autolayout代码模式

[NSLayoutConstraint constraintWithItem:<#(id)#>

attribute:<#(NSLayoutAttribute)#>

relatedBy:<#(NSLayoutRelation)#>

toItem:<#(id)#> attribute:<#(NSLayoutAttribute)#>

multiplier:<#(CGFloat)#> constant:<#(CGFloat)#>]

[NSLayoutConstraint constraintsWithVisualFormat:<#(NSString *)#>

options:<#(NSLayoutFormatOptions)#>

metrics:<#(NSDictionary *)#>

views:<#(NSDictionary *)#>];