早上做了会实验结果如下：
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;
完事...
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