让UIScrollerview一直显示滚动条

一.UIScrollView显示或者隐藏滚动条, 默认有两个属性

@property(nonatomic)         BOOL                         showsHorizontalScrollIndicator; // default YES. show indicator while we are tracking. fades out after tracking
@property(nonatomic)         BOOL                         showsVerticalScrollIndicator;   // default YES. show indicator while we are tracking. fades out after tracking

这两个属性默认是YES, 显示水平和垂直滚动条, 只有当我们滚动的时候显示,当我们不滚动时,慢慢淡出

二.UIScrollView滚动条的样式, 默认有三个样式

@property(nonatomic)         UIScrollViewIndicatorStyle   indicatorStyle;                 // default is UIScrollViewIndicatorStyleDefault

typedef NS_ENUM(NSInteger, UIScrollViewIndicatorStyle) {
    UIScrollViewIndicatorStyleDefault,     // black with white border. good against any background
    UIScrollViewIndicatorStyleBlack,       // black only. smaller. good against a white background
    UIScrollViewIndicatorStyleWhite        // white only. smaller. good against a black background
};

这个属性默认是UIScrollViewIndicatorStyleDefault, 第二个属性UIScrollViewIndicatorStyleBlack 滚动条是黑色, 第三个属性UIScrollViewIndicatorStyleWhite 滚动条是白色

三.UIScrollView滚动条的偏移量

@property(nonatomic)         UIEdgeInsets                 scrollIndicatorInsets;          // default is UIEdgeInsetsZero. adjust indicators inside of insets

四.UIScrollView滚动条一直显示

1. 滚动条是一个UIImageView, 滚动条隐藏是因为设置了alpha属性为0, 所有我们写一个UIImageView的分类

   
   #define noDisableVerticalScrollTag 836913
   #define noDisableHorizontalScrollTag 836914
   
   @implementation UIImageView (WLScrollView)
   
   - (void)setAlpha:(CGFloat)alpha {
       if (self.superview.tag == noDisableVerticalScrollTag) {
           if (alpha == 0 && self.autoresizingMask == UIViewAutoresizingFlexibleLeftMargin) {
               if (self.frame.size.width < 10 && self.frame.size.height > self.frame.size.width) {
                   UIScrollView *sc = (UIScrollView*)self.superview;
                   if (sc.frame.size.height < sc.contentSize.height) {
                       return;
                   }
               }
           }
       }
   
       if (self.superview.tag == noDisableHorizontalScrollTag) {
           if (alpha == 0 && self.autoresizingMask == UIViewAutoresizingFlexibleTopMargin) {
               if (self.frame.size.height < 10 && self.frame.size.height < self.frame.size.width) {
                   UIScrollView *sc = (UIScrollView*)self.superview;
                   if (sc.frame.size.width < sc.contentSize.width) {
                       return;
                   }
               }
           }
       }
   
       [super setAlpha:alpha];
   }
   @end
   

2. 写完分类之后, 我们在初始化UIScrollView的时候设置UIScrollView的tag为836913

3. 要在UIScrollView数据绑定之后, 调用以下方法

   - (void)flashScrollIndicators;             // displays the scroll indicators for a short time. This should be done whenever you bring the scroll view to front.

   即调用以下方法

   [scrollView reloadData];
   [scrollView layoutIfNeeded]; // 第一次进来时没有显示出来，可能是UI没有更新，添加这句，等reloadData刷新完之后，更新UI；
   [scrollView flashScrollIndicators];