Autolayout(自动布局)

VFL语言实现以下界面：

- (void)viewDidLoad

{

[super viewDidLoad];

//创建上面的view

UIView *topView = [[UIView alloc]init];

topView.backgroundColor = [UIColor redColor];

[self.view addSubview:topView];

topView.translatesAutoresizingMaskIntoConstraints = NO;

//创建下面的view

UIView *bottomView = [[UIView alloc]init];

bottomView.backgroundColor = [UIColor blueColor];

[self.view addSubview:bottomView];

bottomView.translatesAutoresizingMaskIntoConstraints = NO;

//利用VFL语言

//创建约束(垂直方向)

NSInteger gap = 20;

NSArray *verticalC = [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-gap-[topView(==50)]-gap-[bottomView(==topView)]" options:NSLayoutFormatAlignAllRight metrics:@{@"gap":@(gap)} views:@{@"topView":topView,@"bottomView":bottomView}];

[self.view addConstraints:verticalC];

NSArray *herizonTopC = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-gap-[topView]-gap-|" options:NSLayoutFormatAlignAllRight metrics:@{@"gap":@(gap)} views:@{@"topView":topView}];

[self.view addConstraints:herizonTopC];

NSLayoutConstraint *contraintBottom = [NSLayoutConstraint constraintWithItem:bottomView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:topView attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0];

[self.view addConstraint:contraintBottom];

}