浏览图片可放大缩放功能

用scrollView加载图片并实现

#pragma mark --------------- scrollView代理 -------------
//要放大缩放的视图
- (UIView *)viewForZoomingInScrollView:(UIScrollView *)scrollView{
    return self.alertImageView;
}
//实现手势放大与缩放
- (void)scrollViewDidZoom:(UIScrollView *)scrollView{
    CGRect frame = self.alertImageView.frame;
    
    frame.origin.y = (self.scrollView.frame.size.height - self.alertImageView.frame.size.height) > 0 ? (self.scrollView.frame.size.height - self.alertImageView.frame.size.height) * 0.5 : 0;
    frame.origin.x = (self.scrollView.frame.size.width - self.alertImageView.frame.size.width) > 0 ? (self.scrollView.frame.size.width - self.alertImageView.frame.size.width) * 0.5 : 0;
    self.alertImageView.frame = frame;
    
    self.scrollView.contentSize = CGSizeMake(self.alertImageView.frame.size.width + 30, self.alertImageView.frame.size.height + 30);
}