BottomSheetViewController.m

- (void)panGesture:(UIPanGestureRecognizer *)recognizer {
    // Bottom Sheet 會隨著手勢移動跟著移動
    [self moveViewWithGesture:recognizer];

    // 當手移開螢幕時，Bottom Sheet 會根據新狀態移動到對應位置 
    if (recognizer.state != UIGestureRecognizerStateEnded) {
        return;
    }

    // 以動畫效果來漸進畫面的移動
    __weak __typeof(self)weakSelf = self;
    [UIView animateKeyframesWithDuration:0.2
                                   delay:0.0
                                 options:UIViewKeyframeAnimationOptionAllowUserInteraction
                              animations:^{
        const Director director = [recognizer velocityInView:weakSelf.view].y >= 0 ? down: up;
        State state = weakSelf.lastStatus;

        if (weakSelf.lastStatus == partial && director == up) {
            state = expanded;

        } else if (weakSelf.lastStatus == expanded && director == up) {
            state = full;
        }

        if (weakSelf.lastStatus == full && director == down) {
            state = expanded;

        } else if (weakSelf.lastStatus == expanded && director == down) {
            state = partial;
        }

        // handle
        if (recognizer.view) {
            if (state == expanded) {
                CGFloat endLocation = recognizer.view.frame.origin.y;
                if (endLocation > expandedViewYPosition &&
                    director == down) {
                    state = partial;

                } else if (endLocation < expandedViewYPosition &&
                           director == up) {
                    state = full;
                }

            } else if (state == partial &&
                weakSelf.lastStatus == partial) {
                CGFloat endLocation = recognizer.view.frame.origin.y;
                if (endLocation < expandedViewYPosition) {
                    state = expanded;
                }
            }
        }

        weakSelf.lastStatus = state;
        [weakSelf moveView:state];
                              } completion:^(BOOL finished) {
                                  // block fires when animaiton has finished
    }];
}

// 依造不同狀態調整不同高度
- (void)moveView:(State)state {
    CGFloat yPosition = fullViewYPosition;
    if (state == partial) {
        yPosition = partialViewYPosition;

    } else if (state == expanded) {
        yPosition = expandedViewYPosition;
    }

    const CGFloat width = self.view.frame.size.width;
    const CGFloat height = self.view.frame.size.height;
    CGRect rect = CGRectMake(0 , yPosition, width, height);

    self.view.frame = rect;
}

// 根據手勢移動到哪邊，view 的 height 就變化多少
- (void)moveViewWithGesture:(UIPanGestureRecognizer *)recognizer {
    const CGPoint translation = [recognizer translationInView:self.view];
    const CGFloat minY = self.view.frame.origin.y;

    if ((minY + translation.y >= fullViewYPosition) &&
        (minY + translation.y <= partialViewYPosition)) {
        const CGFloat width = self.view.frame.size.width;
        const CGFloat height = self.view.frame.size.height;
        const CGRect rect = CGRectMake(0 , minY + translation.y, width, height);
        self.view.frame = rect;
        [recognizer setTranslation:CGPointZero inView:self.view];
    }
}