ios开发图片轮播器以及定时器小问题

一：图片轮播器效果如图:能实现自动轮播，到最后一页时，轮播回来，可以实现拖拽滚动

二：代码：

#import "ViewController.h"
static CGFloat const imageCount = 5;
@interface ViewController ()<UIScrollViewDelegate>
/*scrollView*/
@property (nonatomic,weak)UIScrollView *scroll;
/*pageControl*/
@property (nonatomic,weak)UIPageControl *pageControl;
/*timer*/
@property (nonatomic ,strong)NSTimer *timer;
@property (nonatomic,assign)int  a;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    //0:创建底部视图
    UIView *bottomView = [[UIView alloc]init];
    bottomView.frame = CGRectMake(20, 100, self.view.frame.size.width - 40, 200);
    [self.view addSubview:bottomView];
    
    
    //1:创建滚动视图
    UIScrollView *scrollView = [[UIScrollView alloc]init];
    scrollView.frame  = bottomView.bounds;
    scrollView.bounces = NO;
    scrollView.showsVerticalScrollIndicator = NO;
    scrollView.showsHorizontalScrollIndicator = NO;
    scrollView.pagingEnabled = YES;
    scrollView.delegate = self;
    scrollView.backgroundColor = [UIColor redColor];
    self.scroll = scrollView;
    [bottomView addSubview:scrollView];
    
    //2:创建ImageView
    CGFloat imageWidth = scrollView.frame.size.width;
    CGFloat imageHeight = scrollView.frame.size.height;
    
    for (int i = 0; i < imageCount; i++) {
        
        UIImageView *imageView = [[UIImageView alloc]init];
        imageView.frame = CGRectMake(i *imageWidth, 0, imageWidth, imageHeight);
        imageView.image = [UIImage imageNamed:[NSString stringWithFormat:@"img_0%d.png",i+1]];
        [scrollView addSubview:imageView];
        
    }
    
    //3:设置scroll的contentSize
    scrollView.contentSize = CGSizeMake(imageCount *imageWidth, 0);
    
    
    //4:创建pageControl
    UIPageControl *pageControl = [[UIPageControl alloc]init];
    pageControl.numberOfPages = imageCount;
    pageControl.currentPage = 0;
    pageControl.pageIndicatorTintColor = [UIColor whiteColor];
    pageControl.currentPageIndicatorTintColor = [UIColor redColor];
    pageControl.center = CGPointMake(bottomView.frame.size.width *0.5, bottomView.frame.size.height *0.9);
    self.pageControl = pageControl;
    [bottomView addSubview:pageControl];
    
    //5：添加定时器
    NSTimer *timer = [NSTimer timerWithTimeInterval:2 target:self selector:@selector(changeImage) userInfo:nil repeats:YES];
    NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
    /*
     forMode的参数：
     NSDefaultRunLoopMode
     NSRunLoopCommonModes
     */
    self.timer = timer;
    [runLoop addTimer:timer forMode: NSRunLoopCommonModes];
//    [timer fire];
    
    
    //6:添加UITextView
    UITextView *textView = [[UITextView alloc]init];
    textView.frame = CGRectMake(0, 320, self.view.frame.size.width, 150);
    textView.text = @"凡事看大佛我阿萨德fo一我啊搜ID念佛爱上你都放那艘IDif完加强覅进去我ijefo凡事看大佛我阿萨德fo一我啊搜ID念佛爱上你都放那艘IDif完加强覅进去我ijefo凡事看大佛我阿萨德fo一我啊搜ID念佛爱上你都放那艘IDif完加强覅进去我ijefo凡事看大佛我阿萨德fo一我啊搜ID念佛爱上你都放那艘IDif完加强覅进去我ijefo凡事看大佛我阿萨德fo一我啊搜ID念佛爱上你都放那艘IDif完加强覅进去我ijefo凡事看大佛我阿萨德fo一我啊搜ID念佛爱上你都放那艘IDif完加强覅进去我ijefo凡事看大佛我阿萨德fo一我啊搜ID念佛爱上你都放那艘IDif完加强覅进去我ijefo凡事看大佛我阿萨德fo一我啊搜ID念佛爱上你都放那艘IDif完加强覅进去我ijefo凡事看大佛我阿萨德fo一我啊搜ID念佛爱上你都放那艘IDif完加强覅进去我ijefo凡事看大佛我阿萨德fo一我啊搜ID念佛爱上你都放那艘IDif完加强覅进去我ijefo";
    textView.scrollEnabled = YES;
    textView.backgroundColor = [UIColor redColor];
    [self.view addSubview:textView];
}

- (void)changeImage {
    
    NSInteger page = self.pageControl.currentPage;

    
    
    if (page == imageCount -1) {
        self.a = imageCount -1;
    }
    
    
    if (page == 0) {
        self.a = 0;
    }
    

    if (self.a == imageCount -1) {
        
        page -- ;
       
    }else {
        
        page ++;
    }
    

//    [self.scroll setContentOffset:CGPointMake(page *self.scroll.frame.size.width, 0) animated:YES];
    [UIView animateWithDuration:1.0 animations:^{
        self.scroll.contentOffset = CGPointMake(page *self.scroll.frame.size.width, 0);
    }];
    
}

- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    
        //当下一页滑到>=0.5宽度时，此时就要改变pageControl
        CGFloat page = scrollView.contentOffset.x / scrollView.frame.size.width;
        NSUInteger currentPage = page;
        self.pageControl.currentPage = (page - currentPage) < 0.5 ? currentPage : currentPage +1;
}

//拖拽开始时关闭定时器，定时器一旦关闭，就不能再重新开启
- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView {
    [self.timer invalidate];

}

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate {
    
    self.timer = [NSTimer scheduledTimerWithTimeInterval:2 target:self selector:@selector(changeImage) userInfo:nil repeats:YES];
}


@end

三：知识点总结

1：创建定时器： 1：NSTimer *timer = [NSTimer timerWithTimeInterval:2 target:self selector:@selector(changeImage) userInfo:nil repeats:YES];此方法创建的定时器，必须加到NSRunLoop中，NSRunLoop *runLoop = [NSRunLoop currentRunLoop]; [runLoop addTimer:timer forMode: NSRunLoopCommonModes];第二个参数有两种类型可供选择： forMode的参数： NSDefaultRunLoopMode  NSRunLoopCommonModes，第一个参数为默认参数，当下面有textView，textfield等控件时，拖拽控件，此时轮播器会停止轮播，因为NSRunLoop的原因，NSRunLoop为一个死循环，实时监测有无事件响应，textView或是textField的事件会影响图片轮播，使用第二个参数会解决这个问题 2： self.timer = [NSTimer scheduledTimerWithTimeInterval:2 target:self selector:@selector(changeImage) userInfo:nil repeats:YES];此种创建定时器的方式，默认加到了runloop，且默认为第二个参数。3：定时器方法：[timer fire];此时会立刻执行定时器的方法，若是没有调用此方法，则会等待2秒执行定时器的方法。

2：轮播器的拖拽：在拖拽轮播器时，不影响轮播效果，实现滚动代理的开始拖拽方法，并关闭定时器[self.timer invalidate];定时器一旦被关闭，则不能再重新开启，需要重新创建定时器。在拖拽结束的代理中，再重新创建定时器

3：图片滚动超过宽度的0.5时的计算方法：四舍五入CGFloat page = scrollView.contentOffset.x / scrollView.frame.size.width; NSUInteger currentPage = page; self.pageControl.currentPage = (page - currentPage) < 0.5 ? currentPage : currentPage +1;先求出滚动的浮点数，在转化成整数，做差值，小于0.5的取当前的整数，大于0.5的取当前整数加1

4：设置scrollView的偏移量：1：UIView的动画  2：[self.scroll setContentOffset:CGPointMake(page *self.scroll.frame.size.width, 0) animated:YES];第二种方法，滚动的时间很短，时间小于1秒，第一种就是可以控制动画的时间，和完成动画的回调

5：设置轮播效果：

    NSInteger page = self.pageControl.currentPage;

    if (page == imageCount -1) {

        self.a = imageCount -1;

    }

    if (page == 0) {

        self.a = 0;

    }

    if (self.a == imageCount -1) {

        

        page -- ;

       

    }else {

        

        page ++;

    }

注意：不要在此方法中，设置pageControl的currentPage，因为如果设置了，会立刻偏移到相应的位置，但是此时还在实时监测scrollView的滚动，在此滚动代理中，小于宽度一半时，会取当前的整数页，大于时会取整数+1页，所以此时pageControl会立刻到某页，在回到-1页，再突然回到某页，存在bug。不要设置pageControl的currentPage，让其直接在滚动代理中去设置。