iOS visual format

autoLayout的学习资源实在太少，下面是我个人这一段时间对autoLayout学习的总结。

cocoa & cocoa touch 有三种方式创建自动布局：xib || storyBoard、使用代码创建单个约束、使用visual format创建约束.

关于三种方式的比较，可以在internet找到答案。

官方指出的visual format 语法如下表格：

Symbol	Replacement rule
<visualFormatString>	(<orientation>:)?(<superview><connection>)?<view>(<connection><view>)*(<connection><superview>)?
<orientation>	H|V (H->表示水平方向，V->表示垂直方向)
<superview>	| (这条竖线 代表 当前视图的父视图)
<view>	[<viewName>(<predicateListWithParens>)?] 例：[someView(someViewWidth)]
<connection>	e|-<predicateList>-|-
<predicateList>	<simplePredicate>|<predicateListWithParens>
<simplePredicate>	<metricName>|<positiveNumber>
<predicateListWithParens>	(<predicate>(,<predicate>)*)
<predicate>	(<relation>)?(<objectOfPredicate>)(@<priority>)?
<relation>	==|<=|>=
<objectOfPredicate>	<constant>|<viewName> (see note)
<priority>	<metricName>|<number>
<constant>	<metricName>|<number>
<viewName>	Parsed as a C identifier. This must be a key mapping to an instance of NSView in the passedviews dictionary.
<metricName>	Parsed as a C identifier. This must be a key mapping to an instance ofNSNumber in the passed metrics dictionary.
<number>	As parsed by strtod_l, with the C locale

上面的表格 你也许看的不太懂，要多读多写几个例子，自然而然就明白了。

下面是官方使用visual format的几个简单例子

The following are examples of constraints you can specify using the visual format. Note how the text visually matches the image.

Standard Space

[button]-[textField]

Width Constraint

[button(>=50)]

Connection to Superview

|-50-[purpleBox]-50-|

Vertical Layout

V:[topField]-10-[bottomField]

Flush Views

[maroonView][blueView]

Priority

[button(100@20)]

Equal Widths

[button1(==button2)]

Multiple Predicates

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

A Complete Line of Layout

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

本人写的一个例子，地址：https://github.com/shiyingyan/autoLayout-visualFormat