From cb2c41dccbfa2d5ee68bfeffd72100e1eef84e9a Mon Sep 17 00:00:00 2001
From: Kyle Van Essen <k@squareup.com>
Date: Wed, 22 Jun 2022 12:16:42 -0700
Subject: [PATCH] Introduce performInitialUpdate to allow disabling updates
 during view controller building.

---
 .../Sources/Screen/ScreenViewController.swift |  9 +++--
 .../ViewControllerDescription.swift           | 34 +++++++++++++++----
 .../ViewControllerDescriptionTests.swift      | 18 ++++++++++
 3 files changed, 53 insertions(+), 8 deletions(-)

diff --git a/WorkflowUI/Sources/Screen/ScreenViewController.swift b/WorkflowUI/Sources/Screen/ScreenViewController.swift
index e88f4019a..adfc9ae44 100644
--- a/WorkflowUI/Sources/Screen/ScreenViewController.swift
+++ b/WorkflowUI/Sources/Screen/ScreenViewController.swift
@@ -71,8 +71,13 @@
         /// Convenience to create a view controller description for the given screen
         /// value. See the example on the comment for ScreenViewController for
         /// usage.
-        public final class func description(for screen: ScreenType, environment: ViewEnvironment) -> ViewControllerDescription {
-            return ViewControllerDescription(
+        public final class func description(
+            for screen: ScreenType,
+            environment: ViewEnvironment,
+            performInitialUpdate: Bool = true
+        ) -> ViewControllerDescription {
+            ViewControllerDescription(
+                performInitialUpdate: performInitialUpdate,
                 type: self,
                 build: { self.init(screen: screen, environment: environment) },
                 update: { $0.update(screen: screen, environment: environment) }
diff --git a/WorkflowUI/Sources/ViewControllerDescription/ViewControllerDescription.swift b/WorkflowUI/Sources/ViewControllerDescription/ViewControllerDescription.swift
index f064f4721..936b0070d 100644
--- a/WorkflowUI/Sources/ViewControllerDescription/ViewControllerDescription.swift
+++ b/WorkflowUI/Sources/ViewControllerDescription/ViewControllerDescription.swift
@@ -34,6 +34,15 @@
     /// methods such as `buildViewController()`, `update(viewController:)`, if you are
     /// manually managing your own view controller hierarchy.
     public struct ViewControllerDescription {
+        /// If an initial call to `update(viewController:)` will be performed
+        /// when the view controller is created. Defaults to `true`.
+        ///
+        /// ### Note
+        /// When creating container view controllers that contain other view controllers
+        /// (eg, a navigation stack), you usually want to set this value to `false` to avoid
+        /// duplicate updates to your children if they are created in `init`.
+        public var performInitialUpdate: Bool = true
+
         private let viewControllerType: UIViewController.Type
         private let build: () -> UIViewController
         private let checkViewControllerType: (UIViewController) -> Bool
@@ -43,13 +52,24 @@
         /// build and update a specific view controller type.
         ///
         /// - Parameters:
+        ///   - performInitialUpdate: If an initial call to `update(viewController:)`
+        ///     will be performed when the view controller is created. Defaults to `true`.
+        ///
         ///   - type: The type of view controller produced by this description.
-        ///           Typically, should should be able to omit this parameter, but
-        ///           in cases where type inference has trouble, it’s offered as
-        ///           an escape hatch.
+        ///     Typically, should should be able to omit this parameter, but
+        ///     in cases where type inference has trouble, it’s offered as
+        ///     an escape hatch.
+        ///
         ///   - build: Closure that produces a new instance of the view controller
+        ///
         ///   - update: Closure that updates the given view controller
-        public init<VC: UIViewController>(type: VC.Type = VC.self, build: @escaping () -> VC, update: @escaping (VC) -> Void) {
+        public init<VC: UIViewController>(
+            performInitialUpdate: Bool = true,
+            type: VC.Type = VC.self,
+            build: @escaping () -> VC,
+            update: @escaping (VC) -> Void
+        ) {
+            self.performInitialUpdate = performInitialUpdate
             self.viewControllerType = type
             self.build = build
             self.checkViewControllerType = { $0 is VC }
@@ -66,8 +86,10 @@
         public func buildViewController() -> UIViewController {
             let viewController = build()
 
-            // Perform an initial update of the built view controller
-            update(viewController: viewController)
+            if performInitialUpdate {
+                // Perform an initial update of the built view controller
+                update(viewController: viewController)
+            }
 
             return viewController
         }
diff --git a/WorkflowUI/Tests/ViewControllerDescriptionTests.swift b/WorkflowUI/Tests/ViewControllerDescriptionTests.swift
index f2946e599..1144546e2 100644
--- a/WorkflowUI/Tests/ViewControllerDescriptionTests.swift
+++ b/WorkflowUI/Tests/ViewControllerDescriptionTests.swift
@@ -82,6 +82,24 @@
             XCTAssertFalse(description.canUpdate(viewController: UIViewController()))
         }
 
+        func test_performInitialUpdate() {
+            var updateCount = 0
+            let description = ViewControllerDescription(
+                performInitialUpdate: false,
+                build: { BlankViewController() },
+                update: { _ in updateCount += 1 }
+            )
+
+            XCTAssertEqual(updateCount, 0)
+
+            // Build should not cause an initial update when
+            let viewController = description.buildViewController()
+            XCTAssertEqual(updateCount, 0)
+
+            description.update(viewController: viewController)
+            XCTAssertEqual(updateCount, 1)
+        }
+
         func test_update() {
             var updateCount = 0
             let description = ViewControllerDescription(