Make layout painless.

Introduction

COSLayout is yet another layout library. It's neither a wrapper nor a replacement for Auto Layout. It dose not handle circular references of constraints and constraint priority. Besides that, COSLayout can solve all layout cases. What's more, COSLayout provides some additional benefits: smaller memory footprint, better performance and more intuitive expression.

COSLayout is an abstraction of layout of view. With COSLayout, you can specify view's layout relative to it's superview, sibling views and non-sibling views. Following example specifies a 10-points constraint from view's bottom to superview's bottom:

UIView *view = [[UIView alloc] init];

COSLayout *layout = [COSLayout layoutOfView:view];

[layout addRule:@"bb = 10"];

In the example above, a rule has been added into layout by method addRule:. A rule is expressed in Sample Layout Language or "SLL", which can specify constraints intuitively. The syntax of SLL is very simple, just comma-separated assignment expressions. Each assignment expression specifies a constraint, l-value is constraint name, r-value is constraint value.

COSLayout supports 18 constraints:

COSLayout supports 5 constraint value types:

Note that the percentage has different means for different constraint directions. If current constraint direction is horizontal, the percentage represents the percentage of superview's width, otherwise, the percentage of superview's height. You can also specify direction of percentage by H: (horizontal) or V: (vertical) prefix. For example, H:50% means 50% width of superview, V:30% means 30% height of superview.

Format specifier represents a constraint value given by additional argument. For example, %tt is the space from other view's top to superview's top. Here, the other view is given by additional argument, and the superview is the superview of layout's view. It means that COSLayout can specify constraints between non-sibling views.

COSLayout support 20 format specifiers:

Like percentage constraint value, you can also use H: and V: prefix to specify direction of percentage. For example, H:%p, H:%^p, H:%@p means percentage is horizontal, V:%p, V:%^p, V:%@p means percentage is vertical.

It is worth mentioning that, format specifier also create a dependency between two views: the layout view and the other view given by additional argument. In COSLayout, the dependencies is presented by DAG. So COSLayout do not support the circular dependencies. When superview needs layout, all layouts of subviews will solve it's constraints according to the dependencies.

Constraint value expression

You can apply arithmetic operator between constraint values. Like other languages, SLL supports 5 basic arithmetic operators:

You can also use () to group sub-expression to change the evaluation order of expression.

Examples

In the following example, COSLayout aligns view's top-right corner to superview's top-right corner with 5-points space.

UIView *view = [[UIView alloc] init];

COSLayout *layout = [COSLayout layoutOfView:view];

[layout addRule:@"tt = rr = 5"];

In the following example, COSLayout aligns view's left/bottom/right to superview's left/bottom/right with 10-points space, and make the view's top aligned to superview's center.

UIView *view = [[UIView alloc] init];

COSLayout *layout = [COSLayout layoutOfView:view];

[layout addRule:@"ll = bb = rr = 10, tt = 50%"];