25/05/2015

// 1 UIButton -> clicked

[_baseButton addTarget:self action:@selector(toggleStack:) forControlEvents:UIControlEventTouchUpInside];

// 2 保证UIView 在屏幕中间

[stackView setCenter:CGPointMake(self.view.frame.size.width / 2 - 20 , self.view.frame.size.height / 2  )];

// 3 如何得到一个label的宽度

[label.text sizeWithAttributes:@{NSFontAttributeName : label.font}];