使用AFNetworking的方法

AFHTTPSessionManager *manager = [AFHTTPSessionManager manager];//初始化请求对象
    manager.responseSerializer = [AFHTTPResponseSerializer serializer];//设置服务器允许的请求格式内容
//    manager.responseSerializer.acceptableContentTypes = [NSSet setWithObjects:@"application/json",@"text/html",@"text/json", @"text/javascript,multipart/form-data", nil];
    //上传图片/文字，只能同POST
    [manager POST:kUploadAvatarURL parameters:@{@"user_id":@"1000665",@"sign_key":@"566e37eadb8fa4794f8415db11d828f3"} constructingBodyWithBlock:^(id<AFMultipartFormData>  _Nonnull formData) {
        // 注意：这个name（我的后台给的字段是file）一定要和后台的参数字段一样 否则不成功
        [formData appendPartWithFileData:data name:@"file" fileName:@"aaa.jpg" mimeType:@"image/jpg"];
       // [formData appendPartWithFormData:[@"wfWiEWrgEFA9A78512weF7106A" dataUsingEncoding:NSUTF8StringEncoding]
                                    name:@"aaa"];
    } progress:^(NSProgress * _Nonnull uploadProgress) {
        NSLog(@"uploadProgress = %@",uploadProgress);
    } success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {
        
        NSLog(@"responseObject = %@, task = %@",responseObject,task);
        NSDictionary *obj = [NSJSONSerialization JSONObjectWithData:responseObject options:NSJSONReadingAllowFragments error:nil];
        NSLog(@"obj = %@",obj[@"error_msg"]);
        
    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {
         NSLog(@"error = %@",error);
    }];

还有一个是网上搜到的别人封装的类

RequestPostUploadHelper.h文件

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

@interface RequestPostUploadHelper : NSObject
/**
 *POST 提交 并可以上传图片目前只支持单张
 */
+ (NSString *)postRequestWithURL: (NSString *)url  // IN
                      postParems: (NSMutableDictionary *)postParems // IN 提交参数据集合
                     picFilePath: (NSString *)picFilePath  // IN 上传图片路径
                     picFileName: (NSString *)picFileName;  // IN 上传图片名称

/**
 * 修发图片大小
 */
+ (UIImage *) imageWithImageSimple:(UIImage*)image scaledToSize:(CGSize) newSize;
/**
 * 保存图片
 */
+ (NSString *)saveImage:(UIImage *)tempImage WithName:(NSString *)imageName;
/**
 * 生成GUID
 */
//+ (NSString *)generateUuidString;
@end

RequestPostUploadHelper.m文件

注意：这个文件里面需要修改static NSString * const FORM_FLE_INPUT = @"file"; 把file修改为你后台规定的字段，否则不会成功

#import "RequestPostUploadHelper.h"

@implementation RequestPostUploadHelper


static NSString * const FORM_FLE_INPUT = @"file";//与服务器要求的key 一样

+ (NSString *)postRequestWithURL: (NSString *)url  // IN
                      postParems: (NSMutableDictionary *)postParems // IN
                     picFilePath: (NSString *)picFilePath  // IN上传图片路径
                     picFileName: (NSString *)picFileName  // IN
{
    
    
    NSString *TWITTERFON_FORM_BOUNDARY = @"0xKhTmLbOuNdArY";
    //根据url初始化request
    NSMutableURLRequest* request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:url]
                                                           cachePolicy:NSURLRequestReloadIgnoringLocalCacheData
                                                       timeoutInterval:10];
    //分界线 --AaB03x
    NSString *MPboundary=[[NSString alloc]initWithFormat:@"--%@",TWITTERFON_FORM_BOUNDARY];
    //结束符 AaB03x--
    NSString *endMPboundary=[[NSString alloc]initWithFormat:@"%@--",MPboundary];
    //得到图片的data
    NSData* data;
    
    if(picFilePath){
        
        UIImage *image=[UIImage imageWithContentsOfFile:picFilePath];
        //判断图片是不是png格式的文件
        if (UIImagePNGRepresentation(image)) {
            //返回为png图像。
            data = UIImagePNGRepresentation(image);
        }else {
            //返回为JPEG图像。
            data = UIImageJPEGRepresentation(image, 1.0);
        }
    }
    //http body的字符串
    NSMutableString *body=[[NSMutableString alloc]init];
    //参数的集合的所有key的集合
    NSArray *keys= [postParems allKeys];
    
    //遍历keys
    for(int i=0;i<[keys count];i++)
    {
        //得到当前key
        NSString *key=[keys objectAtIndex:i];
        
        //添加分界线，换行
        [body appendFormat:@"%@\r\n",MPboundary];
        //添加字段名称，换2行
        [body appendFormat:@"Content-Disposition: form-data; name=\"%@\"\r\n\r\n",key];
        //添加字段的值
        [body appendFormat:@"%@\r\n",[postParems objectForKey:key]];
        
        NSLog(@"添加字段的值==%@",[postParems objectForKey:key]);
    }
    
    if(picFilePath){
        ////添加分界线，换行
        [body appendFormat:@"%@\r\n",MPboundary];
        
        //声明pic字段，文件名为boris.png
        [body appendFormat:@"Content-Disposition: form-data; name=\"%@\"; filename=\"%@\"\r\n",FORM_FLE_INPUT,picFileName];
        //声明上传文件的格式
        [body appendFormat:@"Content-Type: image/jpge,image/gif, image/jpeg, image/pjpeg, image/pjpeg\r\n\r\n"];
        NSLog(@"body = %@",body);
    }
    
    //声明结束符：--AaB03x--
    NSString *end=[[NSString alloc]initWithFormat:@"\r\n%@",endMPboundary];
    //声明myRequestData，用来放入http body
    NSMutableData *myRequestData=[NSMutableData data];
    
    //将body字符串转化为UTF8格式的二进制
    [myRequestData appendData:[body dataUsingEncoding:NSUTF8StringEncoding]];
    if(picFilePath){
        //将image的data加入
        [myRequestData appendData:data];
    }
    //加入结束符--AaB03x--
    [myRequestData appendData:[end dataUsingEncoding:NSUTF8StringEncoding]];
    
    //设置HTTPHeader中Content-Type的值
    NSString *content=[[NSString alloc]initWithFormat:@"multipart/form-data; boundary=%@",TWITTERFON_FORM_BOUNDARY];
    //设置HTTPHeader
    [request setValue:content forHTTPHeaderField:@"Content-Type"];
    //设置Content-Length
    [request setValue:[NSString stringWithFormat:@"%lu", (unsigned long)[myRequestData length]] forHTTPHeaderField:@"Content-Length"];
    //设置http body
    [request setHTTPBody:myRequestData];
    //http method
    [request setHTTPMethod:@"POST"];
    
    
    NSHTTPURLResponse *urlResponese = nil;
    NSError *error = [[NSError alloc]init];
    NSData* resultData = [NSURLConnection sendSynchronousRequest:request   returningResponse:&urlResponese error:&error];
    NSString* result= [[NSString alloc] initWithData:resultData encoding:NSUTF8StringEncoding];
    if([urlResponese statusCode] >=200&&[urlResponese statusCode]<300){
        NSLog(@"返回结果=====%@",result);
        return result;
    }
    return nil;
}

/**
 * 修发图片大小
 */
+ (UIImage *) imageWithImageSimple:(UIImage*)image scaledToSize:(CGSize) newSize{
    newSize.height=image.size.height*(newSize.width/image.size.width);
    UIGraphicsBeginImageContext(newSize);
    [image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
    UIImage *newImage=UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return  newImage;
    
}

/**
 * 保存图片
 */
+ (NSString *)saveImage:(UIImage *)tempImage WithName:(NSString *)imageName{
    
    NSData* imageData;
    
    //判断图片是不是png格式的文件
    if (UIImagePNGRepresentation(tempImage)) {
        //返回为png图像。
        imageData = UIImagePNGRepresentation(tempImage);
    }else {
        //返回为JPEG图像。
        imageData = UIImageJPEGRepresentation(tempImage, 1.0);
    }
    NSArray* paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask,YES);
    
    NSString* documentsDirectory = [paths objectAtIndex:0];
    
    NSString* fullPathToFile = [documentsDirectory stringByAppendingPathComponent:imageName];
    
    NSArray *nameAry=[fullPathToFile componentsSeparatedByString:@"/"];
    NSLog(@"===fullPathToFile===%@",fullPathToFile);
    NSLog(@"===FileName===%@",[nameAry objectAtIndex:[nameAry count]-1]);
    
    [imageData writeToFile:fullPathToFile atomically:NO];
    return fullPathToFile;
}

///**
// * 生成GUID
// */
//+ (NSString *)generateUuidString{
//    // create a new UUID which you own
//    CFUUIDRef uuid = CFUUIDCreate(kCFAllocatorDefault);
//
//    // create a new CFStringRef (toll-free bridged to NSString)
//    // that you own
//    NSString *uuidString = (NSString *)CFUUIDCreateString(kCFAllocatorDefault, uuid);
//
//    // transfer ownership of the string
//    // to the autorelease pool
//    [uuidString autorelease];
//
//    // release the UUID
//    CFRelease(uuid);
//
//    return uuidString;
//}
@end

.m文件中调用类方法

NSString *picString = [RequestPostUploadHelper saveImage:littleImg WithName:@"avatar.jpg"];
    NSMutableDictionary *params = [NSMutableDictionary dictionary];
    [params setObject:[UserLoginStateManager shareManager].model.userID forKey:@"user_id"];
    [params setObject:[UserLoginStateManager shareManager].model.sign_key forKey:@"sign_key"];
    [RequestPostUploadHelper postRequestWithURL:kUploadAvatarURL postParems:params picFilePath:picString picFileName:@"avatar.jpg"];