iOS修行---Auto Layout学习1

Auto Layout学习1

使用步骤：1.利用NSLayoutConstraint类创建约束对象  2.将约束对象添加到View上

例子：将一个正方形view加入vc中

UIView *centerView = [[UIView alloc]init];    [self.view addSubview:centerView];    centerView.backgroundColor = UIColorFromHex(0xFFEC8B, 1.0);    centerView.translatesAutoresizingMaskIntoConstraints = NO;注：UIColorFromHex是我自己定义的宏
下面加入约束
NSLayoutConstraint *widthConstraint = [NSLayoutConstraint constraintWithItem:centerView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:150.0];    [self.view addConstraint:widthConstraint];        NSLayoutConstraint *heightConstraint = [NSLayoutConstraint constraintWithItem:centerView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:150];    [self.view addConstraint:heightConstraint];        NSLayoutConstraint *topConstraint = [NSLayoutConstraint constraintWithItem:centerView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:200];    [self.view addConstraint:topConstraint];    NSLayoutConstraint *leftConstraint = [NSLayoutConstraint constraintWithItem:centerView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:100];    [self.view addConstraint:leftConstraint];这里简单介绍一下
constraintWithItem: 这里是要被约束的对象
attribute: 这里是约束种类（有好多种，下个文章分析吧）
relatedBy: 关系大小
typedef NS_ENUM(NSInteger, NSLayoutRelation) {    NSLayoutRelationLessThanOrEqual = -1,    NSLayoutRelationEqual = 0,    NSLayoutRelationGreaterThanOrEqual = 1,};toItem: 建立约束关系的对象
attribute: 约束种类
multiplier：倍数
constant：常量
约束公式： 被约束对象.约束attribute  =  (关系对象.约束attribute) * 倍数multiplier + 常量constant； 
详细属性关系，使用例子下章说吧。。。因为我自己还没搞懂
补充iOS8以后新版约束方法
NSLayoutConstraint *centerX = [NSLayoutConstraint constraintWithItem:centerView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0];    centerX.active = YES;