components/BottomSheet/src/private/MDCDraggableView.m

// Copyright 2017-present the Material Components for iOS authors. All Rights Reserved.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#import "MDCDraggableView.h"

static void CancelGestureRecognizer(UIGestureRecognizer *gesture) {
  if (gesture.enabled) {
    // Setting enabled to NO while a gesture recognizer is currently recognizing a gesture will
    // transition it to a cancelled state.
    gesture.enabled = NO;
    gesture.enabled = YES;
  }
}

@interface MDCDraggableView ()<UIGestureRecognizerDelegate>
@property(nonatomic) UIPanGestureRecognizer *dragRecognizer;
@property(nonatomic, strong) UIScrollView *scrollView;
@end

@implementation MDCDraggableView

- (instancetype)initWithFrame:(CGRect)frame scrollView:(UIScrollView *)scrollView {
  self = [super initWithFrame:frame];
  if (self) {
    _scrollView = scrollView;
    _dragRecognizer = [[UIPanGestureRecognizer alloc] initWithTarget:self
                                                              action:@selector(didPan:)];
    _dragRecognizer.maximumNumberOfTouches = 1;
    _dragRecognizer.delegate = self;
    [self addGestureRecognizer:_dragRecognizer];

    self.clipsToBounds = YES;
  }
  return self;
}

#pragma mark - Gesture handling

- (void)didPan:(UIPanGestureRecognizer *)recognizer {
  CGPoint point = [recognizer translationInView:self.superview];

  // Ensure that dragging the sheet past the maximum height results in an exponential decay on the
  // translation. This gives the same effect as when you overscroll a scrollview.
  CGFloat newHeight = CGRectGetMaxY(self.superview.bounds) - CGRectGetMinY(self.frame);
  if (newHeight > [self.delegate maximumHeightForDraggableView:self]) {
    point.y -= point.y / 1.2;
  }

  self.center = CGPointMake(self.center.x, self.center.y + point.y);
  [recognizer setTranslation:CGPointZero inView:self.superview];

  CGPoint velocity = [recognizer velocityInView:self.superview];
  velocity.x = 0;

  if (recognizer.state == UIGestureRecognizerStateBegan) {
    [self.delegate draggableViewBeganDragging:self];
  } else if (recognizer.state == UIGestureRecognizerStateEnded) {
    [self.delegate draggableView:self draggingEndedWithVelocity:velocity];
  }
}

#pragma mark - UIGestureRecognizerDelegate

- (BOOL)gestureRecognizerShouldBegin:(UIPanGestureRecognizer *)recognizer {
  // When opening the control center while a MDCBottomSheet is displayed recognizer happens to be
  // an object of _UISystemGestureGateGestureRecognizer which doesn't have velocityInView: and it
  // crashes the app.
  if (recognizer != self.dragRecognizer) {
    return NO;
  }

  CGPoint velocity = [recognizer velocityInView:self.superview];
  velocity.x = 0;

  if ([self.delegate draggableView:self shouldBeginDraggingWithVelocity:velocity]) {
    // If dragging the pane, don't allow the content to scroll at the same time.
    CancelGestureRecognizer(self.scrollView.panGestureRecognizer);
    return YES;
  } else {
    return NO;
  }
}

// Allow the drag recogniser to recognize alongside the embedded scrollview's pan gesture.
- (BOOL)gestureRecognizer:(__unused UIGestureRecognizer *)recognizer
    shouldRecognizeSimultaneouslyWithGestureRecognizer:
        (UIGestureRecognizer *)otherGestureRecognizer {
  if (otherGestureRecognizer == self.scrollView.panGestureRecognizer) {
    return YES;
  }
  return NO;
}

@end