More unit tests

Sources/FadeInText/FadeInTextController.swift

@@ -38,6 +38,7 @@ public class FadeInTextController: ObservableObject {
   @objc
   private func onFrameUpdate(_ displayLink: CADisplayLink) {
     guard let startTime, !self.chunks.isEmpty else {
+      self.tearDownDisplayLink()
       return
     }
     let time = CACurrentMediaTime() - startTime
@@ -52,8 +53,12 @@ public class FadeInTextController: ObservableObject {
     }
     self.text = updatedString
     if newResult.shouldAnimationFinish {
-      self.displayLink?.invalidate()
-      self.displayLink = nil
+      self.tearDownDisplayLink()
     }
   }
+
+  private func tearDownDisplayLink() {
+    self.displayLink?.invalidate()
+    self.displayLink = nil
+  }
 }

Sources/FadeInText/Interpolator/LinearInterpolator.swift

@@ -18,7 +18,7 @@ public final class LinearInterpolator: Interpolator {
 
     var newOpacities = [Double]()
     for i in 0..<numberOfChunks {
-      let startTime = Double(i) * (config.appearanceDuration / Double(numberOfChunks))
+      let startTime = Double(i) * config.appearanceDuration / Double(numberOfChunks)
       let newOpacity = max(min(1.0, (currentTime - startTime) / config.fadeInDuration), 0.0)
       newOpacities.append(newOpacity)
     }

Tests/FadeInTextTests/LinearInterpolatorTests.swift

@@ -0,0 +1,27 @@
+//
+//  File.swift
+//  
+//
+//  Created by Jun Yan on 12/26/24.
+//
+
+import Foundation
+import XCTest
+@testable import FadeInText
+
+class LinearInterpolatorTests: XCTestCase {
+
+  func test_interpolation() {
+    let interpolator = LinearInterpolator(config: .init(fadeInDuration: 0.4, appearanceDuration: 3))
+
+    var result = interpolator.interpolate(currentTime: 0, numberOfChunks: 5)
+    XCTAssertFalse(result.shouldAnimationFinish)
+    XCTAssertTrue(result.opacities.allSatisfy({ $0 == 0.0 }))
+
+    result = interpolator.interpolate(currentTime: 0.3, numberOfChunks: 5)
+    XCTAssertFalse(result.shouldAnimationFinish)
+    XCTAssertTrue(result.opacities.first! > 0)
+    XCTAssertTrue(result.opacities.filter { $0 == 0.0 }.count == 4)
+
+  }
+}