Add API for keyboard observation on ScrollView

BlueprintUICommonControls.podspec

@@ -14,4 +14,9 @@ Pod::Spec.new do |s|
   s.source_files = 'BlueprintUICommonControls/Sources/**/*.swift'
 
   s.dependency 'BlueprintUI'
+
+  s.test_spec 'Tests' do |test_spec|
+    test_spec.source_files = 'BlueprintUICommonControls/Tests/**/*.swift'
+    test_spec.framework = 'XCTest'
+  end
 end

BlueprintUICommonControls/Sources/Internal/KeyboardObserver.swift

@@ -0,0 +1,178 @@
+//
+//  KeyboardObserver.swift
+//  BlueprintUICommonControls
+//
+//  Created by Kyle Van Essen on 2/16/20.
+//
+
+import UIKit
+
+
+protocol KeyboardObserverDelegate : AnyObject {
+
+    func keyboardFrameWillChange(
+        for observer : KeyboardObserver,
+        animationDuration : Double,
+        options : UIView.AnimationOptions
+    )
+}
+
+/**
+ Implementation borrowed from Listable:
+ https://github.com/kyleve/Listable/blob/master/Listable/Sources/Internal/KeyboardObserver.swift
+ 
+ iOS Docs for keyboard management:
+ https://developer.apple.com/library/archive/documentation/StringsTextFonts/Conceptual/TextAndWebiPhoneOS/KeyboardManagement/KeyboardManagement.html
+ */
+final class KeyboardObserver {
+
+    private let center : NotificationCenter
+
+    weak var delegate : KeyboardObserverDelegate?
+
+    //
+    // MARK: Initialization
+    //
+
+    init(center : NotificationCenter = .default) {
+
+        self.center = center
+
+        /// We need to listen to both `will` and `keyboardDidChangeFrame` notifications. Why?
+        /// When dealing with an undocked or floating keyboard, moving the keyboard
+        /// around the screen does NOT call `willChangeFrame`; only `didChangeFrame` is called.
+        /// Before calling the delegate, we compare `old.endingFrame != new.endingFrame`,
+        /// which ensures that the delegate is notified if the frame really changes, and
+        /// prevents duplicate calls.
+
+        self.center.addObserver(self, selector: #selector(keyboardFrameChanged(_:)), name: UIWindow.keyboardWillChangeFrameNotification, object: nil)
+        self.center.addObserver(self, selector: #selector(keyboardFrameChanged(_:)), name: UIWindow.keyboardDidChangeFrameNotification, object: nil)
+    }
+
+    private var latestNotification : NotificationInfo?
+
+    //
+    // MARK: Handling Changes
+    //
+
+    enum KeyboardFrame : Equatable {
+
+        /// The current frame does not overlap the current view at all.
+        case nonOverlapping
+
+        /// The current frame does overlap the view, by the provided rect, in the view's coordinate space.
+        case visible(frame: CGRect)
+    }
+
+    /// How the keyboard overlaps the view provided. If the view is not on screen (eg, no window),
+    /// or the observer has not yet learned about the keyboard's position, this method returns nil.
+    func currentFrame(in view : UIView) -> KeyboardFrame? {
+
+        guard view.window != nil else {
+            return nil
+        }
+
+        guard let notification = self.latestNotification else {
+            return nil
+        }
+
+        let frame = view.convert(notification.endingFrame, from: nil)
+
+        if frame.intersects(view.bounds) {
+            return .visible(frame: frame)
+        } else {
+            return .nonOverlapping
+        }
+    }
+
+    //
+    // MARK: Receiving Updates
+    //
+
+    private func receivedUpdatedKeyboardInfo(_ new : NotificationInfo) {
+
+        let old = self.latestNotification
+
+        self.latestNotification = new
+
+        /// Only communicate a frame change to the delegate if the frame actually changed.
+
+        if let old = old {
+            guard old.endingFrame != new.endingFrame else {
+                return
+            }
+        }
+
+        /**
+         Create an animation curve with the correct curve for showing or hiding the keyboard.
+         
+         This is unfortunately a private UIView curve. However, we can map it to the animation options' curve
+         like so: https://stackoverflow.com/questions/26939105/keyboard-animation-curve-as-int
+         */
+        let animationOptions = UIView.AnimationOptions(rawValue: new.animationCurve << 16)
+
+        self.delegate?.keyboardFrameWillChange(
+            for: self,
+            animationDuration: new.animationDuration,
+            options: animationOptions
+        )
+    }
+
+    //
+    // MARK: Notification Listeners
+    //
+
+    @objc private func keyboardFrameChanged(_ notification : Notification) {
+
+        do {
+            let info = try NotificationInfo(with: notification)
+            self.receivedUpdatedKeyboardInfo(info)
+        } catch {
+            assertionFailure("Blueprint could not read system keyboard notification. This error needs to be fixed in Blueprint. Error: \(error)")
+        }
+    }
+}
+
+extension KeyboardObserver
+{
+    struct NotificationInfo : Equatable {
+
+        var endingFrame : CGRect = .zero
+
+        var animationDuration : Double = 0.0
+        var animationCurve : UInt = 0
+
+        init(with notification : Notification) throws {
+
+            guard let userInfo = notification.userInfo else {
+                throw ParseError.missingUserInfo
+            }
+
+            guard let endingFrame = (userInfo[UIResponder.keyboardFrameEndUserInfoKey] as? NSValue)?.cgRectValue else {
+                throw ParseError.missingEndingFrame
+            }
+
+            self.endingFrame = endingFrame
+
+            guard let animationDuration = (userInfo[UIResponder.keyboardAnimationDurationUserInfoKey] as? NSNumber)?.doubleValue else {
+                throw ParseError.missingAnimationDuration
+            }
+
+            self.animationDuration = animationDuration
+
+            guard let animationCurve = (userInfo[UIResponder.keyboardAnimationCurveUserInfoKey] as? NSNumber)?.uintValue else {
+                throw ParseError.missingAnimationCurve
+            }
+
+            self.animationCurve = animationCurve
+        }
+
+        enum ParseError : Error, Equatable {
+
+            case missingUserInfo
+            case missingEndingFrame
+            case missingAnimationDuration
+            case missingAnimationCurve
+        }
+    }
+}