MessageSend:
1,在方法沒有實現(xiàn)的時候不會報錯蹄溉，是因為OC運行時機制，在運行的時候才會動態(tài)監(jiān)測灵妨，而c或者C＋＋這類語言必須在定義的時候明確指明其類型财饥，而不是等到運行過程動態(tài)監(jiān)測，這類語言叫做靜態(tài)語言
[cat play];
[cat playWithKid:@"zhangdanfeng"];
[IMCat play];
[IMCat playWithKid:@"zhangdanfeng"];
2,用稍微底層的方法performSeletor實現(xiàn)：
[cat performSelector:@selector(play)];
[cat performSelector:@selector(playWithKid:) withObject:@"zhangdanfeng"];
Class catClass = [cat class];
[catClass performSelector:@selector(play)];
[[IMCat class] performSelector:@selector(playWithKid:) withObject:@"zhangdanfeng"];
3,底層實際就是轉(zhuǎn)成了messageSend方法,貌似iOS8之后底層方法被禁用了务嫡，如果需要打開甲抖，需要在Build Settings中禁用enable strict checking of Obj-c messageSend calls選項
objc_msgSend(cat, @selector(play));
objc_msgSend(cat, @selector(playWithKid:),@"zhangdanfeng");
objc_msgSend([IMCat class], @selector(play));
objc_msgSend([IMCat class], @selector(playWithKid:),@"zhangdanfeng");
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