使用AutoLayOut的注意事项

一定要注意: // 不要将AutoresizingMask转为Autolayout的约束

blueView.translatesAutoresizingMaskIntoConstraints = NO;

[self.view addSubview:blueView];

还有先添加父视图再添加约束

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

NSLayoutConstraint *widthConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:100];
[blueView addConstraint:widthConstraint];
NSLayoutConstraint *heightConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:100];
[blueView addConstraint:heightConstraint];

NSLayoutConstraint *rightConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1.0 constant:-30];
[self.view addConstraint:rightConstraint];

NSLayoutConstraint *bottomConstraint = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeBottom multiplier:1.0 constant:-20];
[self.view addConstraint:bottomConstraint];

右边和下边注意相对父视图有可能是负值!