IOS倒计时秒杀

IOS中倒计时的方式有很多种:

1、使用NSTimer实现倒计时，需要注意的是NSTimer会持有对象，可能造成死锁

2、使用performSelector:<#(nonnull SEL)#> withObject:<#(nullable id)#> afterDelay:<#(NSTimeInterval)#>

3、使用GCD dispatch_after+迭代

4、使用GCD dispatch_source

下面是对使用dispatch_source实现倒计时的封装：

#import <Foundation/Foundation.h>

typedef void(^CountdownTimerBlock)(NSInteger days,NSInteger hours,NSInteger minutes,NSInteger seconds);

@interface CountdownTimer : NSObject

/**
 初始化倒计时器

 @param timeInterval 倒计时时间戳，以秒计算

 @return 返回初始化对象
 */
-(instancetype)initWithTimeInterval:(NSTimeInterval)timeInterval;


/**
 启动倒计时

 @param timerBlock 倒计时每调用一次执行的block
 */
-(void)startCountdownTimer:(CountdownTimerBlock)timerBlock;


/**
 关闭倒计时
 */
-(void)stopCountdownTimer;

@end

#import "CountdownTimer.h"
@interface CountdownTimer(){
    dispatch_source_t _timer;
}

@property(nonatomic,assign)NSTimeInterval timeInterval;

@end

@implementation CountdownTimer

-(instancetype)init{
    @throw [NSException exceptionWithName:NSInternalInconsistencyException
                                   reason:@"must use initWithTimeInterval:(NSTimeInterval)timeInterval instead"
                                 userInfo:nil];
}

-(instancetype)initWithTimeInterval:(NSTimeInterval)timeInterval{
    if(self=[super init]){
        _timeInterval=timeInterval;
    }
    return self;
}

-(void)startCountdownTimer:(CountdownTimerBlock)timerBlock{
    if(_timer==nil){
        __block int timeout = _timeInterval;
        if(timeout!=0){
            dispatch_queue_t queue=dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
            _timer=dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
            //每秒执行一次
            dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), 1.0*NSEC_PER_SEC, 0);
            dispatch_source_set_event_handler(_timer, ^{
                if(timeout<=0){
                    dispatch_source_cancel(_timer);
                    _timer=nil;
                    if(timerBlock){
                        dispatch_async(dispatch_get_main_queue(), ^{
                            timerBlock(0,0,0,0);
                        });
                    }
                }else{
                    int days = (int)(timeout/(3600*24));
                    int hours = (int)((timeout-days*24*3600)/3600);
                    int minutes = (int)(timeout-days*24*3600-hours*3600)/60;
                    int seconds = timeout-days*24*3600-hours*3600-minutes*60;
                    if(timerBlock){
                        dispatch_async(dispatch_get_main_queue(), ^{
                            timerBlock(days,hours,minutes,seconds);
                        });
                    }
                }
                timeout--;
            });
            dispatch_resume(_timer);
        }
    }
}

-(void)stopCountdownTimer{
    if(_timer){
        dispatch_source_cancel(_timer);
        _timeInterval=0;
        _timer = nil;
    }
}

@end

dispatch_after

performSelector:<#(nonnull SEL)#> withObject:<#(nullable id)#> afterDelay:<#(NSTimeInterval)#>

performSelector:<#(nonnull SEL)#> withObject:<#(nullable id)#> afterDelay:<#(NSTimeInterval)#>