NSLayoutConstraint-代码实现自动布局的函数用法说明

[NSLayoutConstraint constraintWithItem:(id)item                              attribute:(NSLayoutAttribute)attribute                              relatedBy:(NSLayoutRelation)relation                                 toItem:(id)otherItem                              attribute:(NSLayoutAttribute)otherAttribute                             multiplier:(CGFloat)multiplier                               constant:(CGFloat)constant]

参数说明:

第一个参数:指定约束左边的视图view1

第二个参数:指定view1的属性attr1，具体属性见文末。

第三个参数:指定左右两边的视图的关系relation，具体关系见文末。

第四个参数:指定约束右边的视图view2

第五个参数:指定view2的属性attr2，具体属性见文末。

第六个参数:指定一个与view2属性相乘的乘数multiplier

第七个参数:指定一个与view2属性相加的浮点数constant

这个函数的对照公式为:

view1.attr1 <relation> view2.attr2 * multiplier + constant

注意:

1.若是你想设置的约束里不须要第二个view，要将第四个参数设为nil，第五个参数设为NSLayoutAttributeNotAnAttribute

举例:

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[NSLayoutConstraint constraintWithItem:view1                              attribute:NSLayoutAttributeLeft                              relatedBy:NSLayoutRelationEqual                                 toItem:view2                              attribute:NSLayoutAttributeRight                             multiplier:1                               constant:10]

翻译过来就是:view1的左侧，在，view2的右侧，再多10个点，的地方。

附视图的属性和关系的值:

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typedef NS_ENUM(NSInteger, NSLayoutRelation) {     NSLayoutRelationLessThanOrEqual = -1,          //小于等于     NSLayoutRelationEqual = 0,                     //等于     NSLayoutRelationGreaterThanOrEqual = 1,        //大于等于 }; typedef NS_ENUM(NSInteger, NSLayoutAttribute) {     NSLayoutAttributeLeft = 1,                     //左侧     NSLayoutAttributeRight,                        //右侧     NSLayoutAttributeTop,                          //上方     NSLayoutAttributeBottom,                       //下方     NSLayoutAttributeLeading,                      //首部     NSLayoutAttributeTrailing,                     //尾部     NSLayoutAttributeWidth,                        //宽度     NSLayoutAttributeHeight,                       //高度     NSLayoutAttributeCenterX,                      //X轴中心     NSLayoutAttributeCenterY,                      //Y轴中心     NSLayoutAttributeBaseline,                     //文本底标线                                                                                                                                                          NSLayoutAttributeNotAnAttribute = 0            //没有属性 };

NSLayoutAttributeLeft/NSLayoutAttributeRight 和NSLayoutAttributeLeading/NSLayoutAttributeTrailing的区别是left/right永远是指左右，而leading/trailing在某些从右至左习惯的地区会变成，leading是右边，trailing是左边。

屏幕兼容的问题

为了让咱们的应用在不容尺寸的屏幕下都能 “正常”的表示，咱们尽可能不要把数据写死。

大多数可视元素都是一个矩形区域，固然这个矩形区域有坐标的，咱们有了这个区域坐标就能肯定可视元素的现实位置了。

可是iphone5 和之前的屏幕不同了，在之前的设备中，咱们能够添加一个 xx.@2x.png 来适应retina屏幕，可是iphoen5咋办呢？

ios 引入了 Auto Layout 的东东，这个要和UIViewAutoresizing 区分下。

看下面代码

- (void)viewDidLoad
{
    [super viewDidLoad];
        
    UIView *aView = [[UIView alloc] init];
    aView.backgroundColor = [UIColor redColor];

//为了避免和autosizing冲突，咱们设置No

    [aView setTranslatesAutoresizingMaskIntoConstraints:NO];
    [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);

//NSDictionaryOfVariableBindings  宏  其实 NSDictionaryOfVariableBindings(v1, v2, v3) 等效于 [NSDictionary dictionaryWithObjectsAndKeys:v1, @"v1", v2, @"v2", v3, @"v3", nil];    
    [self.view addConstraints:
    [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.view addConstraints:
     [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.view addConstraint:
     [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,好像是这样的！我的以为！

  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.view addConstraint:
     [NSLayoutConstraint constraintWithItem:bView
                                  attribute:NSLayoutAttributeTop
                                  relatedBy:NSLayoutRelationEqual
                                     toItem:aView
                                  attribute:NSLayoutAttributeTop
                                 multiplier:1
                                   constant:0]];
    
    [aView release];
    [bView release];
}

constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:

Create a constraint of the form "view1.attr1 <relation> view2.attr2 * multiplier + constant".

属性

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