界面布局之VFL

VFL（Visual Format Language）

简单说明：
H 横向
V 纵向
| 父视图边缘
- 默认距离
[viewName(30)] view用[]扩住，不能有self之类的关键字（会报错，原因后续）,view的宽或者高为30
- 10 - 两者间距10
[viewName(>=60@700)] view的宽或者高大于等于60（>=,==,<=），优先级为700（优先级最大为1000，数值越大越优先满足）
[view1][view2(==view1)] view1后接view2，且view2的宽或者高于view1相等

PS：view的translatesAutoresizingMaskIntoConstraints一定要设置为NO。

    UINavigationBar * LHNavigation = [[UINavigationBar alloc] init];    LHNavigation.translatesAutoresizingMaskIntoConstraints = NO;    UITabBar * LHTabBar = [[UITabBar alloc] init];    LHTabBar.translatesAutoresizingMaskIntoConstraints = NO;    UITableView * LHTable = [[UITableView alloc] init];    LHTable.translatesAutoresizingMaskIntoConstraints = NO;    //添加垂直方向的约束    NSNumber *margin = @20;    //等同于 NSDictionary *mertrics = [NSDictionary dictionaryWithObjectsAndKeys:20, @"margin", nil]    NSDictionaryOfVariableBindings(margin);    NSDictionary *mertrics = NSDictionaryOfVariableBindings(margin);    NSString *vflV = @"V:|-margin-[LHNavigation]-[LHTable]-[LHTabBar]-|";    NSDictionary * views = NSDictionaryOfVariableBindings(LHNavigation,LHTabBar,LHTable);    NSArray * constraintsV = [NSLayoutConstraint constraintsWithVisualFormat:vflV options:0 metrics:mertrics views:views];    [self.view addConstraints:constraintsV] ;