objective-c - 在iOSAutolayout的代码中出了错,请大家看看(代码不多,刚刚学AutoLayout)

Question

先谢谢了,请帮我分析下,谢谢

主要代码如下:

-(void)loadView
    {
        [super loadView];
        self.leftView.translatesAutoresizingMaskIntoConstraints = NO;
        self.rightView.translatesAutoresizingMaskIntoConstraints = NO;
        
        [self.view addSubview:self.leftView];
        [self.view addSubview:self.rightView];
    }
- (void)updateViewConstraints
    {
        NSLayoutConstraint *width = [NSLayoutConstraint constraintWithItem:self.leftView attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:50];     
        NSLayoutConstraint *height = [NSLayoutConstraint constraintWithItem:self.leftView attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:self.leftView attribute:NSLayoutAttributeWidth multiplier:1.0 constant:0];
        NSLayoutConstraint *left = [NSLayoutConstraint constraintWithItem:self.leftView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:20.f];
        NSLayoutConstraint *top = [NSLayoutConstraint constraintWithItem:self.leftView attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.leftView attribute:NSLayoutAttributeLeft multiplier: 1.f constant:0];
    
        [NSLayoutConstraint activateConstraints:@[width, height,left, top]];
        [super updateViewConstraints];
    }

输出:

    *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** +[NSLayoutConstraint constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]: A multiplier of 0 or a nil second item together with a location for the first attribute creates an illegal constraint of a location equal to a constant. Location attributes must be specified in pairs'

Answer 1

NSLayoutConstraint *left = [NSLayoutConstraint constraintWithItem:self.leftView attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0 constant:20.f];

NSLayoutAttributeLeft 这个属性的toItem 不能为nil