NSTimer、NSDate

###iOS时间那点事
经典看前必先看：iOS时间那点事NSCalendar + NSDateComponents
iOS 时区日期处理及定时 (NSDate,NSCalendar,NSTimer,NSTimeZone)

NSTimer

NSTimer 定时器用法总结

自动加一秒(dateByAddingComponents和dateByAddingTimeInterval)

参考：时间与日期处理
涉及知识点：
NSDate的生成
方法①通过NSDateFormatter的dateFromString:方法对dateString生成NSDate；

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

NSString *dateString = @"2016-08-15 22:55:03";
NSDate *date = [dateFormatter dateFromString:dateString];

方法②通过NSCalendar的dateFromComponents:方法对dateComponents生成NSDate;(dateComponents由分个字段生成)
[greCalendar dateFromComponents:dateComponents];

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    NSCalendar *greCalendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    
NSDateComponents *dateComponents = [[NSDateComponents alloc] init];
[dateComponents setYear:2016];
[dateComponents setMonth:8];
[dateComponents setDay:15];
NSDate *operationDate = [greCalendar dateFromComponents:dateComponents];

NSDate转NSString

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NSDate *operationDate;

- (void)updateTimeExample {
    NSCalendar *greCalendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    NSDateComponents *dateComponents = [[NSDateComponents alloc] init];
    [dateComponents setYear:2016];
    [dateComponents setMonth:8];
    [dateComponents setDay:15];
    operationDate = [greCalendar dateFromComponents:dateComponents];
    
    if (!self.timer) {
        self.timer = [NSTimer scheduledTimerWithTimeInterval:1.0 target: self selector:@selector(addOneSecondToDate:) userInfo:nil repeats:YES];
    }
}
    
- (void)addOneSecondToDate:(NSTimer *)timer {
//    NSDate *operationDate = [timer userInfo];
    NSCalendar *greCalendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
    
    NSDateComponents *offsetDateComponents = [[NSDateComponents alloc] init];
    [offsetDateComponents setMinute:1];
    [offsetDateComponents setSecond:1];
    NSDate *finalDate = [greCalendar dateByAddingComponents:offsetDateComponents toDate:operationDate options:0];
    operationDate = finalDate;
    
    //NSDate转NSString
    NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
    [dateFormatter setDateFormat:@"yyyy年MM月dd HH:mm:ss"];
    NSString *dateString = [dateFormatter stringFromDate:finalDate];
    
    self.networkTimeLabel.text = dateString;
}

#####NSJSONSerialization 去除不必要的空格回车
NSJSONSerialization介绍
NSJSONSerialization四个枚举什么意思

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// 要去除不必要的空格回车，只需将参数Option设置为0即可。如果设置NSJSONWritingPrettyPrinted会使格式化的json更加可读，即添加空格和换行，但如果设为0，则不添加空格和换行
    NSData *data = [NSJSONSerialization dataWithJSONObject:dictionary
                                                   options:0
                                                     error:nil];
    NSString *JSON = [[NSString alloc] initWithData:data
                                           encoding:NSUTF8StringEncoding];

######颜色渐变 CAGradientLayer
CAGradientLayer简介