Animate dimming view opacity based on sheet height

SheetImagePicker.xcodeproj/project.pbxproj

@@ -18,6 +18,7 @@
 		D610D2F6233945C0009EB06A /* SheetImagePicker.framework in Embed Frameworks */ = {isa = PBXBuildFile; fileRef = D610D2D1233945AF009EB06A /* SheetImagePicker.framework */; settings = {ATTRIBUTES = (CodeSignOnCopy, RemoveHeadersOnCopy, ); }; };
 		D610D2FD23394E00009EB06A /* SheetContainerViewController.swift in Sources */ = {isa = PBXBuildFile; fileRef = D610D2FC23394E00009EB06A /* SheetContainerViewController.swift */; };
 		D610D2FF23395975009EB06A /* Detent.swift in Sources */ = {isa = PBXBuildFile; fileRef = D610D2FE23395975009EB06A /* Detent.swift */; };
+		D6B61D63233A748300809DE7 /* MathHelpers.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6B61D62233A748300809DE7 /* MathHelpers.swift */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXContainerItemProxy section */
@@ -58,6 +59,7 @@
 		D610D2F0233945BB009EB06A /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
 		D610D2FC23394E00009EB06A /* SheetContainerViewController.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = SheetContainerViewController.swift; sourceTree = "<group>"; };
 		D610D2FE23395975009EB06A /* Detent.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = Detent.swift; sourceTree = "<group>"; };
+		D6B61D62233A748300809DE7 /* MathHelpers.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = MathHelpers.swift; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -102,6 +104,7 @@
 			isa = PBXGroup;
 			children = (
 				D610D2D4233945AF009EB06A /* SheetImagePicker.h */,
+				D6B61D62233A748300809DE7 /* MathHelpers.swift */,
 				D610D2FC23394E00009EB06A /* SheetContainerViewController.swift */,
 				D610D2FE23395975009EB06A /* Detent.swift */,
 				D610D2D5233945AF009EB06A /* Info.plist */,
@@ -246,6 +249,7 @@
 			files = (
 				D610D2FD23394E00009EB06A /* SheetContainerViewController.swift in Sources */,
 				D610D2FF23395975009EB06A /* Detent.swift in Sources */,
+				D6B61D63233A748300809DE7 /* MathHelpers.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;
 		};

SheetImagePicker/MathHelpers.swift

@@ -0,0 +1,30 @@
+//
+//  MathHelpers.swift
+//  SheetImagePicker
+//
+//  Created by Shadowfacts on 9/24/19.
+//  Copyright © 2019 Shadowfacts. All rights reserved.
+//
+
+import UIKit
+
+func clamp(_ value: CGFloat, from: CGFloat, to: CGFloat) -> CGFloat {
+    if value < from {
+        return from
+    } else if value > to {
+        return to
+    } else {
+        return value
+    }
+}
+
+func smoothstep(value: CGFloat, from: CGFloat, to: CGFloat) -> CGFloat {
+    let x = clamp((value - from) / (to - from), from: 0, to: 1)
+    // 3x^2 - 2x^3
+    return 3 * pow(x, 2) - 2 * pow(x, 3)
+}
+
+func lerp(_ value: CGFloat, min: CGFloat, max: CGFloat, from: CGFloat, to: CGFloat) -> CGFloat {
+    let value = clamp((value - min) / (max - min), from: 0, to: 1)
+    return value * (to - from) + from
+}

SheetImagePicker/SheetContainerViewController.swift

@@ -31,6 +31,9 @@ public class SheetContainerViewController: UIViewController {
     var topDetent: (detent: Detent, offset: CGFloat) {
         return detents.map { ($0, $0.offset(in: view)) }.min(by: { $0.1 < $1.1 })!
     }
+    var bottomDetent: (detent: Detent, offset: CGFloat) {
+        return detents.map { ($0, $0.offset(in: view)) }.max(by: { $0.1 < $1.1 })!
+    }
     
     public var minimumDetentJumpVelocity: CGFloat = 500
     public var maximumStretchDistance: CGFloat = 15
@@ -39,6 +42,8 @@ public class SheetContainerViewController: UIViewController {
     lazy var initialConstant: CGFloat = view.bounds.height / 2
     
     var dimmingView: UIView!
+    public var minimumDimmingAlpha: CGFloat = 0
+    public var maximumDimmingAlpha: CGFloat = 0.75
 
     public init(content: UIViewController) {
         self.content = content
@@ -56,7 +61,7 @@ public class SheetContainerViewController: UIViewController {
         dimmingView = UIView()
         dimmingView.translatesAutoresizingMaskIntoConstraints = false
         dimmingView.backgroundColor = .systemGray
-        dimmingView.alpha = 0.5
+        dimmingView.alpha = (maximumDimmingAlpha - minimumDimmingAlpha) / 2
         view.addSubview(dimmingView)
         NSLayoutConstraint.activate([
             dimmingView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
@@ -88,27 +93,14 @@ public class SheetContainerViewController: UIViewController {
         case .changed:
             let translation = recognizer.translation(in: content.view)
             var realOffset = initialConstant + translation.y
-            if realOffset < topDetent.offset {
-                func clamp(_ value: CGFloat, from: CGFloat, to: CGFloat) -> CGFloat {
-                    if value < from {
-                        return from
-                    } else if value > to {
-                        return to
-                    } else {
-                        return value
-                    }
-                }
-                func smoothstep(value: CGFloat, from: CGFloat, to: CGFloat) -> CGFloat {
-                    let x = clamp((value - from) / (to - from), from: 0, to: 1)
-                    // 3x^2 - 2x^3
-                    return 3 * pow(x, 2) - 2 * pow(x, 3)
-                }
             let topOffset = topDetent.offset
+            if realOffset < topOffset {
                 let smoothed = smoothstep(value: realOffset, from: topOffset, to: 0)
                 realOffset = topOffset - smoothed * maximumStretchDistance
                 
             }
             topConstraint.constant = realOffset
+            dimmingView.alpha = lerp(realOffset, min: topOffset, max: bottomDetent.offset, from: maximumDimmingAlpha, to: minimumDimmingAlpha)
 
         case .ended:
             let velocity = recognizer.velocity(in: view)
@@ -130,6 +122,7 @@ public class SheetContainerViewController: UIViewController {
                 
                 UIView.animate(withDuration: 0.35, delay: 0, usingSpringWithDamping: 0.6, initialSpringVelocity: springVelocity, animations: {
                     self.view.layoutIfNeeded()
+                    self.dimmingView.alpha = lerp(springToDetent.1, min: self.topDetent.offset, max: self.bottomDetent.offset, from: self.maximumDimmingAlpha, to: self.minimumDimmingAlpha)
                 }, completion: { (finished) in
                     self.delegate?.sheetContainer(self, didSnapToDetent: springToDetent.0)
                 })