约束表达式

    NSDictionary *views = NSDictionaryOfVariableBindings(_userNameView,_pwdView);
    NSDictionary *metrics = @{@"hDistance":@"15",@"vDistance":@"20",@"vHeight":@"40"};
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-hDistance-[_userNameView]-hDistance-|" options:0 metrics:metrics views:views]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-vDistance-[_userNameView(vHeight)]|" options:0 metrics:metrics views:views]];
    
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-hDistance-[_pwdView]-hDistance-|" options:0 metrics:metrics views:views]];
//    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-80-[_pwdView(_userNameView)]|" options:0 metrics:metrics views:views]];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[_userNameView]-20-[_pwdView(_userNameView)]|" options:0 metrics:metrics views:views]];

        V:|-vDistance-[_userNameView(vHeight)]-20-_pwdView(_userNameView)|