Faster text sizing

Author: lkzhao

Sources/UIComponent/Components/View/Text.swift

@@ -15,10 +15,16 @@ public enum TextContent {
     case attributedString(NSAttributedString)
 }
 
+/// A shared UILabel instance used for sizing text when `useSharedLabelForSizing` is true.
+private let layoutLabel = UILabel()
+
 /// A `Text` component represents a piece of text with styling and layout information.
 /// It can be initialized with either a plain `String` or an `NSAttributedString`.
 /// It also supports the new swift `AttributedString` from iOS 15 for more complex styling.
 public struct Text: Component {
+    /// A flag to determine if a shared UILabel should be used for sizing.
+    static var useSharedLabelForSizing = true
+
     /// Environment-injected font used when rendering plain strings.
     @Environment(\.font) var font
     /// Environment-injected text color used when rendering plain strings.
@@ -110,6 +116,25 @@ public struct Text: Component {
         case .attributedString(let string):
             attributedString = string
         }
+        if Self.useSharedLabelForSizing, Thread.isMainThread {
+            layoutLabel.numberOfLines = numberOfLines
+            layoutLabel.lineBreakMode = lineBreakMode
+            switch content {
+            case .string(let string):
+                layoutLabel.font = font
+                layoutLabel.text = string
+            case .attributedString(let string):
+                layoutLabel.font = nil
+                layoutLabel.attributedText = string
+            }
+            let size = layoutLabel.sizeThatFits(constraint.maxSize)
+            return TextRenderNode(
+                attributedString: attributedString,
+                numberOfLines: numberOfLines,
+                lineBreakMode: lineBreakMode,
+                size: size.bound(to: constraint)
+            )
+        }
         if numberOfLines != 0 || isSwiftAttributedString {
             // Slower route
             //

Tests/UIComponentTests/LazyLayoutTest.swift

@@ -97,6 +97,45 @@ struct LazyLayoutTests {
         #expect(fixedSizeLayoutTime * 2 < rawLayoutTime)
     }
 
+    @Test func testPerfImprovment() throws {
+        Text.useSharedLabelForSizing = false
+        let oldLayoutTime = measureTime {
+            VStack {
+                for _ in 0..<10000 {
+                    Text(text1)
+                }
+            }
+        }
+        let oldLayoutTimeWithNumberOfLines = measureTime {
+            VStack {
+                for _ in 0..<10000 {
+                    Text(text1, numberOfLines: 1)
+                }
+            }
+        }
+        Text.useSharedLabelForSizing = true
+        let newLayoutTime = measureTime {
+            VStack {
+                for _ in 0..<10000 {
+                    Text(text1)
+                }
+            }
+        }
+        let newLayoutTimeWithNumberOfLines = measureTime {
+            VStack {
+                for _ in 0..<10000 {
+                    Text(text1, numberOfLines: 1)
+                }
+            }
+        }
+        print("Layout 10000 text with old method used \(oldLayoutTime)s.")
+        print("Layout 10000 text with new method used \(newLayoutTime)s.")
+        print("Layout 10000 text with old method used \(oldLayoutTimeWithNumberOfLines)s with number of lines.")
+        print("Layout 10000 text with new method used \(newLayoutTimeWithNumberOfLines)s with number of lines.")
+        #expect(newLayoutTime < oldLayoutTime)
+        #expect(newLayoutTimeWithNumberOfLines < oldLayoutTimeWithNumberOfLines)
+    }
+
     func measureTime(_ component: () -> any Component) -> TimeInterval {
         let view = UIView()
         let startTime = CACurrentMediaTime()

Tests/UIComponentTests/UIComponentTests.swift

@@ -4,7 +4,7 @@ import XCTest
 
 @testable import UIComponent
 
-let text1 = "This is a test of layout methods"
+let text1 = "This is a test of layout methods, performance, and correctness."
 let text2 = "This is a test of layout"
 let font = UIFont.systemFont(ofSize: 16)
 let maxSize = CGSize(width: 100, height: CGFloat.infinity)