前几日尝试了各种UIButton
UIButton *btn0=[UIButton buttonWithType:UIButtonTypeContactAdd];
UIButton *btn1=[UIButtonbuttonWithType:UIButtonTypeCustom];
UIButton *btn2=[UIButtonbuttonWithType:UIButtonTypeDetailDisclosure];
UIButton *btn3=[UIButtonbuttonWithType:UIButtonTypeInfoDark];
UIButton *btn4=[UIButtonbuttonWithType:UIButtonTypeInfoLight];
UIButton *btn5=[UIButtonbuttonWithType:UIButtonTypeRoundedRect];
其中UIButtonTypeCustom类的btn2默认为无色透明，与Alpha值设为0的UIButtonTypeRoundedRect类btn5不同的是，
btn2是真正意义上的透明（虽看不见，但知道其存在，可触摸，点击），btn5一旦alpha设置为0即消失不见，无法被触摸。
小记一笔。
