1.N行完全自适应：

        UILabel *testLabel = [[UILabel alloc] initWithFrame:CGRectMake(10, 30, 100, 21)];
        NSString *txt = @"dffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff";
        testLabel.numberOfLines = 0; ///相当于不限制行数
        testLabel.text = txt;

这样不行，还需要调用 [testLabel sizeToFit];

2.限制在N行内自适应：

        UILabel *testLabel = [[UILabel alloc] initWithFrame:CGRectMake(10, 30, 100, 21)];
        NSString *txt = @"dffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff";
        testLabel.numberOfLines = 3;   ////限制在3行内自适应
        testLabel.text = txt;
         [testLabel sizeToFit];

结果不起作用，全部在一行显示了。

3.为了实现2的需求，需要这么做：

        CGSize maxSize = CGSizeMake(100, 21*3);
        UILabel *testLabel = [[UILabel alloc] initWithFrame:CGRectMake(10, 30, 100, 21)];
        NSString *txt = @"dffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff";
        CGSize labelSize = [txt sizeWithFont:testLabel.font constrainedToSize:maxSize lineBreakMode: UILineBreakModeTailTruncation];

        testLabel.frame = CGRectMake(testLabel.frame.origin.x, testLabel.frame.origin.y, labelSize.width, labelSize.height);
        testLabel.text = txt;