● H:[cancelButton(72)]-12-[acceptButton(50)]
● canelButton宽72,acceptButton宽50,它们之间间距12

● H:[wideView(>=60@700)]
● wideView宽度大于等于60point,该约束条件优先级为700(优先级最大值为1000,优先级越高的约束越先被满足)

● V:[redBox]-[yellowBox(==redBox)]
● 竖直方向上,先有一个redBox,其下方紧接一个高度等于redBox高度的yellowBox

● H:|-10-[Find]-[FindNext]-[FindField(>=20)]-|

● 水平方向上,Find距离父view左边缘默认间隔宽度,之后是FindNext距离Find间隔默认宽度; 再之后是宽度不小于20的FindField,它和FindNext以及父view右边缘的间距都是默认宽度。 (竖线“|” 表示superview的边缘)

● 使用VFL来创建约束数组

+ (NSArray *)constraintsWithVisualFormat:(NSString
*)format options:(NSLayoutFormatOptions)opts metrics: (NSDictionary *)metrics views:(NSDictionary *)views; 

● format :VFL语句
● opts :约束类型(一般可以传0)
● metrics :VFL语句中用到的具体数值 ● views :VFL语句中用到的控件

● 创建一个字典(内部包含VFL语句中用到的控件)的快捷宏定义 NSDictionaryOfVariableBindings(...)

#import "ViewController.h"@interface ViewController ()@end@implementation ViewController- (void)viewDidLoad {[super viewDidLoad];// 1.添加两个控件到父控件上// 1.1添加蓝色ViewUIView *blueView = [[UIView alloc] init];blueView.backgroundColor = [UIColor blueColor];[self.view addSubview:blueView];// 1.1添加红色ViewUIView *redView = [[UIView alloc] init];redView.backgroundColor = [UIColor redColor];[self.view addSubview:redView];// 2.禁用auturezingblueView.translatesAutoresizingMaskIntoConstraints = NO;redView.translatesAutoresizingMaskIntoConstraints = NO;// 3.添加约束/*lFormat : VFL语句options: 对齐方式metrics: VFL语句中用到的变量值views: VFL语句中用到的控件*/// 设置蓝色View距离左边和右边有20的的间距  X 和 宽度
//    NSArray *blueViewH = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-20-[blueView]-20-|" options:0 metrics:nil views:@{@"blueView" : blueView}];int margin = 20;NSArray *blueViewH = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-margin-[blueView]-margin-|" options:0 metrics:@{@"margin": @(margin)} views:@{@"blueView" : blueView}];[self.view addConstraints:blueViewH];// 设置蓝色View距离顶部有20的间距, 并且高度等于50   Y 和高度// 设置红色View距离蓝色底部有20的间距, 并且红色View的高度等于蓝色View的高度 Y 和高度// 并且设置红色和蓝色右对齐NSArray *blueViewV = [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-20-[blueView(50)]-20-[redView(==blueView)]" options:NSLayoutFormatAlignAllRight metrics:nil views:@{@"blueView" : blueView, @"redView": redView}];[self.view addConstraints:blueViewV];// 注意: 在VFL语句中, 是不支持乘除法
//    NSArray *redVeiwH = [NSLayoutConstraint constraintsWithVisualFormat:@"H:[redView(==blueView )]" options:0 metrics:nil views:@{@"blueView" : blueView, @"redView": redView}];
//    [self.view addConstraints:redVeiwH];
    NSLayoutConstraint *redVeiwW = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0];[self.view addConstraint:redVeiwW];
}@end

[添加了约束的view layoutIfNeeded];

}];