批量上传图片

满足一下功能点：

（1）异步上传

（2）所有的请求都完成之后再通知回调

（3）请求返回的结果必须按照请求发出的数据顺序一致

- (NSURLSessionUploadTask*)uploadTaskWithImage:(UIImage*)image completion:(void (^)(NSURLResponse *response, id responseObject, NSError *error))completionBlock {
// 构造 NSURLRequest
NSError* error = NULL;
NSMutableURLRequest *request = [[AFHTTPRequestSerializer serializer] multipartFormRequestWithMethod:@"POST" URLString:[self uploadUrl] parameters:nil constructingBodyWithBlock:^(id<AFMultipartFormData> formData) {
NSData* imageData = UIImageJPEGRepresentation(image, 1.0);
[formData appendPartWithFileData:imageData name:@"file" fileName:@"someFileName" mimeType:@"multipart/form-data"];
} error:&error];

// 将 NSURLRequest 与 completionBlock 包装为 NSURLSessionUploadTask
AFURLSessionManager *manager = [[AFURLSessionManager alloc] initWithSessionConfiguration:[NSURLSessionConfiguration defaultSessionConfiguration]];
NSURLSessionUploadTask *uploadTask = [manager uploadTaskWithStreamedRequest:request progress:^(NSProgress * _Nonnull uploadProgress) {
} completionHandler:completionBlock];

return uploadTask;
}

- (IBAction)runDispatchTest:(id)sender {
// 需要上传的数据
NSArray* images = [self images];

// 准备保存结果的数组，元素个数与上传的图片个数相同，先用 NSNull 占位
NSMutableArray* result = [NSMutableArray array];
for (UIImage* image in images) {
[result addObject:[NSNull null]];
}

dispatch_group_t group = dispatch_group_create();

for (NSInteger i = 0; i < images.count; i++) {

dispatch_group_enter(group);

NSURLSessionUploadTask* uploadTask = [self uploadTaskWithImage:images[i] completion:^(NSURLResponse *response, NSDictionary* responseObject, NSError *error) {
if (error) {
NSLog(@"第 %d 张图片上传失败: %@", (int)i + 1, error);
dispatch_group_leave(group);
} else {
NSLog(@"第 %d 张图片上传成功: %@", (int)i + 1, responseObject);
@synchronized (result) { // NSMutableArray 是线程不安全的，所以加个同步锁
result[i] = responseObject; //block会保存自动变量
}
dispatch_group_leave(group);
}
}];
[uploadTask resume];
}

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"上传完成!");
for (id response in result) {
NSLog(@"%@", response);
}
});
}