feat: Move view cache to Blueprint hierarchy

.gitignore

@@ -11,6 +11,9 @@ xcuserdata/
 Pods/
 *.xcworkspace
 
+# Tuist
+Derived/
+
 # SPM
 
 # Disabled so that we can commit the generated schemes that live in

BlueprintUI/Sources/BlueprintView/BlueprintView.swift

@@ -40,6 +40,8 @@ public final class BlueprintView: UIView {
 
     private var sizesThatFit: [SizeConstraint: CGSize] = [:]
 
+    private lazy var viewCache: TypeKeyedCache = .init()
+
     /// A base environment used when laying out and rendering the element tree.
     ///
     /// Some keys will be overridden with the traits from the view itself. Eg, `windowSize`, `safeAreaInsets`, etc.
@@ -525,6 +527,13 @@ public final class BlueprintView: UIView {
 
         var environment = inherited.merged(prioritizing: environment)
 
+        /// We're a root `BlueprintView`, so pass in the view measurer for our downstream
+        /// dependencies if we didn't inherit a measurement cache.
+
+        if environment.inheritedViewCache == nil {
+            environment.viewCache = viewCache
+        }
+
         if let displayScale = window?.screen.scale {
             environment.displayScale = displayScale
         }

BlueprintUI/Sources/Element/UIViewElement.swift

@@ -90,13 +90,23 @@ extension UIViewElement {
 
     /// Defer to the reused measurement view to provide the size of the element.
     public var content: ElementContent {
-
         ElementContent { constraint, environment in
-            UIViewElementMeasurer.shared.measure(
-                element: self,
-                constraint: constraint,
-                environment: environment
-            )
+            let view = environment.viewCache.value {
+                makeUIView()
+            }
+
+            let bounds = CGRect(origin: .zero, size: constraint.maximum)
+
+            /// Ensure that during measurement / sizing, the inherited `Environment` is available
+            /// to any child `BlueprintView`s. We must manually wire this property up, as the
+            /// measurement views are not in the view hierarchy.
+
+            view.nativeViewNodeBlueprintEnvironment = environment
+            defer { view.nativeViewNodeBlueprintEnvironment = nil }
+
+            updateUIView(view, with: UIViewElementContext(isMeasuring: true, environment: environment))
+
+            return size(bounds.size, thatFits: view)
         }
     }
 
@@ -129,54 +139,3 @@ public struct UIViewElementContext {
     /// The environment the element is rendered in.
     public var environment: Environment
 }
-
-/// An private type which caches `UIViewElement` views to be reused for sizing and measurement.
-private final class UIViewElementMeasurer {
-
-    /// The standard shared cache.
-    static let shared = UIViewElementMeasurer()
-
-    /// Provides the size for the provided element by using a cached measurement view.
-    func measure(
-        element: some UIViewElement,
-        constraint: SizeConstraint,
-        environment: Environment
-    ) -> CGSize {
-
-        let bounds = CGRect(origin: .zero, size: constraint.maximum)
-
-        let view = measurementView(for: element)
-
-        /// Ensure that during measurement / sizing, the inherited `Environment` is available
-        /// to any child `BlueprintView`s. We must manually wire this property up, as the
-        /// measurement views are not in the view hierarchy.
-
-        view.nativeViewNodeBlueprintEnvironment = environment
-        defer { view.nativeViewNodeBlueprintEnvironment = nil }
-
-        element.updateUIView(view, with: .init(isMeasuring: true, environment: environment))
-
-        return element.size(bounds.size, thatFits: view)
-    }
-
-    func measurementView<ViewElement: UIViewElement>(for element: ViewElement) -> ViewElement.UIViewType {
-        let key = Key(
-            elementType: ObjectIdentifier(ViewElement.self)
-        )
-
-        if let existing = views[key] {
-            return existing as! ViewElement.UIViewType
-        } else {
-            let new = element.makeUIView()
-            views[key] = new
-            return new
-        }
-    }
-
-    private var views: [Key: UIView] = [:]
-
-    private struct Key: Hashable {
-        let elementType: ObjectIdentifier
-    }
-}
-

BlueprintUI/Sources/Environment/Keys/ViewCacheKey.swift

@@ -0,0 +1,55 @@
+import Foundation
+
+extension Environment {
+    private enum ViewCacheKey: EnvironmentKey {
+        static let defaultValue: TypeKeyedCache? = nil
+    }
+
+    private static let fallback = TypeKeyedCache()
+
+    var inheritedViewCache: TypeKeyedCache? {
+        self[ViewCacheKey.self]
+    }
+
+    public internal(set) var viewCache: TypeKeyedCache {
+        get {
+            if let inheritedViewCache {
+                return inheritedViewCache
+            } else {
+                #if DEBUG
+                do {
+                    /// Set a breakpoint on this `throw` if you'd like to understand where this error is occurring.
+                    ///
+                    /// We throw a caught error so that program execution can continue, and folks can opt
+                    /// in or out of stopping on the error.
+                    throw ViewCacheErrors.fallingBackToStaticCache
+                } catch {
+
+                    /// **Warning**: Blueprint is falling back to a static `TypeKeyedCache`,
+                    /// which will result in prototype measurement values being retained for
+                    /// the lifetime of the application, which can result in memory leaks.
+                    ///
+                    /// If you are seeing this error, ensure you're passing the Blueprint `Environment`
+                    /// properly through your element hierarchies – you should almost _never_ be
+                    /// passing an `.empty` environment to methods, and instead passing an inherited
+                    /// environment which will be passed to you by callers or a parent view controller,
+                    /// screen, or element.
+                    ///
+                    /// To learn more, see https://github.com/square/Blueprint/tree/main/Documentation/TODO.md.
+
+                }
+                #endif
+
+                return Self.fallback
+            }
+        }
+
+        set {
+            self[ViewCacheKey.self] = newValue
+        }
+    }
+
+    private enum ViewCacheErrors: Error {
+        case fallingBackToStaticCache
+    }
+}

BlueprintUI/Sources/Measuring/TypeKeyedCache.swift

@@ -0,0 +1,34 @@
+import UIKit
+
+/// A cache that uses the value's type as the key.
+public final class TypeKeyedCache {
+
+    private var views: [Key: AnyObject] = [:]
+
+    // Intentionally internal. Not intended for public instantiation.
+    init() {}
+
+    // Returns the cached value for the Value type if it exists. If it doesn't
+    // exist, it creates a new instance of Value, caches it, and returns it.
+    //
+    // - Parameter create: A closure that creates a new instance of Value. It
+    // is only invoked when a cached value does not exist. The created value is
+    // cached for future usage.
+    public func value<Value: AnyObject>(_ create: () -> Value) -> Value {
+        let key = Key(
+            elementType: ObjectIdentifier(Value.self)
+        )
+
+        if let existing = views[key] {
+            return existing as! Value
+        } else {
+            let new = create()
+            views[key] = new
+            return new
+        }
+    }
+
+    private struct Key: Hashable {
+        let elementType: ObjectIdentifier
+    }
+}

BlueprintUICommonControls/Sources/AttributedLabel.swift

@@ -128,16 +128,22 @@ public struct AttributedLabel: Element, Hashable {
     /// You can check if this value should be false via `NSAttributedString.needsNormalizingForView(...)`
     public var needsTextNormalization: Bool = true
 
-    private static let prototypeLabel = LabelView()
-
     public var content: ElementContent {
 
         // We create this outside of the measurement block so it's called fewer times.
         let text = displayableAttributedText
 
         return ElementContent { constraint, environment -> CGSize in
-            let label = Self.prototypeLabel
-            label.update(model: self, text: text, environment: environment, isMeasuring: true)
+            let label = environment.viewCache.value {
+                LabelView()
+            }
+
+            label.update(
+                model: self,
+                text: text,
+                environment: environment,
+                isMeasuring: true
+            )
             return label.sizeThatFits(constraint.maximum)
         }
     }

SampleApp/Sources/AccessibilityViewController.swift

@@ -34,8 +34,8 @@ final class AccessibilityViewController: UIViewController {
 
             Row {
                 Button(
-                    onTap: {
-                        self.firstTrigger.focus()
+                    onTap: { [weak self] in
+                        self?.firstTrigger.focus()
                     },
                     wrapping: Label(text: "Focus on First", configure: { label in
                         label.color = .systemBlue
@@ -75,9 +75,9 @@ final class AccessibilityViewController: UIViewController {
         .accessibilityContainer()
         .inset(uniform: 20)
         .centered()
-        .onAppear {
+        .onAppear { [weak self] in
             DispatchQueue.main.asyncAfter(deadline: .now() + 0.15) {
-                self.secondTrigger.focus()
+                self?.secondTrigger.focus()
             }
         }
     }

SampleApp/Sources/TextLinkViewController.swift

@@ -33,8 +33,8 @@ final class TextLinkViewController: UIViewController {
         return Column(alignment: .fill, minimumSpacing: 20) {
             label
             Label(text: "Custom link handling:")
-            label.onLinkTapped {
-                self.presentAlert(message: $0.absoluteString)
+            label.onLinkTapped { [weak self] in
+                self?.presentAlert(message: $0.absoluteString)
             }
 
             AttributedLabel(attributedText: NSAttributedString(string: "https://squareup.com")) {