在9.0之前的iOS版本上模仿UIStackView`填充比例'布局方法

The iOS 9.0 comes with UIStackView which makes it easier to layout views according to their content size. For example, to place 3 buttons in a row in accordance with their content width you can simply embed them in stack view, set axis horizontal and distribution - fill proportionally.The iOS 9.0 comes with UIStackView which makes