ImageEditorPinchGestureRecognizer.swift

//
// Copyright 2019 Signal Messenger, LLC
// SPDX-License-Identifier: AGPL-3.0-only
//

import SignalServiceKit

struct ImageEditorPinchState {
    let centroid: CGPoint
    let distance: CGFloat
    let angleRadians: CGFloat

    static var empty: ImageEditorPinchState {
        return ImageEditorPinchState(centroid: .zero, distance: 1.0, angleRadians: 0)
    }
}

// This GR:
//
// * Tries to fail quickly to avoid conflicts with other GRs, especially pans/swipes.
// * Captures a bunch of useful "pinch state" that makes using this GR much easier
//   than UIPinchGestureRecognizer.
class ImageEditorPinchGestureRecognizer: UIGestureRecognizer {

    weak var referenceView: UIView?

    var pinchStateStart = ImageEditorPinchState.empty

    var pinchStateLast = ImageEditorPinchState.empty

    // MARK: - Touch Handling

    private var gestureBeganLocation: CGPoint?

    private func failAndReset() {
        state = .failed
        gestureBeganLocation = nil
    }

    override func touchesBegan(_ touches: Set<UITouch>, with event: UIEvent) {
        super.touchesBegan(touches, with: event)

        if state == .possible {
            if gestureBeganLocation == nil {
                gestureBeganLocation = centroid(forTouches: event.allTouches)
            }

            switch touchState(for: event) {
            case .possible:
                // Do nothing
                break
            case .invalid:
                failAndReset()
            case .valid(let pinchState):
                state = .began
                pinchStateStart = pinchState
                pinchStateLast = pinchState
            }
        } else {
            failAndReset()
        }
    }

    override func touchesMoved(_ touches: Set<UITouch>, with event: UIEvent) {
        super.touchesMoved(touches, with: event)

        switch state {
        case .began, .changed:
            switch touchState(for: event) {
            case .possible:
                if let gestureBeganLocation {
                    let location = centroid(forTouches: event.allTouches)

                    // If the initial touch moves too much without a second touch,
                    // this GR needs to fail - the gesture looks like a pan/swipe/etc.,
                    // not a pinch.
                    let distance = location.distance(gestureBeganLocation)
                    let maxDistance: CGFloat = 10.0
                    guard distance <= maxDistance else {
                        failAndReset()
                        return
                    }
                }

            case .invalid:
                failAndReset()

            case .valid(let pinchState):
                state = .changed
                pinchStateLast = pinchState
            }
        default:
            failAndReset()
        }
    }

    override func touchesEnded(_ touches: Set<UITouch>, with event: UIEvent) {
        super.touchesEnded(touches, with: event)

        switch state {
        case .began, .changed:
            switch touchState(for: event) {
            case .possible:
                failAndReset()
            case .invalid:
                failAndReset()
            case .valid(let pinchState):
                state = .ended
                pinchStateLast = pinchState
            }
        default:
            failAndReset()
        }
    }

    override func touchesCancelled(_ touches: Set<UITouch>, with event: UIEvent) {
        super.touchesCancelled(touches, with: event)

        state = .cancelled
    }

    enum TouchState {
        case possible
        case valid(pinchState: ImageEditorPinchState)
        case invalid
    }

    private func touchState(for event: UIEvent) -> TouchState {
        guard let allTouches = event.allTouches else {
            owsFailDebug("Missing allTouches")
            return .invalid
        }
        // Note that we use _all_ touches.
        if allTouches.count < 2 {
            return .possible
        }
        guard let pinchState = pinchState() else {
            return .invalid
        }
        return .valid(pinchState: pinchState)
    }

    private func pinchState() -> ImageEditorPinchState? {
        guard let referenceView else {
            owsFailDebug("Missing view")
            return nil
        }
        guard numberOfTouches == 2 else {
            return nil
        }
        // We need the touch locations _with a stable ordering_.
        // The only way to ensure the ordering is to use location(ofTouch:in:).
        let location0 = location(ofTouch: 0, in: referenceView)
        let location1 = location(ofTouch: 1, in: referenceView)

        let centroid = CGPoint.scale(CGPoint.add(location0, location1), factor: 0.5)
        let distance = location0.distance(location1)

        // The valence of the angle doesn't matter; we're only going to be using
        // changes to the angle.
        let delta = CGPoint.subtract(location1, location0)
        let angleRadians = atan2(delta.y, delta.x)
        return ImageEditorPinchState(
            centroid: centroid,
            distance: distance,
            angleRadians: angleRadians,
        )
    }

    private func centroid(forTouches touches: Set<UITouch>?) -> CGPoint {
        guard let view = self.view else {
            owsFailDebug("Missing view")
            return .zero
        }
        guard let touches else {
            return .zero
        }
        guard touches.count > 0 else {
            return .zero
        }
        var sum = CGPoint.zero
        for touch in touches {
            let location = touch.location(in: view)
            sum = CGPoint.add(sum, location)
        }

        let centroid = CGPoint.scale(sum, factor: 1 / CGFloat(touches.count))
        return centroid
    }
}