Add didChange observation

Author: n8chur

ViewEnvironmentUI/Sources/UIView+ViewEnvironmentPropagatingObject.swift

@@ -29,6 +29,10 @@ extension UIView: ViewEnvironmentPropagatingObject {
     @_spi(ViewEnvironmentWiring)
     public func setNeedsApplyEnvironment() {
         setNeedsLayout()
+
+        if let observing = self as? ViewEnvironmentObserving {
+            observing.environmentDidChange()
+        }
     }
 }
 

ViewEnvironmentUI/Sources/UIViewController+ViewEnvironmentPropagatingObject.swift

@@ -37,6 +37,10 @@ extension UIViewController: ViewEnvironmentPropagatingObject {
     @_spi(ViewEnvironmentWiring)
     public func setNeedsApplyEnvironment() {
         viewIfLoaded?.setNeedsLayout()
+
+        if let observing = self as? ViewEnvironmentObserving {
+            observing.environmentDidChange()
+        }
     }
 }
 

ViewEnvironmentUI/Sources/ViewEnvironmentObserving.swift

@@ -57,12 +57,20 @@ public protocol ViewEnvironmentObserving: ViewEnvironmentCustomizing {
     ///
     func apply(environment: ViewEnvironment)
 
-    /// Consumers _must_ call this function when the envirnoment should be re-applied, e.g. in
+    /// Consumers _must_ call this function when the environment should be re-applied, e.g. in
     /// `viewWillLayoutSubviews()` for `UIViewController`s and `layoutSubviews()` for `UIView`s.
     ///
     /// This will call ``apply(environment:)`` on the receiver if the node has been flagged for needing update.
     ///
     /// - Tag: ViewEnvironmentObserving.applyEnvironmentIfNeeded
     ///
     func applyEnvironmentIfNeeded()
+
+    func environmentDidChange()
+}
+
+extension ViewEnvironmentObserving {
+    public func apply(environment: ViewEnvironment) {}
+
+    public func environmentDidChange() {}
 }

ViewEnvironmentUI/Sources/ViewEnvironmentPropagationNode.swift

@@ -22,7 +22,7 @@ import ViewEnvironment
 /// environment as it flows between two nodes.
 ///
 @_spi(ViewEnvironmentWiring)
-public struct ViewEnvironmentPropagationNode: ViewEnvironmentCustomizing {
+public class ViewEnvironmentPropagationNode: ViewEnvironmentCustomizing, ViewEnvironmentObserving {
     public typealias EnvironmentAncestorProvider = () -> ViewEnvironmentPropagating?
 
     public typealias EnvironmentDescendantsProvider = () -> [ViewEnvironmentPropagating]
@@ -39,14 +39,28 @@ public struct ViewEnvironmentPropagationNode: ViewEnvironmentCustomizing {
         didSet { setNeedsEnvironmentUpdate() }
     }
 
+    public var environmentDidChangeObserver: ((ViewEnvironment) -> Void)? {
+        didSet { setNeedsEnvironmentUpdate() }
+    }
+
+    public var applyEnvironment: (ViewEnvironment) -> Void {
+        didSet { setNeedsEnvironmentUpdate() }
+    }
+
+    private var needsEnvironmentUpdate: Bool = true
+
     public init(
         environmentAncestor: @escaping EnvironmentAncestorProvider = { nil },
         environmentDescendants: @escaping EnvironmentDescendantsProvider = { [] },
-        customizeEnvironment: @escaping (inout ViewEnvironment) -> Void = { _ in }
+        customizeEnvironment: @escaping (inout ViewEnvironment) -> Void = { _ in },
+        environmentDidChange: ((ViewEnvironment) -> Void)? = nil,
+        applyEnvironment: @escaping (ViewEnvironment) -> Void = { _ in }
     ) {
         self.environmentAncestorProvider = environmentAncestor
         self.environmentDescendantsProvider = environmentDescendants
         self.customizeEnvironment = customizeEnvironment
+        self.environmentDidChangeObserver = environmentDidChange
+        self.applyEnvironment = applyEnvironment
     }
 
     public var environmentAncestor: ViewEnvironmentPropagating? { environmentAncestorProvider() }
@@ -58,6 +72,26 @@ public struct ViewEnvironmentPropagationNode: ViewEnvironmentCustomizing {
     }
 
     public func setNeedsEnvironmentUpdate() {
+        needsEnvironmentUpdate = true
+
         setNeedsEnvironmentUpdateOnAppropriateDescendants()
     }
+
+    public func environmentDidChange() {
+        guard let didChange = environmentDidChangeObserver else { return }
+
+        didChange(environment)
+    }
+
+    public func applyEnvironmentIfNeeded() {
+        guard needsEnvironmentUpdate else { return }
+
+        needsEnvironmentUpdate = false
+
+        apply(environment: environment)
+    }
+
+    public func apply(environment: ViewEnvironment) {
+        applyEnvironment(environment)
+    }
 }

WorkflowUI/Sources/Hosting/WorkflowHostingController.swift

@@ -181,13 +181,13 @@ public final class WorkflowHostingController<ScreenType, Output>: UIViewControll
 }
 
 extension WorkflowHostingController: ViewEnvironmentCustomizing, ViewEnvironmentObserving {
-    public func apply(environment: ViewEnvironment) {
-        update(screen: workflowHost.rendering.value, environment: environment)
-    }
-
     public func customize(environment: inout ViewEnvironment) {
         customizeEnvironment(&environment)
     }
+
+    public func environmentDidChange() {
+        update(screen: workflowHost.rendering.value, environment: environment)
+    }
 }
 
 #endif

WorkflowUI/Sources/Screen/ScreenViewController.swift

@@ -72,8 +72,8 @@ open class ScreenViewController<ScreenType: Screen>: UIViewController {
         self.screen = screen
         let previousEnvironment = self.environment
         _environment = environment
-        setNeedsEnvironmentUpdate()
         screenDidChange(from: previousScreen, previousEnvironment: previousEnvironment)
+        setNeedsEnvironmentUpdate()
     }
 
     open func screenDidChange(from previousScreen: ScreenType, previousEnvironment: ViewEnvironment) {}