DragToSwitchView

DragToSwitchView.swift

import UIKit

public protocol DragToSwitchViewDelegate: class {
    
    func dragToSwitchViewWillStartDragging(dragToSwitchView: DragToSwitchView)
    func dragToSwitchViewDidEndDragging(dragToSwitchView: DragToSwitchView)
    
    func dragToSwitchView(
        dragToSwitchView: DragToSwitchView,
        performSwitchingAnimationWithDuration duration: NSTimeInterval)
    
    func dragToSwitchView(dragToSwitchView: DragToSwitchView, willSwitchToView view: UIView)
    func dragToSwitchView(dragToSwitchView: DragToSwitchView, didSwitchToView view: UIView)
}

extension DragToSwitchViewDelegate {

    func dragToSwitchViewWillStartDragging(dragToSwitchView: DragToSwitchView) {}
    func dragToSwitchViewDidEndDragging(dragToSwitchView: DragToSwitchView) {}
    
    func dragToSwitchView(
        dragToSwitchView: DragToSwitchView,
        performSwitchingAnimationWithDuration duration: NSTimeInterval) {}
    
    func dragToSwitchView(dragToSwitchView: DragToSwitchView, willSwitchToView view: UIView) {}
    func dragToSwitchView(dragToSwitchView: DragToSwitchView, didSwitchToView view: UIView) {}
}

public class DragToSwitchView: UIControl {
    
    public weak var delegate: DragToSwitchViewDelegate?
    
    /// 触发两子视图切换的最小距离
    public var maximumThresholdDistance: CGFloat = 40
    
    /// 两个子视图之间切换所需要的动画时间
    public var transitionDuration: NSTimeInterval = 0.3
    
    override public init(frame: CGRect) {
        super.init(frame: frame)
        clipsToBounds = true
        configureForGestureRecognizer()
    }
    
    required public init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        clipsToBounds = true
        configureForGestureRecognizer()
    }
    
    /**
     配置需要切换的子视图
     DragToSwitchView 会自动设置子视图的横向约束（充满），每个子视图必须自行设置纵向约束
     
     - parameter topHiddenView:    默认被隐藏，放置于上方的子视图
     - parameter bottomVisibleView: 默认被显示，放置于下方的子视图
     */
    public func setup(topHiddenView topHiddenView: UIView, bottomVisibleView: UIView) {
        self.topView?.removeFromSuperview()
        self.bottomView?.removeFromSuperview()
        
        addSubview(topHiddenView)
        addSubview(bottomVisibleView)
        
        self.topView = topHiddenView
        self.bottomView = bottomVisibleView
        
        addConstraints(
            [
                NSLayoutConstraint
                    .constraintsWithVisualFormat("H:|[view]|",
                        options: .DirectionLeadingToTrailing,
                        metrics: nil,
                        views: ["view": topHiddenView]),
                NSLayoutConstraint
                    .constraintsWithVisualFormat("H:|[view]|",
                        options: .DirectionLeadingToTrailing,
                        metrics: nil,
                        views: ["view": bottomVisibleView])
                ]
                .flatten()
                .map { $0 }
        )
        
        transitionStatus = .TopViewAboveBottomView(topView: topHiddenView, bottomView: bottomVisibleView)
        bringSubviewToFront(topHiddenView)
        invalidateIntrinsicContentSize()
        superview?.layoutIfNeeded()
    }
    
    // MARK: Transition
    
    private weak var topView: UIView?
    private weak var bottomView: UIView?
    
    private var topViewBottomConstraint: NSLayoutConstraint?
    private var bottomViewBotomConstraint: NSLayoutConstraint?
    private var viewHeightConstraint: NSLayoutConstraint?
    
    /**
     切换状态
     
     - None:                   空，未配置子视图
     - TopViewAboveBottomView: TopView（不可见） 在 BottomView（可见） 之上
     - TopViewCoverBottomView: TopView（可见) 完整覆盖于 BottomView（不可见）之上
     - PrepareForSwapping:     准备进行子视图交换动画
     - TopViewOverBottomView:  TopView 被下拉（部分可见）覆盖了部分 BottomView（部分可见）
     */
    private enum TransitionStatus {
        case None
        case TopViewAboveBottomView(topView: UIView, bottomView: UIView)
        case TopViewCoverBottomView(topView: UIView, bottomView: UIView)
        case PrepareForSwapping
        case TopViewOverBottomView(yAxisDistance: CGFloat)
    }
    
    private var transitionStatus: TransitionStatus = .None {
        didSet {
            switch transitionStatus {
            case .TopViewAboveBottomView(let topView, let bottomView):
                if
                    let bottomViewBottomMargin = bottomViewBotomConstraint,
                    let topViewBottomMargin = topViewBottomConstraint,
                    let viewHeight = viewHeightConstraint {
                    removeConstraints([bottomViewBottomMargin, viewHeight, topViewBottomMargin])
                }
                topViewBottomConstraint = NSLayoutConstraint(
                    item: topView,
                    attribute: .Bottom,
                    relatedBy: .Equal,
                    toItem: self,
                    attribute: .Top,
                    multiplier: 1,
                    constant: 0)
                viewHeightConstraint = NSLayoutConstraint(
                    item: self,
                    attribute: .Height,
                    relatedBy: .Equal,
                    toItem: bottomView,
                    attribute: .Height,
                    multiplier: 1,
                    constant: 0)
                bottomViewBotomConstraint = NSLayoutConstraint(
                    item: bottomView,
                    attribute: .Bottom,
                    relatedBy: .Equal,
                    toItem: self,
                    attribute: .Bottom,
                    multiplier: 1,
                    constant: 0)
                addConstraints(
                    [
                        topViewBottomConstraint!,
                        viewHeightConstraint!,
                        bottomViewBotomConstraint!,
                        topViewBottomConstraint!
                    ]
                )
            case .TopViewCoverBottomView(let topView, _):
                if let constraint = topViewBottomConstraint {
                    removeConstraint(constraint)
                }
                if let constraint = viewHeightConstraint {
                    removeConstraint(constraint)
                }
                viewHeightConstraint = NSLayoutConstraint(
                    item: self,
                    attribute: .Height,
                    relatedBy: .Equal,
                    toItem: topView,
                    attribute: .Height,
                    multiplier: 1,
                    constant: 0)
                topViewBottomConstraint = NSLayoutConstraint(
                    item: topView,
                    attribute: .Bottom,
                    relatedBy: .Equal,
                    toItem: self,
                    attribute: .Bottom,
                    multiplier: 1,
                    constant: 0)
                
                addConstraints(
                    [
                        viewHeightConstraint!,
                        topViewBottomConstraint!
                    ]
                )
            case .TopViewOverBottomView(let yAxisDistance):
                viewHeightConstraint?.constant = yAxisDistance
                topViewBottomConstraint?.constant = yAxisDistance * 1.1
            default:
                break
            }
        }
    }
    
    // MARK:
    
    private var verticalPanGestureRecognizer: UIPanGestureRecognizer!
    private var panStartPoint: CGPoint?
    
    private func configureForGestureRecognizer() {
        verticalPanGestureRecognizer = UIPanGestureRecognizer(target: self, action: #selector(self.dynamicType.handleVerticalPan(_:)))
        verticalPanGestureRecognizer.delegate = self
        addGestureRecognizer(verticalPanGestureRecognizer)
    }
}

// MARK: Geusture method

extension DragToSwitchView {
    
    @objc
    private func handleVerticalPan(sender: UIPanGestureRecognizer) {
        guard
            let topView = topView,
            let bottomView = bottomView else {
                return
        }
        
        switch sender.state {
        case .Began:
            handlePanBegan(sender)
        case .Changed:
            handlePanChanged(sender, currentTopView: topView, currentBottomView: bottomView)
        case .Ended, .Cancelled, .Failed:
            handlePanEnded(sender, currentTopView: topView, currentBottomView: bottomView)
        case .Possible:
            break
        }
    }
    
    private func handlePanBegan(sender: UIPanGestureRecognizer) {
        delegate?.dragToSwitchViewWillStartDragging(self)
        panStartPoint = sender.locationInView(self)
    }
    
    private func handlePanChanged(
        sender: UIPanGestureRecognizer,
        currentTopView: UIView,
        currentBottomView: UIView) {
        let yAxisDistance: CGFloat = {
            guard let startPoint = panStartPoint else {
                fatalError()
            }
            let currentPoint = sender.locationInView(self)
            return currentPoint.y - startPoint.y
        }()
        
        if yAxisDistance < 0 {
            return
        }
        
        if yAxisDistance < maximumThresholdDistance {
            transitionStatus = .TopViewOverBottomView(yAxisDistance: yAxisDistance)
            invalidateIntrinsicContentSize()
            superview?.layoutIfNeeded()
            return
        }
        
        // 准备进行切换动画，禁用手势识别
        transitionStatus = .PrepareForSwapping
        sender.enabled = false
        
        swapTopAndBottomViews(
            currentTopView,
            currentBottomView: currentBottomView) { () -> Void in
                sender.enabled = true
        }
    }
    
    private func handlePanEnded(
        sender: UIPanGestureRecognizer,
        currentTopView: UIView,
        currentBottomView: UIView) {
        delegate?.dragToSwitchViewDidEndDragging(self)
        panStartPoint = nil
        
        switch transitionStatus {
        case .TopViewOverBottomView:
            sender.enabled = false
            transitionStatus = .TopViewAboveBottomView(
                topView: currentTopView,
                bottomView: currentBottomView)
            
            UIView.animateWithDuration(transitionDuration,
                                       animations: { () -> Void in
                                        self.superview?.layoutIfNeeded()
                }, completion: { (_) -> Void in
                    sender.enabled = true
            })
        default:
            return
        }
    }
    
    private func swapTopAndBottomViews(
        currentTopView: UIView,
        currentBottomView: UIView,
        complitionHandler: () -> Void) {
        let animationDuration = transitionDuration
        delegate?.dragToSwitchView(self, performSwitchingAnimationWithDuration: animationDuration)
        
        UIView.animateWithDuration(
            animationDuration,
            delay: 0,
            options: .CurveEaseIn,
            animations: { () -> Void in
                // 首先将 TopView 降到底部，覆盖住 BottomView
                self.transitionStatus = .TopViewCoverBottomView(
                    topView: currentTopView,
                    bottomView: currentBottomView)
                self.invalidateIntrinsicContentSize()
                self.superview?.layoutIfNeeded()
                self.delegate?.dragToSwitchView(self, willSwitchToView: currentTopView)
        }) { (_) -> Void in
            // 然后将 BottomView 升到 TopView 头部，并置换两者，至此完成切换
            (self.topView, self.bottomView) = (self.bottomView, self.topView)
            self.transitionStatus = .TopViewAboveBottomView(
                topView: self.topView!,
                bottomView: self.bottomView!)
            self.invalidateIntrinsicContentSize()
            self.superview?.layoutIfNeeded()
            self.delegate?.dragToSwitchView(self, didSwitchToView: currentTopView)
            self.bringSubviewToFront(self.topView!)
            complitionHandler()
        }
    }
}

extension DragToSwitchView: UIGestureRecognizerDelegate {
    
    public override func gestureRecognizerShouldBegin(gestureRecognizer: UIGestureRecognizer) -> Bool {
        if gestureRecognizer == verticalPanGestureRecognizer {
            let velocity = verticalPanGestureRecognizer.velocityInView(self)
            return fabs(velocity.y) > fabs(velocity.x)
        }
        return true
    }
    
    public func gestureRecognizer(
        gestureRecognizer: UIGestureRecognizer,
        shouldRecognizeSimultaneouslyWithGestureRecognizer otherGestureRecognizer: UIGestureRecognizer)
        -> Bool {
            return true
    }
}