//    NSDictionary * viewsDictionary = NSDictionaryOfVariableBindings(self.segmentControl);
    NSDictionary * viewsDictionary = [[NSDictionary alloc]initWithObjectsAndKeys:self.segmentControl ,@"segmentControl", nil];
    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[segmentControl]" options:0 metrics:nil views:viewsDictionary]];


//  第一句显示错误，运行不通过，显示不合法的key，原因如下：
//  terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unable to parse constraint format: segmentControl is not a key in the views dictionary.  

我想知道，为什么第一句不对，而第二句对的。