`這次的專案裡面有涉及到商品的倒計時功能，UIcollectionview中羅列商品進行倒計時，以前頂多就是密碼找回，獲取驗證碼，或者輪播圖簡單的使用了一下NSTimer。之前還因為timer使用不恰當造成了記憶體持續飆高，直接crash的情況。。。銷燬timer需要使用invalidate，直接設定nil並不能銷燬。

這次使用GCD定時器進行功能簡單實現，其中也是找了一些資料，看了其他大神的程式碼，然後自己寫了一個demo，僅作為筆記方便以後自己檢視，完善。
demo地址:https://github.com/w467364316/GoodTimerDemo.git

放程式碼：
工具類.h

@interface CutDown : NSObject

-(void)creatTimerWit:(void(^)())completeBlock;

-(void)destroyTimer;
@end

建立定時器，設定間隔時間為0.1秒

@interface CutDown ()

@property(nonatomic,strong) dispatch_source_t timer;

@end

@implementation CutDown
-(void)creatTimerWit:(void(^)())completeBlock{

    if (_timer == nil
 
) {
        dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
        _timer = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER, 0, 0, queue);
        dispatch_source_set_timer(_timer, dispatch_walltime(NULL, 0), 0.1*NSEC_PER_SEC, 0);
        dispatch_source_set_event_handler(_timer, ^{
           dispatch_async
 
(dispatch_get_main_queue(), ^{
               completeBlock();
           });
        });
        dispatch_resume(_timer);
    }
}

-(void)destroyTimer{
    if (_timer) {
        dispatch_source_cancel(_timer);
        _timer = nil;
    }

}
@end

viewController內部，在viewDidLoad方法中呼叫cutDown 建立定時器

- (void)viewDidLoad {

    [super viewDidLoad];
    // Do any additional setup after loading the view from its nib.

    self.myCollection.backgroundColor = [UIColor whiteColor];
    self.dataArray = [NSMutableArray array];
    [self.myCollection registerNib:[UINib nibWithNibName:@"TimerCollectionViewCell" bundle:nil] forCellWithReuseIdentifier:@"timeCell"];

    NSString *path = [[NSBundle mainBundle] pathForResource:@"GoodsTimes" ofType:@".plist"];
    NSArray *array = [NSArray arrayWithContentsOfFile:path];

    for (NSDictionary *dic in array) {
        GoodTimeModel *model = [GoodTimeModel initWithDic:dic];
        [self.dataArray addObject:model];
    }
    //開始進行計時
    self.cutDown = [[CutDown alloc] init];
    __weak typeof(self) weakSelf = self;
    [self.cutDown creatTimerWit:^{
        [weakSelf updateVisialCells];
    }];
}

GoodTimeModel引數

@interface GoodTimeModel : NSObject
@property(nonatomic,copy) NSString *number;//序號
@property(nonatomic,copy) NSString *beginTime;//開始時間
@property(nonatomic,copy) NSString *time;//時間
@property(nonatomic,copy) NSString *minSecond;//毫秒

+(instancetype)initWithDic:(NSDictionary*)dic;
-(instancetype)initWithDic:(NSDictionary*)dic;

@end

更新當前檢視內的cell的顯示資訊

/**
 *  更新cell
 */
-(void)updateVisialCells{

    NSArray *cells = self.myCollection.visibleCells;
    for (TimerCollectionViewCell *cell in cells) {
        GoodTimeModel *model = self.dataArray[cell.tag];
        int minSec = [model.minSecond intValue];
        if (minSec <=0) {
            minSec = 9;
        }else{
            minSec --;
        }
        model.minSecond = [NSString stringWithFormat:@"%d",minSec];
        cell.model = model;
    }
}

設定cell部分

-(UICollectionViewCell*)collectionView:(UICollectionView *)collectionView cellForItemAtIndexPath:(NSIndexPath *)indexPath{

    TimerCollectionViewCell *cell = [collectionView dequeueReusableCellWithReuseIdentifier:@"timeCell" forIndexPath:indexPath];
    GoodTimeModel *model = self.dataArray[indexPath.row];
    cell.model = model;
    cell.tag = indexPath.row;
    return cell;
}

TimerCollectionViewCell中進行計算顯示剩餘時間

-(void)setModel:(GoodTimeModel *)model{

    _model = model;
    self.numerLabel.text = model.number;
    self.minSecondLabel.text = model.minSecond;
    [self setTimeWithLastTime:model.time beginTime:model.beginTime];
}

/**
 *  開始進行倒計時
 *
 *  @param time 結束時間
 */
-(void)setTimeWithLastTime:(NSString*)time beginTime:(NSString*)beginTime{

    NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
    [formatter setDateFormat:@"yyyy-MM-dd HH:mm:ss"];

    NSDate *endDate = [formatter dateFromString:time];
    NSDate *nowDate = [NSDate date];

    NSDate *beginDate = [formatter dateFromString:beginTime];
    NSTimeInterval beginTimeInterval = [beginDate timeIntervalSinceDate:nowDate];

    //剩餘時間
    NSTimeInterval timeInterval = [endDate timeIntervalSinceDate:nowDate];
    self.minSecondLabel.hidden = YES;
    if (timeInterval <=0) {
        //活動結束
        self.timerLabel.text = @"活動結束";
    }else if (beginTimeInterval > 0){
         //活動未開始
          self.timerLabel.text = @"活動未開始";
    }else{
        self.minSecondLabel.hidden = NO;
        int day = (int)timeInterval/(3600*24);
        int hours = (int)(timeInterval - day*3600*24)/3600;
        int minus = (timeInterval - day*24*3600 - hours*3600)/60;
        int second = (timeInterval - day*3600*24 - hours*3600- minus*60);

        //小時
        NSString *finalHours = [NSString stringWithFormat:@"%d",day*24 + hours];
        if ([finalHours intValue] <10) {
            finalHours = [NSString stringWithFormat:@"0%@",finalHours];
        }
        //分鐘
        NSString *finalMinutes = [NSString stringWithFormat:@"%d",minus];
        if ([finalMinutes intValue] <10) {
            finalMinutes = [NSString stringWithFormat:@"0%@",finalMinutes];
        }
        //秒
        NSString *finalSeconds = [NSString stringWithFormat:@"%d",second];
        if ([finalSeconds intValue] < 10) {
            finalSeconds = [NSString stringWithFormat:@"0%@",finalSeconds];
        }
        self.timerLabel.text = [NSString stringWithFormat:@"%@:%@:%@",finalHours,finalMinutes,finalSeconds];
    }
}

-(void)dealloc{

    NSLog(@"class = %s",object_getClassName(self));
    [_cutDown destroyTimer];
}