# 12. 缓存 NSDateFormatter

**作者**: [*拿破仑的*风火轮\_](https://weibo.com/u/2293476232)

为什么要缓存 `NSDateFormatter` ?

> Creating a date formatter is not a cheap operation. If you are likely to use a formatter frequently, it is typically more efficient to cache a single instance than to create and dispose of multiple instances. One approach is to use a static variable.

思路: 利用 `NSCache`, 以 `stringFormatter+NSLocale` 的 `localeIdentifier` 为 `key` 缓存 `NSDateFormatter`. 当`UIApplicationDidReceiveMemoryWarningNotification` 或 `NSCurrentLocaleDidChangeNotification` 释放 `NSCache` 缓存的对象.

代码参考 [BTNSDateFormatterFactory.m](https://github.com/BrooksWon/BTNSDateFormatterFactory/blob/master/BTNSDateFormatterFactory/BTNSDateFormatterFactory.m), 核心实现代码如下:

```
- (NSDateFormatter *)dateFormatterWithFormat:(NSString *)format andLocale:(NSLocale *)locale {
    @synchronized(self) {
        NSString *key = [NSString stringWithFormat:@"%@|%@", format, locale.localeIdentifier];

        NSDateFormatter *dateFormatter = [loadedDataFormatters objectForKey:key];
        if (!dateFormatter) {
            dateFormatter = [[NSDateFormatter alloc] init];
            dateFormatter.dateFormat = format;
            dateFormatter.locale = locale;
            [loadedDataFormatters setObject:dateFormatter forKey:key];
        }

        return dateFormatter;
    }
}
```