ios NSLayoutConstraint

为了让我们的应用在不同尺寸的屏幕下都能 “正常”的表示，我们尽量不要把数据写死。大多数可视元素都是一个矩形区域，当然这个矩形区域有坐标的，我们有了这个区域坐标就能确定可视元素的现实位置了。但是iphone5和以前的屏幕不一样了，在以前的设备中，我们可以添加一个 xx.@2x.png来适应retina屏幕，但是iphoen5咋办呢？ios6引入了 Auto Layout的东东，这个要和UIViewAutoresizing区分下。

1.看下面代码:

-(void)viewDidLoad

{

[superviewDidLoad];

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

aView.backgroundColor = [UIColor redColor];

//为了不和autosizing冲突，我们设置No

[aView  setTranslatesAutoresizingMaskIntoConstraints:NO];//不把AutoresizingMask转化为Constraints

[self.view  addSubview:aView];

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

bView.backgroundColor = [UIColor blueColor];

[bView  setTranslatesAutoresizingMaskIntoConstraints:NO];

[self.view  addSubview:bView];

NSDictionary*views =NSDictionaryOfVariableBindings(aView, bView);

/* This macro is a helper for making view dictionariesfor +constraintsWithVisualFormat:options:metrics:views:.

NSDictionaryOfVariableBindings(v1, v2, v3) isequivalent（等效于）  to [NSDictionarydictionaryWithObjectsAndKeys:v1, @"v1", v2, @"v2", v3,@"v3", nil];

*/

[self.viewaddConstraints:

[NSLayoutConstraint   constraintsWithVisualFormat:@"H:|-(>=50)-[aView(100)]"

options:0

metrics:nil

views:views]];

[self.view  addConstraints:

[NSLayoutConstraint  constraintsWithVisualFormat:@"V:|-(>=100)-[aView(50)]"

options:0

metrics:nil

views:views]];

[self.viewaddConstraints:

[NSLayoutConstraint   constraintsWithVisualFormat:@"H:[bView(==aView)]"

options:0

metrics:nil

views:views]];

[self.view addConstraints:

[NSLayoutConstraint   constraintsWithVisualFormat:@"V:[bView(==aView)]"

options:0

metrics:nil

views:views]];

[self.viewaddConstraint:

[NSLayoutConstraint  constraintWithItem:bView

attribute:NSLayoutAttributeLeft

relatedBy:NSLayoutRelationEqual

toItem:aView

attribute:NSLayoutAttributeRight

multiplier:1

constant:10]];

//添加一个限制等效于bView.frame.origin.x = (aView.frame.origin.x +aView.frame.size.width) * 1  + 10！它是一种依赖关系，bView依赖aView，这样就算aView变了，bView也会跟着变换。

[self.viewaddConstraint:

[NSLayoutConstraint  constraintWithItem:bView

attribute:NSLayoutAttributeTop

relatedBy:NSLayoutRelationEqual

toItem:aView

attribute:NSLayoutAttributeTop

multiplier:1

constant:0]];

[aView release];

[bView release];

}

2.constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:

/* Create constraints explicitly.  Constraints are of the form "view1.attr1 = view2.attr2 * multiplier
+ constant"

If your equation does not have a second view and attribute, use nil and NSLayoutAttributeNotAnAttribute.

*/

3.属性

typedefNS_ENUM(NSInteger, NSLayoutAttribute) {

NSLayoutAttributeLeft = 1,

NSLayoutAttributeRight,

NSLayoutAttributeTop,

NSLayoutAttributeBottom,

NSLayoutAttributeLeading,

NSLayoutAttributeTrailing,

NSLayoutAttributeWidth,

NSLayoutAttributeHeight,

NSLayoutAttributeCenterX,

NSLayoutAttributeCenterY,

NSLayoutAttributeBaseline,

NSLayoutAttributeNotAnAttribute = 0

};

4.关系

typedefNS_ENUM(NSInteger, NSLayoutRelation) {

NSLayoutRelationLessThanOrEqual = -1,

NSLayoutRelationEqual = 0,

NSLayoutRelationGreaterThanOrEqual = 1,

};

最后的结果就是 “view1.attr1 <= 或者 ==   或者 >=  view2.attr2 * multiplier + constant”

5.Visual Format Language  (视觉形式语言)