iOS自定义滚动条

引言

最近一直在做数据通信相关的工作，导致了UI上的一些bug一直没有解决。这两天终于能腾出点时间大概看了一下Redmine上的bug，发现有很多bug都是与系统滚动条有关系的。所以索性就关注一下这个小小的滚动条。

为什么要自定义ScrollIndictor

原有的ScrollIndictor当然是系统提供的滚动条，但是为什么会有bug出现呢。这和现有的的需求有关系，需求定义是当现有页面内容超过一屏高度的时候，滚动条需要常显示，不能消失。小于一屏就不需要显示了。这和系统滚动条的显示行为不太一致。所以起初我们单纯的考虑，直接修改系统滚动条，让他常显示不就OK了。但是经过几轮测试过后，发现系统定义成让其消失是为了弥补滚动条时常时短的bug。系统控件存在问题怎么办？洪荒之力－－自定义吧。

现有滚动条显示方案

在谈自定义方案之前，还想和大家分享下现有的解决方法－－让系统滚动条常显。这个方法可谓是剑走偏锋，不过思路还是蛮新奇的。具体思路如下：

1.定义UIImageView的分类

2.重写setAlpha方法，从新定义UIImagView的隐藏行为，如果有tag值符合的View令其隐藏行为失效。

3.设置TableView，CollectionView， ScrollVIew的Tag值等于 noDisableVerticalScrollTag 或者 noDisableHorizontalScrollTag。

因为ScrollVIew的滚动条就是一个UIImageView，但是我们不能拿到这个滚动条的实例和隐藏掉用时机的。但是我们可以重新定义UIImageView的行为方法，控制起显示和隐藏的过程。具体代码如下。

#define noDisableVerticalScrollTag 836913

#define noDisableHorizontalScrollTag 836914

#import "UIImageView+ForScrollView.h"

@implementation UIImageView (ForScrollView)

- ( 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

现有方案存在的问题

上述的方案堪称是一劳永逸了，其他地方不需要修改任何代码就可以达到需求。但是测试时发现，在ScrollView滚动到底部的时候，滚动条会突然变长。而且快速改变滚动方向的时候，滚动条会出现闪烁的效果。这都会影响用户体验，必须要修改掉这样的问题。

开始自定义之旅

由于系统中用的最多的就是CollectionView，所以就先从这个CollecitonView的ScrollIncidtor开始吧。自定义可以采用分类也可以采用基类的形式。都各有利弊。采用分类在可以避免与工程中其他的类出现耦合的情况，代码更加集中好管理。但是不能重写父类的方法，例如reloadData。如果强行重写的话可能导致系统时序的混乱。所以我这里才用的是基类的方案。

其中比较难理解的就是滚动条在滑动到顶端和底端的时候，都要有个弹力的效果。这个效果的计算方法当时想的比较复杂。但是最后实现的时候发现也不是很难。具体的说明参见注释，代码如下。

#import "BaseCollectionView.h"
#import "Constants.h"

@implementation BaseCollectionView{
    UIView *scrollIndicatorView;
    CGFloat contentSizeHeight;
}

- ( void)drawRect:( CGRect)rect {
    [ self enableCustomVerticalScrollIndicatorWithColor:[ UIColor lightGrayColor]];
}

- ( void)reloadData{
    [ super reloadData];

    [ self setNeedsDisplay];
}

- ( UIView *)createIndicatorViewWithFrame:( CGRect) frame{
    UIView *indicator = [[ UIView alloc] initWithFrame:frame];
    indicator. layer. cornerRadius = ScrollIndicatorWidth/ 2.0f;
    //    viewScrollIndicator.alpha = 0.0f;
    //    viewScrollIndicator.layer.borderWidth = 1.0f;
    //    viewScrollIndicator.layer.borderColor = indicatorColor.CGColor;

    [ self addSubview:indicator];

    return indicator;
}

//Calculate the real height of scroll indictor accroding to the content size.
- ( CGFloat)getNormalScrollIndictorHeight{
    CGFloat percent = self. frame. size. height / self. contentSize. height;
    float normalHeight = MAX( 0.0f,(percent * self.frame.size.height));
    return normalHeight;
}

- ( void)enableCustomVerticalScrollIndicatorWithColor:( UIColor *)indicatorColor
{
    self. showsVerticalScrollIndicator = NO;

    float height = [ self getNormalScrollIndictorHeight];
    CGRect frame = CGRectMake( self. frame. size. width - ScrollIndicatorWidth - ScrollIndicatorRightSpace, 0.0f, ScrollIndicatorWidth, height);

    if( scrollIndicatorView == nil){
        scrollIndicatorView = [ self createIndicatorViewWithFrame:frame];
        [ self addKVOObservers];
    }
    else{
        scrollIndicatorView. frame = frame;
    }

    [ scrollIndicatorView setBackgroundColor:[indicatorColor colorWithAlphaComponent: 0.75]];

    //If content size is larger than frame size, the indictor will be displayed.
    if( self. frame. size. height >= self. contentSize. height){
        scrollIndicatorView. alpha = 0;
    }
    else{
        scrollIndicatorView. alpha = 1.0;
    }

    [ self refreshVerticalScrollIndicator];
}

- ( void)refreshVerticalScrollIndicator
{
    if ( self. contentSize. height <= 0) {
        return;
    }

    //Get the current frame of scroll indicator
    CGRect rect = scrollIndicatorView. frame;

    //Get the normal height of Indicator
    float normalHeight = [ self getNormalScrollIndictorHeight];

    //Calculate the real content offset ratio.
    CGFloat maxConentOffset = self. contentSize. height - self. frame. size. height;
    CGFloat offsetRatio = self. contentOffset. y / maxConentOffset;

    //Calculate the indictor offset
    CGFloat maxIndicatorOffset = ( self. frame. size. height - normalHeight);
    CGFloat indicatorOffset = offsetRatio * maxIndicatorOffset;

    //if scrolling out of top limitation, the scroll indictor will be compressed.
    if (indicatorOffset < 0) {
        rect. size. height = normalHeight + indicatorOffset;
    }
    //if scrolling out of bottom limitation, the scroll indictor will be compressed again.
    else if(indicatorOffset > self. frame. size. height - normalHeight){
        rect. size. height = normalHeight- (indicatorOffset - maxIndicatorOffset);

        //        indicatorOffset = self.frame.size.height - normalHeight;
    }
    else{
        rect. size. height = normalHeight;
    }

    rect. origin. y = self. contentOffset. y + MAX( 0.0f,indicatorOffset);

    if (rect. size. height < ScrollIndicatorMinHeight) {
        rect. size. height = ScrollIndicatorMinHeight;
    }

    scrollIndicatorView. frame = rect;
}

- ( void)dealloc
{
    [ self removeKVOObservers];
}

#pragma mark - KVO

- ( void)addKVOObservers
{
    [ self addObserver: self forKeyPath: @"contentSize" options:( NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld) context: NULL];
    [ self addObserver: self forKeyPath: @"contentOffset" options:( NSKeyValueObservingOptionNew | NSKeyValueObservingOptionOld) context: NULL];
}

- ( void)removeKVOObservers
{
    [ self removeObserver: self forKeyPath: @"contentSize"];
    [ self removeObserver: self forKeyPath: @"contentOffset"];
}

- ( void) observeValueForKeyPath: ( NSString *) keyPath ofObject: ( id) object change: ( NSDictionary *) change context: ( void *) context
{
    if ( self. contentSize. width > 0.0f) {
        [ self refreshVerticalScrollIndicator];

        /*
         UIView *viewScrollIndicator = [self getViewForHorizontalScrollIndicator];
         CGRect rect = self.frame;
         CGFloat pourcent = self.contentOffset.x / self.contentSize.width;
         viewScrollIndicator.hidden = self.contentSize.width < self.frame.size.width;
         rect.size.width = self.frame.size.width * (self.frame.size.width / self.contentSize.width);
         rect.origin.x = pourcent * self.frame.size.width;
         viewScrollIndicator.frame = rect;
         */
    }
}

@end