此函数调用中的第二个参数:
dispatch_time(DISPATCH_TIME_NOW, NSEC_PER_SEC)
导致编译器错误:
'UInt64' is not convertible to 'Int64'
斯威夫特参考:
func dispatch_time(when: dispatch_time_t, delta: Int64) -> dispatch_time_t
var NSEC_PER_SEC: UInt64 { get } /* nanoseconds per second */