如何在Objective C中使用从iOS 9创建的约束？

// 创建视图
UIView *view1 = [[UIView alloc] init];
view1.translatesAutoresizingMaskIntoConstraints = NO;
view1.backgroundColor = [UIColor redColor];
[self.view addSubview:view1];

UIView *view2 = [[UIView alloc] init];
view2.translatesAutoresizingMaskIntoConstraints = NO;
view2.backgroundColor = [UIColor blueColor];
[self.view addSubview:view2];

// 创建约束对象
NSLayoutConstraint *constraint1 = [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeading multiplier:1.0 constant:20.0];
NSLayoutConstraint *constraint2 = [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:20.0];
NSLayoutConstraint *constraint3 = [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:view2 attribute:NSLayoutAttributeWidth multiplier:0.5 constant:0.0];
NSLayoutConstraint *constraint4 = [NSLayoutConstraint constraintWithItem:view1 attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:view2 attribute:NSLayoutAttributeHeight multiplier:1.0 constant:0.0];

// 添加约束
[self.view addConstraints:@[constraint1, constraint2, constraint3, constraint4]];

// 激活约束
[NSLayoutConstraint activateConstraints:@[constraint1, constraint2, constraint3, constraint4]];