輪播圖+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;">如何優化性能？</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;">無限循環實現了，但是我們知道如果圖片過多這些圖片勢必全部加載到內存，這是我們不願意看到的，此時我們需要優化上面的方案。其實從上面的方案我們也可以看出端倪，我們完全沒必要創建n+2個UIImageView，其實3個已經足夠（事實上也可以用兩個實現，大家不妨自己思考一下），只要一直保持顯示中間的UIImageView，滾動時動態更改三個UIImageView的圖片即可。例如三個UIImageView默認放圖片5、圖片1、圖片2，當前顯示中間的UIImageView，也就是圖片1,。如果向後滾動那麼就會顯示圖片2，當圖片2顯示完整後迅速重新設置三個UIImageView的內容爲圖片1、圖片2、圖片3，然後通過contentOffset設置顯示中間的UIImageView，也就是圖片2。繼續向後看到圖片3，當圖片3滾動完成迅速重新設置3個UIImageView的內容爲圖片2、圖片3、圖片4，然後設置contentOffset顯示中間的UIImageView，也就是圖片3。當然，向前滾動原理完全一樣，如此就給用戶一種循環錯覺，而且不佔用過多內存。</p>

分享一個實現輪播圖的效果

#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;//圖片數據
    int _currentImageIndex;//當前圖片索引
    int _imageCount;//圖片總數
}

@end

@implementation KCMainViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    //加載數據
    [self loadImageData];
    //添加滾動控件
    [self addScrollView];
    //添加圖片控件
    [self addImageViews];
    //添加分頁控件
    [self addPageControl];
    //添加圖片信息描述控件
    [self addLabel];
    //加載默認圖片
    [self setDefaultImage];
}

#pragma mark 加載圖片數據
-(void)loadImageData{
    //讀取程序包路徑中的資源文件
    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];
    //設置代理
    _scrollView.delegate=self;
    //設置contentSize
    _scrollView.contentSize=CGSizeMake(IMAGEVIEW_COUNT*SCREEN_WIDTH, SCREEN_HEIGHT) ;
    //設置當前顯示的位置爲中間圖片
    [_scrollView setContentOffset:CGPointMake(SCREEN_WIDTH, 0) animated:NO];
    //設置分頁
    _scrollView.pagingEnabled=YES;
    //去掉滾動條
    _scrollView.showsHorizontalScrollIndicator=NO;
}

#pragma mark 添加圖片三個控件
-(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 設置默認顯示圖片
-(void)setDefaultImage{
    //加載默認圖片
    _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;
    //設置當前頁
    _pageControl.currentPage=_currentImageIndex;
    NSString *imageName=[NSString stringWithFormat:@"%i.jpg",_currentImageIndex];
    _label.text=_imageData[imageName];
}

#pragma mark 添加分頁控件
-(void)addPageControl{
    _pageControl=[[UIPageControl alloc]init];
    //注意此方法可以根據頁數返回UIPageControl合適的大小
    CGSize size= [_pageControl sizeForNumberOfPages:_imageCount];
    _pageControl.bounds=CGRectMake(0, 0, size.width, size.height);
    _pageControl.center=CGPointMake(SCREEN_WIDTH/2, SCREEN_HEIGHT-100);
    //設置顏色
    _pageControl.pageIndicatorTintColor=[UIColor colorWithRed:193/255.0 green:219/255.0 blue:249/255.0 alpha:1];
    //設置當前頁顏色
    _pageControl.currentPageIndicatorTintColor=[UIColor colorWithRed:0 green:150/255.0 blue:1 alpha:1];
    //設置總頁數
    _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 滾動停止事件
-(void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView{
    //重新加載圖片
    [self reloadImage];
    //移動到中間
    [_scrollView setContentOffset:CGPointMake(SCREEN_WIDTH, 0) animated:NO];
    //設置分頁
    _pageControl.currentPage=_currentImageIndex;
    //設置描述
    NSString *imageName=[NSString stringWithFormat:@"%i.jpg",_currentImageIndex];
    _label.text=_imageData[imageName];
}

#pragma mark 重新加載圖片
-(void)reloadImage{
    int leftImageIndex,rightImageIndex;
    CGPoint offset=[_scrollView contentOffset];
    if (offset.x>SCREEN_WIDTH) { //向右滑動
        _currentImageIndex=(_currentImageIndex+1)%_imageCount;
    }else if(offset.x<SCREEN_WIDTH){ //向左滑動
        _currentImageIndex=(_currentImageIndex+_imageCount-1)%_imageCount;
    }
    //UIImageView *centerImageView=(UIImageView *)[_scrollView viewWithTag:2];
    _centerImageView.image=[UIImage imageNamed:[NSString stringWithFormat:@"%i.jpg",_currentImageIndex]];
    
    //重新設置左右圖片
    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