Auto Layout學習1

使用步驟：1.利用NSLayoutConstraint類創建約束對象  2.將約束對象添加到View上

例子：將一個正方形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是我自己定義的宏
下面加入約束
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];這里簡單介紹一下
constraintWithItem: 這里是要被約束的對象
attribute: 這里是約束種類（有好多種，下個文章分析吧）
relatedBy: 關系大小
typedef NS_ENUM(NSInteger, NSLayoutRelation) {    NSLayoutRelationLessThanOrEqual = -1,    NSLayoutRelationEqual = 0,    NSLayoutRelationGreaterThanOrEqual = 1,};toItem: 建立約束關系的對象
attribute: 約束種類
multiplier：倍數
constant：常量
約束公式： 被約束對象.約束attribute  =  (關系對象.約束attribute) * 倍數multiplier + 常量constant； 
詳細屬性關系，使用例子下章說吧。。。因為我自己還沒搞懂
補充iOS8以后新版約束方法
NSLayoutConstraint *centerX = [NSLayoutConstraint constraintWithItem:centerView attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0];    centerX.active = YES;