(lldb) po [success compare:@"yes"]
$1 = 0 <nil>
(lldb) p [success compare:@"yes"]
(int) $2 = 0
(lldb) p [success isEqualToString:@"yes"] 安全写法
(char) $3 = '\0'
(lldb) p success
(NSString *) $4 = 0x00000000
(lldb) p [success compare:@"yes"] == 0 //NSOrderedSame
(bool) $5 = true