NSLayoutConstraint 代码中的约束

当我们处理框架时，如果约束不是太复杂，我们最好在代码中使用 Interface Builder 或 NSLayoutConstraint 使其足够小，而不是导入 Masonry 或 SnapKit。

例如：

• Objective-C 的

    // 1. create views
    UIView *blueView = [[UIView alloc] init];
    blueView.backgroundColor = [UIColor blueColor];
    [self.view addSubview:blueView];

    UIView *redView = [[UIView alloc] init];
    redView.backgroundColor = [UIColor redColor];
    [self.view addSubview:redView];

    // 2. forbid Autoresizing
    blueView.translatesAutoresizingMaskIntoConstraints = NO;
    redView.translatesAutoresizingMaskIntoConstraints = NO;

    // 3. make contraints
    // 3.1 blueView
    NSLayoutConstraint *blueLeft = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1 constant:20];
    [self.view addConstraint:blueLeft];

    NSLayoutConstraint *blueTop = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1 constant:20];
    [self.view addConstraint:blueTop];

    NSLayoutConstraint *blueRight = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1 constant:-20];
    [self.view addConstraint:blueRight];

    NSLayoutConstraint *blueHeight = [NSLayoutConstraint constraintWithItem:blueView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1 constant:50];
    [self.view addConstraint:blueHeight];

    // 3.2 redView
    NSLayoutConstraint *redTop = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeBottom multiplier:1 constant:20];
    [self.view addConstraint:redTop];

    NSLayoutConstraint *redRight = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeRight multiplier:1 constant:-20];
    [self.view addConstraint:redRight];

    NSLayoutConstraint *redHeight = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeHeight multiplier:1 constant:0];
    [self.view addConstraint:redHeight];

    NSLayoutConstraint *redWidth = [NSLayoutConstraint constraintWithItem:redView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:blueView attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0];
    [self.view addConstraint:redWidth];