轮播图+性能分析

<h2 id="autoid-4-1-0" style="font-size: 21px; letter-spacing: -1px; margin: 24px 0px 12px; padding: 6px 0px; line-height: 21px; color: rgb(51, 51, 51); font-family: Georgia, 'Times New Roman', Times, sans-serif; background-image: none; background-position: 0% 100%; background-repeat: repeat-x;">如何优化性能？</h2><p style="line-height: 25.2000007629395px; margin: 10px auto; color: rgb(51, 51, 51); font-family: Georgia, 'Times New Roman', Times, sans-serif; font-size: 14px;">无限循环实现了，但是我们知道如果图片过多这些图片势必全部加载到内存，这是我们不愿意看到的，此时我们需要优化上面的方案。其实从上面的方案我们也可以看出端倪，我们完全没必要创建n+2个UIImageView，其实3个已经足够（事实上也可以用两个实现，大家不妨自己思考一下），只要一直保持显示中间的UIImageView，滚动时动态更改三个UIImageView的图片即可。例如三个UIImageView默认放图片5、图片1、图片2，当前显示中间的UIImageView，也就是图片1,。如果向后滚动那么就会显示图片2，当图片2显示完整后迅速重新设置三个UIImageView的内容为图片1、图片2、图片3，然后通过contentOffset设置显示中间的UIImageView，也就是图片2。继续向后看到图片3，当图片3滚动完成迅速重新设置3个UIImageView的内容为图片2、图片3、图片4，然后设置contentOffset显示中间的UIImageView，也就是图片3。当然，向前滚动原理完全一样，如此就给用户一种循环错觉，而且不占用过多内存。</p>

分享一个实现轮播图的效果

#import "KCMainViewController.h"
#define SCREEN_WIDTH 320
#define SCREEN_HEIGHT 568
#define IMAGEVIEW_COUNT 3

@interface KCMainViewController ()<UIScrollViewDelegate>{
    UIScrollView *_scrollView;
    UIImageView *_leftImageView;
    UIImageView *_centerImageView;
    UIImageView *_rightImageView;
    UIPageControl *_pageControl;
    UILabel *_label;
    NSMutableDictionary *_imageData;//图片数据
    int _currentImageIndex;//当前图片索引
    int _imageCount;//图片总数
}

@end

@implementation KCMainViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    //加载数据
    [self loadImageData];
    //添加滚动控件
    [self addScrollView];
    //添加图片控件
    [self addImageViews];
    //添加分页控件
    [self addPageControl];
    //添加图片信息描述控件
    [self addLabel];
    //加载默认图片
    [self setDefaultImage];
}

#pragma mark 加载图片数据
-(void)loadImageData{
    //读取程序包路径中的资源文件
    NSString *path=[[NSBundle mainBundle] pathForResource:@"imageInfo" ofType:@"plist"];
    _imageData=[NSMutableDictionary dictionaryWithContentsOfFile:path];
    _imageCount=(int)_imageData.count;
}

#pragma mark 添加控件
-(void)addScrollView{
    _scrollView=[[UIScrollView alloc]initWithFrame:[UIScreen mainScreen].bounds];
    [self.view addSubview:_scrollView];
    //设置代理
    _scrollView.delegate=self;
    //设置contentSize
    _scrollView.contentSize=CGSizeMake(IMAGEVIEW_COUNT*SCREEN_WIDTH, SCREEN_HEIGHT) ;
    //设置当前显示的位置为中间图片
    [_scrollView setContentOffset:CGPointMake(SCREEN_WIDTH, 0) animated:NO];
    //设置分页
    _scrollView.pagingEnabled=YES;
    //去掉滚动条
    _scrollView.showsHorizontalScrollIndicator=NO;
}

#pragma mark 添加图片三个控件
-(void)addImageViews{
    _leftImageView=[[UIImageView alloc]initWithFrame:CGRectMake(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT)];
    _leftImageView.contentMode=UIViewContentModeScaleAspectFit;
    [_scrollView addSubview:_leftImageView];
    _centerImageView=[[UIImageView alloc]initWithFrame:CGRectMake(SCREEN_WIDTH, 0, SCREEN_WIDTH, SCREEN_HEIGHT)];
    _centerImageView.contentMode=UIViewContentModeScaleAspectFit;
    [_scrollView addSubview:_centerImageView];
    _rightImageView=[[UIImageView alloc]initWithFrame:CGRectMake(2*SCREEN_WIDTH, 0, SCREEN_WIDTH, SCREEN_HEIGHT)];
    _rightImageView.contentMode=UIViewContentModeScaleAspectFit;
    [_scrollView addSubview:_rightImageView];

}
#pragma mark 设置默认显示图片
-(void)setDefaultImage{
    //加载默认图片
    _leftImageView.image=[UIImage imageNamed:[NSString stringWithFormat:@"%i.jpg",_imageCount-1]];
    _centerImageView.image=[UIImage imageNamed:[NSString stringWithFormat:@"%i.jpg",0]];
    _rightImageView.image=[UIImage imageNamed:[NSString stringWithFormat:@"%i.jpg",1]];
    _currentImageIndex=0;
    //设置当前页
    _pageControl.currentPage=_currentImageIndex;
    NSString *imageName=[NSString stringWithFormat:@"%i.jpg",_currentImageIndex];
    _label.text=_imageData[imageName];
}

#pragma mark 添加分页控件
-(void)addPageControl{
    _pageControl=[[UIPageControl alloc]init];
    //注意此方法可以根据页数返回UIPageControl合适的大小
    CGSize size= [_pageControl sizeForNumberOfPages:_imageCount];
    _pageControl.bounds=CGRectMake(0, 0, size.width, size.height);
    _pageControl.center=CGPointMake(SCREEN_WIDTH/2, SCREEN_HEIGHT-100);
    //设置颜色
    _pageControl.pageIndicatorTintColor=[UIColor colorWithRed:193/255.0 green:219/255.0 blue:249/255.0 alpha:1];
    //设置当前页颜色
    _pageControl.currentPageIndicatorTintColor=[UIColor colorWithRed:0 green:150/255.0 blue:1 alpha:1];
    //设置总页数
    _pageControl.numberOfPages=_imageCount;
    
    [self.view addSubview:_pageControl];
}

#pragma mark 添加信息描述控件
-(void)addLabel{
    
    _label=[[UILabel alloc]initWithFrame:CGRectMake(0, 10, SCREEN_WIDTH,30)];
    _label.textAlignment=NSTextAlignmentCenter;
    _label.textColor=[UIColor colorWithRed:0 green:150/255.0 blue:1 alpha:1];

    [self.view addSubview:_label];
}

#pragma mark 滚动停止事件
-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView{
    //重新加载图片
    [self reloadImage];
    //移动到中间
    [_scrollView setContentOffset:CGPointMake(SCREEN_WIDTH, 0) animated:NO];
    //设置分页
    _pageControl.currentPage=_currentImageIndex;
    //设置描述
    NSString *imageName=[NSString stringWithFormat:@"%i.jpg",_currentImageIndex];
    _label.text=_imageData[imageName];
}

#pragma mark 重新加载图片
-(void)reloadImage{
    int leftImageIndex,rightImageIndex;
    CGPoint offset=[_scrollView contentOffset];
    if (offset.x>SCREEN_WIDTH) { //向右滑动
        _currentImageIndex=(_currentImageIndex+1)%_imageCount;
    }else if(offset.x<SCREEN_WIDTH){ //向左滑动
        _currentImageIndex=(_currentImageIndex+_imageCount-1)%_imageCount;
    }
    //UIImageView *centerImageView=(UIImageView *)[_scrollView viewWithTag:2];
    _centerImageView.image=[UIImage imageNamed:[NSString stringWithFormat:@"%i.jpg",_currentImageIndex]];
    
    //重新设置左右图片
    leftImageIndex=(_currentImageIndex+_imageCount-1)%_imageCount;
    rightImageIndex=(_currentImageIndex+1)%_imageCount;
    _leftImageView.image=[UIImage imageNamed:[NSString stringWithFormat:@"%i.jpg",leftImageIndex]];
    _rightImageView.image=[UIImage imageNamed:[NSString stringWithFormat:@"%i.jpg",rightImageIndex]];
}

@end