UI布局术语

horizontal, vertical

top, left, bottom, right

UIEdgeInsets

margin与padding

Interpreting Values

Values in Auto Layout are always in points. However, the exact meaning of these measurements can vary depending on the attributes involved and the view’s layout direction. 

Auto Layout Attributes	Value	Notes
Height Width	The size of the view.	These attributes can be assigned constant values or combined with other Height and Width attributes. These values cannot be negative.
Top Bottom Baseline	The values increase as you move down the screen.	These attributes can be combined only with Center Y, Top, Bottom, and Baseline attributes.
Leading Trailing	The values increase as you move towards the trailing edge. For a left-to-right layout directions, the values increase as you move to the right. For a right-to-left layout direction, the values increase as you move left.	These attributes can be combined only with Leading, Trailing, or Center X attributes.
Left Right	The values increase as you move to the right.	These attributes can be combined only with Left, Right, and Center X attributes. Avoid using Left and Right attributes. Use Leading and Trailing instead. This allows the layout to adapt to the view’s reading direction. By default the reading direction is determined based on the current language set by the user. However, you can override this where necessary. In iOS, set the semanticContentAttribute property on the view holding the constraint (the nearest common ancestor of all views affected by the constraint) to specify whether the content’s layout should be flipped when switching between left-to-right and right-to-left languages.
Center X Center Y	The interpretation is based on the other attribute in the equation.	Center X can be combined with Center X, Leading, Trailing, Right, and Left attributes. Center Y can be combined with Center Y, Top, Bottom, and Baseline attributes.