NSLayoutConstraint的简单应用

    UIView *topView = [[UIView alloc] init];
    topView.backgroundColor = [UIColor redColor];
    [self.view addSubview:topView];
    topView.translatesAutoresizingMaskIntoConstraints = NO;

    [self.view addConstraints:@[

                                [NSLayoutConstraint constraintWithItem:topView
                                                             attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:],
                                [NSLayoutConstraint constraintWithItem:topView
                                                             attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0 constant:-],
                                [NSLayoutConstraint constraintWithItem:topView
                                                             attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:],
                                [NSLayoutConstraint constraintWithItem:topView
                                                             attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:]
                                ]];

topView在父视图的左边间距10，下边间距-10，宽度为100，高度为20的实现代码