自动计算高度和宽度的关键点无外乎三个因素，文字内容，宽度或高度，再者就是字号，通过这三个因素，去自动计算对应的高度或宽度

自动计算高度

-(CGRect)contentRectWithString:(NSString *)string labelWidth:(CGFloat)labelWidth AndStringFont:(int)fontSize
{
    CGRect tempRect = [string boundingRectWithSize:CGSizeMake(labelWidth, MAXFLOAT)
                                           options:NSStringDrawingUsesLineFragmentOrigin
                                        attributes:[NSDictionary dictionaryWithObjectsAndKeys:[UIFont systemFontOfSize:fontSize],NSFontAttributeName, nil] context:nil];
    
    return tempRect;
}

自动计算宽度

-(CGRect)contentRectWithString:(NSString *)string labelHeight:(CGFloat)labelHeight AndStringFont:(int)fontSize
{
    CGRect tempRect = [string boundingRectWithSize:CGSizeMake(MAXFLOAT, labelHeight)
                                          options:NSStringDrawingUsesLineFragmentOrigin
                                       attributes:[NSDictionary dictionaryWithObjectsAndKeys:[UIFont systemFontOfSize:fontSize],NSFontAttributeName, nil] context:nil];
    
    return tempRect;
}

示例代码

#import "ViewController.h"
#import "Masonry.h"

@interface ViewController ()
@property (weak, nonatomic) IBOutlet UILabel *myLabel;

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    self.myLabel = (UILabel *)[self.view viewWithTag:3];
    
    [self.view addSubview:self.myLabel];
    
    self.myLabel.numberOfLines = 0;
    
    NSString *str = @"豫章故郡，洪都新府。星分翼轸，地接衡庐。襟三江而带五湖，控蛮荆而引瓯越。物华天宝，龙光射牛斗之墟；人杰地灵，徐孺下陈蕃之榻。雄州雾列，俊采星驰。台隍枕夷夏之交，宾主尽东南之美。都督阎公之雅望，棨戟遥临；宇文新州之懿范，襜帷暂驻。十旬休假，胜友如云；千里逢迎，高朋满座。腾蛟起凤，孟学士之词宗；紫电青霜，王将军之武库。家君作宰，路出名区；童子何知，躬逢胜饯。时维九月，序属三秋。潦水尽而寒潭清，烟光凝而暮山紫。俨骖騑于上路，访风景于崇阿。临帝子之长洲，得仙人之旧馆。层峦耸翠，上出重霄；飞阁流丹，下临无地。鹤汀凫渚，穷岛屿之萦回；桂殿兰宫，列冈峦之体势。披绣闼，俯雕甍，山原旷其盈视，川泽纡其骇瞩。闾阎扑地，钟鸣鼎食之家；舸舰迷津，青雀黄龙之舳。云销雨霁，";
    
    
    CGRect labelRect = [self contentRectWithString:str labelWidth:200.0 AndStringFont:15.0];

    float height = labelRect.size.height;

    [self.myLabel mas_updateConstraints:^(MASConstraintMaker *make) {
        
        make.height.mas_equalTo(height);
        
    }];
    
    self.myLabel.text = str;
    

}

-(CGRect)contentRectWithString:(NSString *)string labelWidth:(CGFloat)labelWidth AndStringFont:(int)fontSize
{
    CGRect tempRect = [string boundingRectWithSize:CGSizeMake(labelWidth, MAXFLOAT)
                                           options:NSStringDrawingUsesLineFragmentOrigin
                                        attributes:[NSDictionary dictionaryWithObjectsAndKeys:[UIFont systemFontOfSize:fontSize],NSFontAttributeName, nil] context:nil];
    
    return tempRect;
}