问尝试使用约束以编程方式移动UIPickerView，但始终得到此错误消息

EN

#import <UIKit/UIKit.h>

@interface ViewController : UIViewController
@property (strong, nonatomic) IBOutlet UIPickerView *pickerView;

@end

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    NSLayoutConstraint *constraint = [NSLayoutConstraint
                                      constraintWithItem:_pickerView
                                      attribute:NSLayoutAttributeTop
                                      relatedBy:NSLayoutRelationEqual
                                      toItem:self.view
                                      attribute:NSLayoutAttributeTop
                                      multiplier:1.0f
                                      constant:216.f];    
    [self.view addConstraint:constraint];    
}

2013-09-19 10:44:19.582 Constrain2[3092:c07] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSLayoutConstraint:0x71808f0 V:[UIPickerView:0x7180fd0(216)]>",
    "<NSLayoutConstraint:0x71805d0 V:|-(216)-[UIPickerView:0x7180fd0]   (Names: '|':UIView:0x7181290 )>",
    "<NSAutoresizingMaskLayoutConstraint:0x7184d50 h=--& v=--& V:[UIView:0x7181290(416)]>",
    "<NSLayoutConstraint:0x7181740 UIPickerView:0x7180fd0.bottom == UIView:0x7181290.bottom>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x71808f0 V:[UIPickerView:0x7180fd0(216)]>