placeHolderLabel.font = self.font;,设置多么大就是多大的，

 if ( placeHolderLabel == nil )

        {

            placeHolderLabel = [[UILabel alloc] initWithFrame:CGRectMake(8,8,self.bounds.size.width - 16,0)];

            placeHolderLabel.lineBreakMode = UILineBreakModeWordWrap;

            placeHolderLabel.numberOfLines = 0;

            placeHolderLabel.font = self.font;

            placeHolderLabel.backgroundColor = [UIColorclearColor];

            placeHolderLabel.textColor = self.placeholderColor;

            placeHolderLabel.alpha = 0;

            placeHolderLabel.tag = 999;

            [self addSubview:placeHolderLabel];

        }