NSDateFormatter
NSDateFormatter 不是唯一一个创建的开销就很昂贵的类，但是它却是常用的、开销大到 Apple 会特别建议应该缓存和重复使用实例的一个。
一种通用的缓存 NSDateFormatter 的方法是使用 -[NSThread threadDictionary]（因为 NSDateFormatter 不是线程安全的）：
+ (NSDateFormatter *)cachedDateFormatter {
NSMutableDictionary *threadDictionary = [[NSThread currentThread] threadDictionary];
NSDateFormatter *dateFormatter = [threadDictionary objectForKey:@"cachedDateFormatter"];
if (dateFormatter == nil) {
dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setLocale:[NSLocale currentLocale]];
[dateFormatter setDateFormat: @"YYYY-MM-dd HH:mm:ss"];
[threadDictionary setObject:dateFormatter forKey:@"cachedDateFormatter"];
}
return dateFormatter;
}
错误示例 每次转化都重新新建一个
//将时间转化为MM-dd HH：mm格式
- (NSString *)getTimeString:(double)timeString
{
if (timeString < 10000) {
return @"";
}
NSDate *timeDate = [NSDate dateWithTimeIntervalSince1970:timeString];
NSDateFormatter *format = [[NSDateFormatter alloc] init];
NSCalendar *calendar = [[NSCalendar alloc] initWithCalendarIdentifier:NSGregorianCalendar];
unsigned int unitFlag = NSYearCalendarUnit;
NSDate *nowDate = [NSDate date];
NSDateComponents *components = [calendar components:unitFlag fromDate:timeDate toDate:nowDate options:0];
NSInteger years = [components year];
if (years > 0) {
[format setDateFormat:@"yyyy-MM-dd HH:mm"];
}else{
[format setDateFormat:@"MM-dd HH:mm"];
}
return [format stringFromDate:timeDate];
}
