1.第一步操作：

注意，下面的宽度的指定，使用约束左边右边或者直接约束宽度都可以。

2.代码如下：

#import "ViewController.h"

@interface ViewController ()

@property (weak, nonatomic) IBOutlet UILabel *label;

@property (nonatomic, copy) NSString *str;

@end

@implementation ViewController

- (void)viewDidLoad {

[super viewDidLoad];

[self initData];

[self initUI];

}

#pragma mark - init

- (void)initData {

_str = @"asdhasiodhas dhasiudasidhasbkdj ahsdjkahsdjkasdh ajksdhas jkdahskjdhnasjk adhs ";

}

- (void)initUI {

//CGSize titleSize = [_str sizeWithFont:[UIFont systemFontOfSize:15] constrainedToSize:CGSizeMake(320, MAXFLOAT) lineBreakMode:UILineBreakModeWordWrap];

NSDictionary *attributes = @{NSFontAttributeName:[UIFont systemFontOfSize:20]};

// 动态获取label的size    // 100限宽

CGSize titleSize = [_str boundingRectWithSize:CGSizeMake(100, 0) options:NSStringDrawingTruncatesLastVisibleLine | NSStringDrawingUsesLineFragmentOrigin | NSStringDrawingUsesFontLeading attributes:attributes context:nil].size;

_label.text = _str;

NSLog(@"%@", NSStringFromCGSize(titleSize));

_label.frame = CGRectMake(_label.frame.origin.x, _label.frame.origin.y, 100, titleSize.height);

}

@end