自动布局

  item1.attribute1 = multiplier * item2.attribute2 + constant

  + (id)constraintWithItem:(id)item1
                 attribute:(NSLayoutAttribute)attribute1
                 relatedBy:(NsLayoutRelation)relation
                    toItem:(id)item2
                 attribute:(NSLayoutAttribute)attribute2
                multiplier:(CGFloat)multiplier
                  constant:(CGFloat)constant;

  Button.centerX = Superview.centerX
  // =>
  [NSLayoutConstraint constraintWithItem:button
                 attribute:NSLayoutAttributeCenterX
                 relatedBy:NsLayoutRelationEqual
                    toItem:superview
                 attribute:NSLayoutAttributeCenterX
                multiplier:1.0
                  constant:0.0];

  Button.bottom = Superview.bottom - <padding>
  // =>
  [NSLayoutConstraint constraintWithItem:button
                 attribute:NSLayoutAttributeBottom
                 relatedBy:NsLayoutRelationEqual
                    toItem:superview
                 attribute:NSLayoutAttributeBottom
                multiplier:1.0
                  constant:-padding];

  [button]-[textField]

  [button(>=50)]

  |-50-[purpleBox]-50-|

  V:[topField]-10-[bottomField]

  [maroonView][blueView]

  [button(100@20)]

  [button1(==button2)]

  [flexibleButton(>=70,<=100)]

  |-[find]-[findNext]-[findField(>=20)]-|