ios - UILabel を UICollectionViewCell の contentView のボタンに固定する Autolayout Constraint

Question

ビジュアル フォーマット言語を使用して、単純な制約を UICollectionViewCell サブクラスに追加しようとしています。UILabel を contentView の下部に固定したいだけです。ここに私の初期コードがあります:

- (id)initWithFrame:(CGRect)frame {
    self = [super initWithFrame:frame];
    if(self) {
        _label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, 128, 50)];
        _label.font = [[UIFont preferredFontForTextStyle:UIFontTextStyleSubheadline1] fontWithSize:20.0];
        _label.lineBreakMode = NSLineBreakByWordWrapping;
        _label.numberOfLines = 2;
       _label.contentMode = UIViewContentModeBottom;
       _label.backgroundColor = [UIColor blackColor];
       [self.contentView addSubview:_label];
        [self.contentView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[_label]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(_label)]];
    }
    return self;
}

アプリを実行したときに表示されるログ メッセージの例を次に示します。

Unable to simultaneously satisfy constraints.
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:0xa3aa620 V:[UILabel:0xa3aa270]-(0)-|   (Names: '|':UIView:0xa3aa010 )>",
    "<NSAutoresizingMaskLayoutConstraint:0x8b9b2d0 h=--& v=--& UILabel:0xa3aa270.midY == + 25>",
    "<NSAutoresizingMaskLayoutConstraint:0x8b9bba0 h=--& v=--& V:[UILabel:0xa3aa270(50)]>",
    "<NSAutoresizingMaskLayoutConstraint:0x8bd1290 h=-&- v=-&- UIView:0xa3aa010.height == STStationGroupCollectionViewCell:0xa3a9f40.height>",
    "<NSAutoresizingMaskLayoutConstraint:0x8bb7ee0 h=--& v=--& V:[STStationGroupCollectionViewCell:0xa3a9f40(128)]>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0xa3aa620 V:[UILabel:0xa3aa270]-(0)-|   (Names: '|':UIView:0xa3aa010 )>

どんな助けでも大歓迎です。ありがとう！