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:10],
[NSLayoutConstraint constraintWithItem:topView
attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0 constant:-10],
[NSLayoutConstraint constraintWithItem:topView
attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:100],
[NSLayoutConstraint constraintWithItem:topView
attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:20]
]];