Message* createMessage(const std::string& typeName)
{
  Message* message = NULL;
  const Descriptor* descriptor = DescriptorPool::generated_pool()->FindMessageTypeByName(typeName);
  if (descriptor)
  {
    const Message* prototype = MessageFactory::generated_factory()->GetPrototype(descriptor);
    if (prototype)
    {
      message = prototype->New();
    }
  }
  return message;
}

message Person {
  string name = 1;
  int32 id = 2;
  string email = 3;
}

Person *newP = [[Person alloc] initWithData:data error:nil];

- (instancetype)initWithData:(NSData *)data error:(NSError **)errorPtr {
    return [self initWithData:data extensionRegistry:nil error:errorPtr];
}

- (instancetype)initWithData:(NSData *)data
           extensionRegistry:(GPBExtensionRegistry *)extensionRegistry
                       error:(NSError **)errorPtr {
  if ((self = [self init])) {
    @try {
      [self mergeFromData:data extensionRegistry:extensionRegistry];
          //...
    }
    @catch (NSException *exception) {
      //...  
    }
  }
  return self;
}

- (void)mergeFromData:(NSData *)data extensionRegistry:(GPBExtensionRegistry *)extensionRegistry {
  GPBCodedInputStream *input = [[GPBCodedInputStream alloc] initWithData:data]; //根据传入的`data`构造出数据流对象
  [self mergeFromCodedInputStream:input extensionRegistry:extensionRegistry]; //通过数据流对象进行merge
  [input checkLastTagWas:0]; //校检
  [input release];
}

typedef struct GPBCodedInputStreamState {
const uint8_t *bytes;
size_t bufferSize;
size_t bufferPos;

// For parsing subsections of an input stream you can put a hard limit on
// how much should be read. Normally the limit is the end of the stream,
// but you can adjust it to anywhere, and if you hit it you will be at the
// end of the stream, until you adjust the limit.
size_t currentLimit;
int32_t lastTag;
NSUInteger recursionDepth;
} GPBCodedInputStreamState;

@interface GPBCodedInputStream () {
@package
struct GPBCodedInputStreamState state_;
NSData *buffer_;
}

@interface GPBDescriptor : NSObject<NSCopying>
@property(nonatomic, readonly, strong, nullable) NSArray<GPBFieldDescriptor*> *fields;
@property(nonatomic, readonly, strong, nullable) NSArray<GPBOneofDescriptor*> *oneofs; //用于 repeated 类型的 filed
@property(nonatomic, readonly, assign) GPBFileDescriptor *file;
@end

@interface GPBFieldDescriptor () {
@package
 GPBMessageFieldDescription *description_;
 GPB_UNSAFE_UNRETAINED GPBOneofDescriptor *containingOneof_;

 SEL getSel_;
 SEL setSel_;
 SEL hasOrCountSel_;  // *Count for map<>/repeated fields, has* otherwise.
 SEL setHasSel_;
}

- (void)mergeFromCodedInputStream:(GPBCodedInputStream *)input
               extensionRegistry:(GPBExtensionRegistry *)extensionRegistry {
 GPBDescriptor *descriptor = [self descriptor]; //生成当前 Message 的`Descriptor`实例
 GPBFileSyntax syntax = descriptor.file.syntax; //syntax 标识.proto文件的语法版本 (proto2/proto3)
 NSUInteger startingIndex = 0; //当前位置
 NSArray *fields = descriptor->fields_; //当前 Message 的所有 fileds
 
 //循环解码
 for (NSUInteger i = 0; i < fields.count; ++i) {
  //拿到当前位置的`FieldDescriptor`
     GPBFieldDescriptor *fieldDescriptor = fields[startingIndex];
     //判断当前field的类型
     GPBFieldType fieldType = fieldDescriptor.fieldType;
     if (fieldType == GPBFieldTypeSingle) {
       //`MergeSingleFieldFromCodedInputStream` 函数中解码 Single 类型的 field 的数据
       MergeSingleFieldFromCodedInputStream(self, fieldDescriptor, syntax, input, extensionRegistry);
       //当前位置+1
       startingIndex += 1; 
     } else if (fieldType == GPBFieldTypeRepeated) {
        // ...
       // Repeated 解码操作
     } else {  
       // ...
       // 其他类型解码操作
     }
  }  // for(i < numFields)
}

GPBDescriptor *descriptor = [self descriptor];

//`desciptor`方法定义
- (GPBDescriptor *)descriptor {
 return [[self class] descriptor]; 
}

+ (GPBDescriptor *)descriptor {
 static GPBDescriptor *descriptor = nil;
 if (!descriptor) {
   static GPBMessageFieldDescription fields[] = {
     {
       .name = "name",
       .dataTypeSpecific.className = NULL,
       .number = Person_FieldNumber_Name,
       .hasIndex = 0,
       .offset = (uint32_t)offsetof(Person__storage_, name),
       .flags = GPBFieldOptional,
       .dataType = GPBDataTypeString,
     },
     //...
     //每个field都会在这里定义出`GPBMessageFieldDescription`
   };
   GPBDescriptor *localDescriptor = //这里会根据fileds和其他一系列参数构造出一个`Descriptor`对象
   descriptor = localDescriptor;
 }
 return descriptor;
}

MergeSingleFieldFromCodedInputStream(self, fieldDescriptor, syntax, input, extensionRegistry);
MergeSingleFieldFromCodedInputStream内部提供了一系列宏定义，针对不同的数据类型进行数据解码。
#define CASE_SINGLE_POD(NAME, TYPE, FUNC_TYPE)                             \
   case GPBDataType##NAME: {                                              \
     TYPE val = GPBCodedInputStreamRead##NAME(&input->state_);            \
     GPBSet##FUNC_TYPE##IvarWithFieldInternal(self, field, val, syntax);  \
     break;                                                               \
           }
#define CASE_SINGLE_OBJECT(NAME)                                           \
   case GPBDataType##NAME: {                                              \
     id val = GPBCodedInputStreamReadRetained##NAME(&input->state_);      \
     GPBSetRetainedObjectIvarWithFieldInternal(self, field, val, syntax); \
     break;                                                               \
   }

     CASE_SINGLE_POD(Int32, int32_t, Int32)
  ...
       
#undef CASE_SINGLE_POD
#undef CASE_SINGLE_OBJECT

int32_t GPBCodedInputStreamReadInt32(GPBCodedInputStreamState *state) {
 int32_t value = ReadRawVarint32(state);
 return value;
}

void GPBSetInt32IvarWithFieldInternal(GPBMessage *self,
                                     GPBFieldDescriptor *field,
                                     int32_t value,
                                     GPBFileSyntax syntax) {

 //最终的赋值操作
 //此处`self`为`GPBMessage`实例
 uint8_t *storage = (uint8_t *)self->messageStorage_;
 int32_t *typePtr = (int32_t *)&storage[field->description_->offset];
 *typePtr = value;

}