计算label的行数

self.instructionsString = @"1：说明1\n2：说明2\n3：说明3";
    _instructionsLabel = [[UILabel alloc] init];
    _instructionsLabel.text = self.instructionsString;
    _instructionsLabel.textColor = [UIColor grayColor];
    _instructionsLabel.numberOfLines = 0;
    //计算文字有多少行
    CGFloat labelHeight = [_instructionsLabel sizeThatFits:CGSizeMake(SCREEN_W-20, MAXFLOAT)].height;
    _hang = (labelHeight) / _instructionsLabel.font.lineHeight;
//    NSLog(@"%ld", (long)_hang);