrMinderSize,
@@ -437,7 +437,7 @@ final class DiskMemoryPersistenceTestCase: XCTestCase {
         let measureExpectation = expectation(description: "measure")
         measureExpectation.expectedFulfillmentCount = 2
 
-        performanceMetrics.measureInvokedClosure = { [count = VarBox(0)] identifier, metadata in
+        performanceMetrics.measureInvokedClosure = { [count = Box(0)] identifier, metadata in
             if count.value == 0 {
                 XCTAssertEqual(identifier, performanceMetrics.memoryWriteIdentifier)
                 XCTAssertDumpsEqual(metadata, [performanceMetrics.blobSizeMetadataKey : mrMinderSize,
@@ -477,7 +477,7 @@ final class DiskMemoryPersistenceTestCase: XCTestCase {
         let measureExpectation2 = expectation(description: "measure")
         measureExpectation2.expectedFulfillmentCount = 3
 
-        performanceMetrics.measureInvokedClosure = { [count = VarBox(0)] identifier, metadata in
+        performanceMetrics.measureInvokedClosure = { [count = Box(0)] identifier, metadata in
             if count.value == 0 {
                 XCTAssertEqual(identifier, performanceMetrics.memoryReadIdentifier)
                 // cache miss
@@ -522,7 +522,7 @@ final class DiskMemoryPersistenceTestCase: XCTestCase {
         let measureExpectation = expectation(description: "measure")
         measureExpectation.expectedFulfillmentCount = 2
 
-        performanceMetrics.measureInvokedClosure = { [count = VarBox(0)] identifier, metadata in
+        performanceMetrics.measureInvokedClosure = { [count = Box(0)] identifier, metadata in
             if count.value == 0 {
                 XCTAssertEqual(identifier, performanceMetrics.memoryReadIdentifier)
                 XCTAssertDumpsEqual(metadata, [performanceMetrics.blobSizeMetadataKey : 0,

diff --git a/Tests/AlicerceTests/Utils/BoxTestCase.swift b/Tests/AlicerceTests/Utils/BoxTestCase.swift
@@ -3,24 +3,43 @@ import XCTest
 
 class BoxTestCase: XCTestCase {
 
-    func testBox_ShouldWrapValue() {
+    func test_init_ShouldWrapValueAndAllowMutation() {
 
         let value = 1337
         let box = Box<Int>(value)
 
         XCTAssertEqual(value, box.value)
+
+        let newValue = 7331
+        box.value = newValue
+
+        XCTAssertEqual(newValue, box.value)
     }
 
-    func testVarBox_ShouldWrapValueAndAllowModifying() {
+    func test_propertyWrapper_ShouldWrapValueAndAllowMutation() {
 
         let value = 1337
-        let varBox = VarBox<Int>(value)
+        @Box var box = value
 
-        XCTAssertEqual(value, varBox.value)
+        XCTAssertEqual(value, box)
 
         let newValue = 7331
-        varBox.value = newValue
+        box = newValue
+
+        XCTAssertEqual(newValue, box)
+    }
+
+    func test_dynamicMember_ShouldExposePropertiesInWrappedValue() {
+
+        struct Foo {
+
+            var foo: Int = 1337
+        }
+
+        let box = Box<Foo>(.init())
+        XCTAssertEqual(box.foo, 1337)
 
-        XCTAssertEqual(newValue, varBox.value)
+        @Box var box2 = Foo()
+        XCTAssertEqual(_box2.foo, 1337)
     }
 }
diff --git a/Tests/AlicerceTests/Utils/PthreadLockTestCase.swift b/Tests/AlicerceTests/Utils/PthreadLockTestCase.swift
@@ -112,13 +112,13 @@ class PthreadLockTestCase: XCTestCase {
             return q
         }
 
-        let box = VarBox(0)
+        @Box var box = 0
 
         for _ in 1...numWrites {
             queues.forEach {
                 $0.addOperation {
                     self.lock.lock()
-                    box.value += 1
+                    box += 1
                     self.lock.unlock()
                     writeExpectation.fulfill()
                 }
@@ -130,6 +130,6 @@ class PthreadLockTestCase: XCTestCase {
 
         waitForExpectations(timeout: 1)
 
-        XCTAssertEqual(box.value, numWrites * numQueues)
+        XCTAssertEqual(box, numWrites * numQueues)
     }
 }
diff --git a/Tests/AlicerceTests/Utils/UnfairLockTestCase.swift b/Tests/AlicerceTests/Utils/UnfairLockTestCase.swift
@@ -112,13 +112,13 @@ class UnfairLockTestCase: XCTestCase {
             return q
         }
 
-        let box = VarBox(0)
+        @Box var box = 0
 
         for _ in 1...numWrites {
             queues.forEach {
                 $0.addOperation {
                     self.lock.lock()
-                    box.value += 1
+                    box += 1
                     self.lock.unlock()
                     writeExpectation.fulfill()
                 }
@@ -130,7 +130,7 @@ class UnfairLockTestCase: XCTestCase {
 
         waitForExpectations(timeout: 1)
 
-        XCTAssertEqual(box.value, numWrites * numQueues)
+        XCTAssertEqual(box, numWrites * numQueues)
     }
 }