杂记-下滑关闭图片

要实现下滑关闭图片这个功能，可以将其分解为几个需求：

1. 向下滑动图片
2. 图片随着往下滑动会逐渐变小，往上滑动则会还原至初始大小
3. 往下滑动背景越来越透明，往上滑动则还原至不透明

滑动图片很简单，添加一个拖拽手势即可：

UIPanGestureRecognizer *pan = [[UIPanGestureRecognizer alloc] initWithTarget:self action:@selector(photoPan:)];
pan.delegate = self;
[imageView addGestureRecognizer:pan];

然后就是实现滑动过程中，图片缩小放大动画以及背景色变化，实现代码如下所示：

-(void)photoPan:(UIPanGestureRecognizer *)panGesture
{
    SDBrowserImageView *attchView = (SDBrowserImageView *)panGesture.view;
    CGPoint pointTrans = [panGesture translationInView:_scrollView];
    CGPoint point = [panGesture locationInView:_scrollView];

    NSLog(@"attchView.transform:%@",NSStringFromCGAffineTransform(attchView.transform));
    NSLog(@"x:%f , y:%f",point.x,point.y);

    if (panGesture.state == UIGestureRecognizerStateBegan)
    {
        [[UIApplication sharedApplication] setStatusBarHidden:NO];
        self.startPoint = point;
    }
    else if(panGesture.state == UIGestureRecognizerStateChanged)
    {
        CGFloat deviationY = point.y - _startPoint.y;
        CGFloat scaleTrans = 1 - pointTrans.y/SCREEN_HEIGHT;//缩放比例
        CGFloat scaleAlpha = (1 - deviationY/SCREEN_HEIGHT) > 0 ? (1 - deviationY/SCREEN_HEIGHT):0.2;//背景色变化比例
        self.backgroundColor = [UIColor colorWithWhite:0.1 alpha:scaleAlpha];

        if (deviationY > 0)
        {
            NSLog(@"scaleTrans : %.2f",scaleTrans);
            CGAffineTransform trans = CGAffineTransformMakeTranslation(pointTrans.x, pointTrans.y);
            CGAffineTransform scale = CGAffineTransformScale(attchView.transform, scaleTrans, scaleTrans);
            
            CGAffineTransform newTransform = CGAffineTransformConcat(trans, scale);
            attchView.transform = newTransform;
        }
        else
        {
            CGAffineTransform trans = CGAffineTransformTranslate(attchView.transform, pointTrans.x, pointTrans.y);
            attchView.transform = trans;
        }
    }
    else if(panGesture.state == UIGestureRecognizerStateEnded)
    {
        CGFloat deviationY = point.y - _startPoint.y;
        if (deviationY >= 60)
        {
             [self photoPanFinished:attchView];
        }
        else
        {
            [UIView animateWithDuration:0.2 animations:^{
                attchView.transform = CGAffineTransformIdentity;
            }];
        }
    }
    else if(panGesture.state == UIGestureRecognizerStateCancelled)
    {
        [[UIApplication sharedApplication] setStatusBarHidden:YES];
    }
    else if(panGesture.state == UIGestureRecognizerStateFailed)
    {
        [[UIApplication sharedApplication] setStatusBarHidden:YES];
    }
    //增量置为o
    [panGesture setTranslation:CGPointZero inView:attchView];
}

translationInView:是为了获取每次拖拽间距，往下为正，往上为负，往左为负，往右为正。

startPoint是初始触摸点，locationInView: 是为了获取相对于触摸view的父视图的移动距离。
CGAffineTransformMakeTranslation 是位移变换。
CGAffineTransformScale 是缩放变换。
CGAffineTransformConcat 组合两个动画。
CGAffineTransformIdentity 是动画初始状态。

每次移动一段距离都需要执行setTranslation:方法，将增直量置为0，否则接下来的移动会叠加之前距离。

在拖拽距离超过60，且松手后，会执行photoPanFinished:方法以关闭图片。

在查看图片时，实现下滑以关闭图片过程中，碰到一个问题：Pan手势与UIScrollView自带的Pan手势冲突，只能相应下滑关闭手势，不能相应左右滑动手势，解决办法如下：

- (BOOL)gestureRecognizerShouldBegin:(UIGestureRecognizer *)gestureRecognizer 
{
    if ([gestureRecognizer isKindOfClass:[UIPanGestureRecognizer class]]) {
        
        UIPanGestureRecognizer *pan = (UIPanGestureRecognizer *)gestureRecognizer;
        CGPoint pos = [pan velocityInView:pan.view];
        if (pos.y > 0) {
            return YES;
        }
    }
    
    return NO;
}

其中velocityInView:是用来获取滑动速度的，pos.y > 0 表示速度向下，反之向上。

这个方法还可以解决如下问题：

1. 可以使得向下滑动图片时，scollview不动（只需要多加一行禁用scrollview的代码）
   2.使图片初始不能向上滑动，只能向下滑