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/Detent.swift

@@ -8,7 +8,7 @@
 
 import UIKit
 
-public enum Detent {
+public enum Detent: Equatable {
     case top
     case middle
     case bottom

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

@@ -8,22 +8,42 @@
 
 import UIKit
 
+public protocol SheetContainerViewControllerDelegate {
+    func sheetContainer(_ sheetContainer: SheetContainerViewController, willSnapToDetent detent: Detent) -> Bool
+    func sheetContainer(_ sheetContainer: SheetContainerViewController, didSnapToDetent detent: Detent)
+}
+
+// default no-op implementation
+public extension SheetContainerViewControllerDelegate {
+    func sheetContainer(_ sheetContainer: SheetContainerViewController, willSnapToDetent detent: Detent) -> Bool {
+        return true
+    }
+    func sheetContainer(_ sheetContainer: SheetContainerViewController, didSnapToDetent detent: Detent) {}
+}
+
 public class SheetContainerViewController: UIViewController {
 
+    public var delegate: SheetContainerViewControllerDelegate?
+    
     let content: UIViewController
     
-    public var detents: [Detent] = [.bottom, .middle, .top] {
-        didSet {
-            sortDetents()
-        }
+    public var detents: [Detent] = [.bottom, .middle, .top]
+    var topDetent: (detent: Detent, offset: CGFloat) {
+        return detents.map { ($0, $0.offset(in: view)) }.min(by: { $0.1 < $1.1 })!
     }
-    var sortedDetentOffsets: [CGFloat] = []
+    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
 
     var topConstraint: NSLayoutConstraint!
     lazy var initialConstant: CGFloat = view.bounds.height / 2
     
-    public var minimumDetentJumpVelocity: CGFloat = 500
-    public var maximumStretchDistance: CGFloat = 15
+    var dimmingView: UIView!
+    public var minimumDimmingAlpha: CGFloat = 0
+    public var maximumDimmingAlpha: CGFloat = 0.75
 
     public init(content: UIViewController) {
         self.content = content
@@ -38,6 +58,18 @@ public class SheetContainerViewController: UIViewController {
     override public func viewDidLoad() {
         super.viewDidLoad()
         
+        dimmingView = UIView()
+        dimmingView.translatesAutoresizingMaskIntoConstraints = false
+        dimmingView.backgroundColor = .systemGray
+        dimmingView.alpha = (maximumDimmingAlpha - minimumDimmingAlpha) / 2
+        view.addSubview(dimmingView)
+        NSLayoutConstraint.activate([
+            dimmingView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
+            dimmingView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
+            dimmingView.topAnchor.constraint(equalTo: view.topAnchor),
+            dimmingView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
+        ])
+        
         addChild(content)
         content.didMove(toParent: self)
         view.addSubview(content.view)
@@ -53,18 +85,6 @@ public class SheetContainerViewController: UIViewController {
         content.view.addGestureRecognizer(panGesture)
     }
 
-    public override func viewDidLayoutSubviews() {
-        super.viewDidLayoutSubviews()
-        
-        sortDetents()
-    }
-    
-    private func sortDetents() {
-        sortedDetentOffsets = detents.map {
-            $0.offset(in: view)
-        }.sorted()
-    }
-    
     @objc func panGestureRecognized(_ recognizer: UIPanGestureRecognizer) {
         switch recognizer.state {
         case .began:
@@ -73,62 +93,59 @@ public class SheetContainerViewController: UIViewController {
         case .changed:
             let translation = recognizer.translation(in: content.view)
             var realOffset = initialConstant + translation.y
-            if realOffset < sortedDetentOffsets.first! {
-                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 = sortedDetentOffsets.first!
+            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)
 
-            let springToDetent: CGFloat
+            let springToDetent: (Detent, CGFloat)
             if abs(velocity.y) > minimumDetentJumpVelocity,
-                let offsetInVelocityDirection = nearestDetentOffset(currentOffset: topConstraint.constant, direction: velocity.y) {
-                springToDetent = offsetInVelocityDirection
-            } else if let nearestOffset = nearestDetentOffset(currentOffset: topConstraint.constant) {
-                springToDetent = nearestOffset
+                let detentInVelocityDirection = nearestDetentOffset(currentOffset: topConstraint.constant, direction: velocity.y) {
+                springToDetent = detentInVelocityDirection
+            } else if let nearestDetent = nearestDetentOffset(currentOffset: topConstraint.constant) {
+                springToDetent = nearestDetent
             } else {
                 return
             }
             
-            let springDistance = abs(topConstraint.constant - springToDetent)
-            self.topConstraint.constant = springToDetent
-            let springVelocity = velocity.y / springDistance
+            if delegate?.sheetContainer(self, willSnapToDetent: springToDetent.0) ?? true {
+                let springDistance = abs(topConstraint.constant - springToDetent.1)
+                self.topConstraint.constant = springToDetent.1
+                let springVelocity = velocity.y / springDistance
                 
-            UIView.animate(withDuration: 0.35, delay: 0, usingSpringWithDamping: 0.6, initialSpringVelocity: springVelocity, animations: {
-                self.view.layoutIfNeeded()
-            })
+                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)
+                })
+            }
         default:
             return
         }
     }
     
-    func nearestDetentOffset(currentOffset: CGFloat) -> CGFloat? {
-        return sortedDetentOffsets.min(by: { abs($0 - currentOffset) < abs($1 - currentOffset) })
+    func nearestDetentOffset(currentOffset: CGFloat) -> (Detent, CGFloat)? {
+        return detents.map { ($0, $0.offset(in: view)) }.min { (a, b) -> Bool in
+            return abs(a.1 - currentOffset) < abs(b.1 - currentOffset)
+        }
     }
     
-    func nearestDetentOffset(currentOffset: CGFloat, direction: CGFloat) -> CGFloat? {
+    func nearestDetentOffset(currentOffset: CGFloat, direction: CGFloat) -> (Detent, CGFloat)? {
+        let sorted = detents.map { ($0, $0.offset(in: view)) }.sorted { (a, b) -> Bool in
+            return a.1 < b.1
+        }
         if direction < 0 {
-            return sortedDetentOffsets.last(where: { $0 < currentOffset })
+            return sorted.last(where: { $0.1 < currentOffset })
         } else {
-            return sortedDetentOffsets.first(where: { $0 > currentOffset })
+            return sorted.first(where: { $0.1 > currentOffset })
         }
     }
     

SheetImagePickerTest/ViewController.swift

@@ -14,12 +14,15 @@ class ViewController: UIViewController {
     override func viewDidLoad() {
         super.viewDidLoad()
         // Do any additional setup after loading the view.
+        view.backgroundColor = .green
         
         let content = UIViewController()
         content.view.translatesAutoresizingMaskIntoConstraints = false
         content.view.backgroundColor = .red
         let sheet = SheetContainerViewController(content: content)
-        sheet.view.backgroundColor = .blue
+        sheet.delegate = self
+        sheet.detents = [.bottom, .middle, .top]
+//        sheet.view.backgroundColor = .blue
         
         addChild(sheet)
         sheet.didMove(toParent: self)
@@ -34,3 +37,18 @@ class ViewController: UIViewController {
 
 }
 
+extension ViewController: SheetContainerViewControllerDelegate {
+    func sheetContainer(_ sheetContainer: SheetContainerViewController, willSnapToDetent detent: Detent) -> Bool {
+        if detent == .bottom {
+            UIView.animate(withDuration: 0.35, animations: {
+                sheetContainer.view.transform = CGAffineTransform(translationX: 0, y: sheetContainer.view.bounds.height)
+            }, completion: { (finished) in
+                sheetContainer.removeFromParent()
+                sheetContainer.didMove(toParent: nil)
+                sheetContainer.view.removeFromSuperview()
+            })
+            return false
+        }
+        return true
+    }
+}