SwipeActionTransitioning.swift

//
//  SwipeActionTransitioning.swift
//
//  Created by Jeremy Koch
//  Copyright © 2017 Jeremy Koch. All rights reserved.
//

import UIKit

/**
 Adopt the `SwipeActionTransitioning` protocol in objects that implement custom appearance of actions during transition.
 */
public protocol SwipeActionTransitioning {
    /**
     Tells the delegate that transition change has occured.
     */
    func didTransition(with context: SwipeActionTransitioningContext) -> Void
}

/**
 The `SwipeActionTransitioningContext` type provides information relevant to a specific action as transitioning occurs.
 */
public struct SwipeActionTransitioningContext {
    /// The unique action identifier.
    public let actionIdentifier: String?
    
    /// The button that is changing.
    public let button: UIButton
    
    /// The old visibility percentage between 0.0 and 1.0.
    public let newPercentVisible: CGFloat
    
    /// The new visibility percentage between 0.0 and 1.0.
    public let oldPercentVisible: CGFloat
    
    internal let wrapperView: UIView
    
    internal init(actionIdentifier: String?, button: UIButton, newPercentVisible: CGFloat, oldPercentVisible: CGFloat, wrapperView: UIView) {
        self.actionIdentifier = actionIdentifier
        self.button = button
        self.newPercentVisible = newPercentVisible
        self.oldPercentVisible = oldPercentVisible
        self.wrapperView = wrapperView
    }
    
    /// Sets the background color behind the action button.
    /// 
    /// - parameter color: The background color.
    public func setBackgroundColor(_ color: UIColor?) {
        wrapperView.backgroundColor = color
    }
}

/**
 A scale transition object drives the custom appearance of actions during transition. 
 
 As button's percentage visibility crosses the `threshold`, the `ScaleTransition` object will animate from `initialScale` to `identity`.  The default settings provide a "pop-like" effect as the buttons are exposed more than 50%.
 */
public struct ScaleTransition: SwipeActionTransitioning {
    
    /// Returns a `ScaleTransition` instance with default transition options.
    public static var `default`: ScaleTransition { return ScaleTransition() }
    
    /// The duration of the animation.
    public let duration: Double
    
    /// The initial scale factor used before the action button percent visible is greater than the threshold.
    public let initialScale: CGFloat

    /// The percent visible threshold that triggers the scaling animation.
    public let threshold: CGFloat
    
    /**
     Contructs a new `ScaleTransition` instance.
    
    - parameter duration: The duration of the animation.
    
    - parameter initialScale: The initial scale factor used before the action button percent visible is greater than the threshold.
    
    - parameter threshold: The percent visible threshold that triggers the scaling animation.
    
    - returns: The new `ScaleTransition` instance.
    */
    public init(duration: Double = 0.15, initialScale: CGFloat = 0.8, threshold: CGFloat = 0.5) {
        self.duration = duration
        self.initialScale = initialScale
        self.threshold = threshold
    }
    
    /// :nodoc:
    public func didTransition(with context: SwipeActionTransitioningContext) -> Void {
        if context.oldPercentVisible == 0 {
            context.button.transform = .init(scaleX: initialScale, y: initialScale)
        }
        
        if context.oldPercentVisible < threshold && context.newPercentVisible >= threshold {
            UIView.animate(withDuration: duration) {
                context.button.transform = .identity
            }
        } else if context.oldPercentVisible >= threshold && context.newPercentVisible < threshold {
            UIView.animate(withDuration: duration) {
                context.button.transform = .init(scaleX: self.initialScale, y: self.initialScale)
            }
        }
    }
}