在给UILabel设置宽高的时候，会出现这样的问题，当你的字符串很长的时候，UILabel显示就是省略号了，这样子肯定是不行的，尤其是自定义cell的时候，cell中包含UILabel，这样就需要去动态的计算高度。

给NSString添加一个category

//
//  NSString+LabelWidthAndHeight.h
//  ZiPeiYi
//
//  Created by YouXianMing on 15/12/9.
//  Copyright © 2015年 YouXianMing. All rights reserved.
//

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>

@interface NSString (LabelWidthAndHeight)

/**
 *  Get the string's height with the fixed width.
 *
 *  @param attribute String's attribute, eg. attribute = @{NSFontAttributeName: [UIFont systemFontOfSize:18.f]}
 *  @param width     Fixed width.
 *
 *  @return String's height.
 */
- (CGFloat)heightWithStringAttribute:(NSDictionary <NSString *, id> *)attribute fixedWidth:(CGFloat)width;

/**
 *  Get the string's width.
 *
 *  @param attribute String's attribute, eg. attribute = @{NSFontAttributeName: [UIFont systemFontOfSize:18.f]}
 *
 *  @return String's width.
 */
- (CGFloat)widthWithStringAttribute:(NSDictionary <NSString *, id> *)attribute;

/**
 *  Get a line of text height.
 *
 *  @param attribute String's attribute, eg. attribute = @{NSFontAttributeName: [UIFont systemFontOfSize:18.f]}
 *
 *  @return String's width.
 */
+ (CGFloat)oneLineOfTextHeightWithStringAttribute:(NSDictionary <NSString *, id> *)attribute;

@end

//
//  NSString+LabelWidthAndHeight.m
//  ZiPeiYi
//
//  Created by YouXianMing on 15/12/9.
//  Copyright © 2015年 YouXianMing. All rights reserved.
//

#import "NSString+LabelWidthAndHeight.h"

@implementation NSString (LabelWidthAndHeight)

- (CGFloat)heightWithStringAttribute:(NSDictionary <NSString *, id> *)attribute fixedWidth:(CGFloat)width {
    
    NSParameterAssert(attribute);
    
    CGFloat height = 0;
    
    if (self.length) {
        
        CGRect rect = [self boundingRectWithSize:CGSizeMake(width, MAXFLOAT)
                                         options:NSStringDrawingTruncatesLastVisibleLine |NSStringDrawingUsesLineFragmentOrigin |
                       NSStringDrawingUsesFontLeading
                                      attributes:attribute
                                         context:nil];
        
        height = rect.size.height;
    }
    
    return height;
}

- (CGFloat)widthWithStringAttribute:(NSDictionary <NSString *, id> *)attribute {
    
    NSParameterAssert(attribute);
    
    CGFloat width = 0;
    
    if (self.length) {
        
        CGRect rect = [self boundingRectWithSize:CGSizeMake(MAXFLOAT, 0)
                                         options:NSStringDrawingTruncatesLastVisibleLine |NSStringDrawingUsesLineFragmentOrigin |
                       NSStringDrawingUsesFontLeading
                                      attributes:attribute
                                         context:nil];
        
        width = rect.size.width;
    }
    
    return width;
}

+ (CGFloat)oneLineOfTextHeightWithStringAttribute:(NSDictionary <NSString *, id> *)attribute {
    
    CGFloat height = 0;
    CGRect rect    = [@"One" boundingRectWithSize:CGSizeMake(200, MAXFLOAT)
                                          options:NSStringDrawingTruncatesLastVisibleLine |NSStringDrawingUsesLineFragmentOrigin |
                      NSStringDrawingUsesFontLeading
                                       attributes:attribute
                                          context:nil];
    
    height = rect.size.height;
    return height;
}

@end

给UILabel设置完text之后，用siteToFit方法更新一下。