UIPickerView上的自定义按钮处于禁用状态

Question

我在PickerView上添加了自定义“完成”和“取消”按钮，如：

- (void)customBtnsToPickerView
{
    UIToolbar *toolBar= [[UIToolbar alloc] initWithFrame:CGRectMake(0, 0, 320, 44)];
    [toolBar setBarStyle:UIBarStyleBlackOpaque];

    NSMutableArray *barItems = [[NSMutableArray alloc] init];

    UIBarButtonItem *cancelBtn = [[UIBarButtonItem alloc] initWithTitle:@"Cancel" style:UIBarButtonItemStyleDone target:self action:@selector(cancelActionOfPickerView:)];
    [barItems addObject:cancelBtn];

    UIBarButtonItem *flexSpace = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemFlexibleSpace target:nil action:nil];
    [barItems addObject:flexSpace];

    UIBarButtonItem *doneBtn = [[UIBarButtonItem alloc] initWithTitle:@"Done" style:UIBarButtonItemStyleDone target:self action:@selector(doneActionOfPickerView:)];
    [barItems addObject:doneBtn];

    [toolBar setItems:[NSArray arrayWithArray:barItems]];
    [self.pickerView addSubview:toolBar];
}

但他们总是处于禁用状态。我不知道里面有什么问题。 我正在使用带有IOS 6.1的iphone 5 ......