告警等级
系统有四种内存警告，定义如下：
typedef enum {
OSMemoryNotificationLevelAny = -1,
OSMemoryNotificationLevelNormal = 0,
OSMemoryNotificationLevelWarning = 1,
OSMemoryNotificationLevelUrgent = 2,
OSMemoryNotificationLevelCritical = 3
} OSMemoryNotificationLevel;
内存警告通知：
UIApplicationDidReceiveMemoryWarningNotification