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iOS APP 架构漫谈二

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上一篇《iOS APP 架构漫谈》简单介绍了information flow的概念。这篇文章简单介绍另一个在编程中非常重要的思想或工具——状态机(State machine)。对大多数计算机专业的家伙们来说,这应该是一门比较难学的课程,里面包含一大堆揪心的名字比如DFA,NFA,还有一大堆各种各样的数学符号,又是编译原理的基础。不过很遗憾,似乎在做完编译原理课程作业之后,很多人再也没有实现过或是用过状态机了。本文通过一个游戏demo来简单描述一下状态机在实践中的应用。demo code

背景

首先看下我们的使用场景,假如我们需要设计一套联网对战的小游戏。第一个难题可能是如何建立一个通道,让2个手机相互发送消息。这里我并不打算引入server端开发,希望只是通过客户端来实现这个逻辑,这里使用LeanCloud API来简化这个过程。这样我们可以暂时不考虑技术细节,直接站在业务角度去思考如何建立这个游戏。

业务场景–邀请

正式开始游戏之前,总会有一个邀请的环节。假如我们有2个用户,分别是Host,Guest。Host创建游戏,Guest加入游戏。游戏的整个流程和我们平时玩的对战游戏流程并没有多大不同。

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  1. Host创建游戏,他就相当于进入一个等待队列里面。
  2. Guest加入游戏,他从等待队列中找到一个匹配,比如Host。然后对Host发送join message
  3. Host会收到很多join message。由于我们只是选择1vs1。这里假定Host同意Guest加入游戏。Host向Guest发送join confirm message
  4. Guest收到join confirm message, 向Host发送Go消息,表示Guest已经进入游戏
  5. Host收到Go消息。也进入游戏。

具体实现业务逻辑

现在的构想的逻辑只有5步,但其实还会包含很多逻辑,比如超时机制,重发机制。由于中间状态很多,还可能有我们没有想到过的问题。在面对这种复杂逻辑时,会通过状态机来帮助我们理顺逻辑。这时,我们脑中思考的业务其实是一个状态到一个状态的图。 如下

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上半部分是游戏的创建者,下半部分是游戏的加入者。

一开始,尽量简化模型,这里红色剪头表示我们的正确主流路线,黑色表现出错路线。也就是说,一旦错误,就回到原始Idle状态。

开始写代码

在想清楚所有逻辑,并考虑清楚正常路线和错误路线之后,就可以开始写代码了。为了方便,这里直接使用第三方的状态机框架TransitionKit

定义State(HOST)

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 TKState *idleState = [TKState stateWithName:@"idle"];
  TKState *waitingJoinState = [TKState stateWithName:@"waitingJoin"];
  TKState *waitingConfirmState = [TKState stateWithName:@"waitingConfirm"];
  TKState *goState = [TKState stateWithName:@"go"];

  [waitingConfirmState setDidEnterStateBlock:^(TKState *state, TKTransition *transition) {
      [selfWeak sendJoinConfirm];
  }];

  [goState setDidEnterStateBlock:^(TKState *state, TKTransition *transition) {
      NSLog(@"happy ending");

      [SVProgressHUD showSuccessWithStatus:@"ok"];
  }];

定义Event(HOST)

Event 是建立State到State的路径

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 TKEvent *waitingJoinEvent = [TKEvent eventWithName:CUHostGameManagerWaitingJoinEvent
                           transitioningFromStates:@[idleState]
                                           toState:waitingJoinState];

  TKEvent *receiveInviteEvent = [TKEvent eventWithName:CUHostGameManagerReceiveInviteEvent
                               transitioningFromStates:@[waitingJoinState]
                                               toState:waitingConfirmState];

  TKEvent *receiveConfirmEvent = [TKEvent eventWithName:CUHostGameManagerReceiveConfirmEvent
                                transitioningFromStates:@[waitingConfirmState]
                                                toState:goState];

  TKEvent *disconnectedEvent = [TKEvent eventWithName:CUHostGameManagerDisconnectedEvent
                               transitioningFromStates:nil
                                              toState:idleState];

定义过程(HOST)

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 - (void)startGame {

      NSAssert(self.session.peerId != nil, @"");

      //这里,如果不是idle,我们切换状态机到idle
      if (![self.stateMachine.currentState.name isEqual:@"idle"]) {
          [self fireEvent:CUHostGameManagerDisconnectedEvent userInfo:nil];
      }

      //这里调用LeanCloud 入队
      AVObject *waitingId = [AVObject objectWithClassName:@"waiting_join_Ids"];
      [waitingId setObject:self.session.peerId forKey:@"peerId"];
      [waitingId saveInBackgroundWithBlock:^(BOOL succeeded, NSError *error) {
          //enqueue 之后,进入waitingJoin状态
          [self fireEvent:CUHostGameManagerWaitingJoinEvent userInfo:nil];
      }];
  }

  - (void)sendJoinConfirm {
      //发送加入确认消息给Guest
      AVMessage *message = [AVMessage messageForPeerWithSession:self.session
                                                   toPeerId:self.peerId
                                                    payload:@"join_confirm"];
      [self.session sendMessage:message transient:YES];
  }

  - (void)session:(AVSession *)session didReceiveMessage:(AVMessage *)message
  {
      if ([message.payload isEqualToString:@"join"]) {
          //收到Join(邀请)之后,发送确认消息
          self.peerId = message.fromPeerId;

          //因为LeanCloud的API比较挫,watch 之后才能发送消息,但是我们不知道什么时候才watch成功。。。。
          //好在只是demo,我们只好用这种方式work around,延迟2s发送消息
          [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(sendInviteConfirmRequest:) object:nil];
          [self performSelector:@selector(sendInviteConfirmRequest:)
                      withObject:@[message.fromPeerId]
                      afterDelay:2.0f];
      }
      else if ([message.payload isEqualToString:@"go"]) {
          //收到go消息,流程结束
          [self fireEvent:CUHostGameManagerReceiveConfirmEvent userInfo:nil];
      }
  }

  - (void)sendInviteConfirmRequest:(NSArray *)watchPeerIds {
      [self.session watchPeerIds:watchPeerIds];
      [self fireEvent:CUHostGameManagerReceiveInviteEvent userInfo:nil];
  }

定义State(Guest)

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 TKState *idleState = [TKState stateWithName:@"idle"];
  TKState *waitingReplyState = [TKState stateWithName:@"waitingReply"];
  TKState *goState = [TKState stateWithName:@"go"];

  [waitingReplyState setWillEnterStateBlock:^(TKState *state, TKTransition *transition) {
      [selfWeak searchingGames];
  }];

  [goState setDidEnterStateBlock:^(TKState *state, TKTransition *transition) {
      [selfWeak sendGo];
      NSLog(@"happy ending");
      [SVProgressHUD showSuccessWithStatus:@"ok"];
  }];

定义Event(Guest)

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 TKEvent *searchingEvent = [TKEvent eventWithName:CUGestGameManagerSearchingEvent
                           transitioningFromStates:@[idleState]
                                           toState:waitingReplyState];

  TKEvent *receiveConfirmEvent = [TKEvent eventWithName:CUGestGameManagerReceiveConfirmEvent
                                transitioningFromStates:@[waitingReplyState]
                                                toState:goState];

  TKEvent *disconnectedEvent = [TKEvent eventWithName:CUGestGameManagerDisconnectedEvent
                              transitioningFromStates:nil
                                              toState:idleState];

定义过程(Guest)

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- (void)joinGame {

  if (![self.stateMachine.currentState.name isEqual:@"idle"]) {
    [self fireEvent:CUGestGameManagerDisconnectedEvent userInfo:nil];
  }

  [self fireEvent:CUGestGameManagerSearchingEvent userInfo:nil];
}

- (void)searchingGames {
  AVQuery *query = [AVQuery queryWithClassName:@"waiting_join_Ids"];
  [query orderByDescending:@"updatedAt"];
  [query setLimit:1];

  [query findObjectsInBackgroundWithBlock:^(NSArray *objects, NSError *error) {
    NSMutableArray *installationIds = [[NSMutableArray alloc] init];
    for (AVObject *object in objects) {
      if ([object objectForKey:@"peerId"]) {
        [installationIds addObject:[object objectForKey:@"peerId"]];
      }
    }

    [self.session watchPeerIds:installationIds];

    [NSObject cancelPreviousPerformRequestsWithTarget:self selector:@selector(sendJoinRequest) object:nil];
    [self performSelector:@selector(sendJoinRequest)
               withObject:nil
               afterDelay:2.0f];
  }];
}

- (void)sendJoinRequest {

  for (NSString *item in self.session.watchedPeerIds) {
    AVMessage *message = [AVMessage messageForPeerWithSession:self.session
                                                     toPeerId:item
                                                      payload:@"join"];
    [self.session sendMessage:message transient:YES];
  }
}

- (void)sendGo{
  AVMessage *message = [AVMessage messageForPeerWithSession:self.session
                                                   toPeerId:self.otherPeerId
                                                    payload:@"go"];
  [self.session sendMessage:message transient:YES];
}

最后

state machine 是一个蛮厉害的锤子,只要是一个工具,就肯定会被滥用。。。state machine最大的好处是在于,方便我们思考清楚所有细节,主线,和错误流程。避免因为考虑不周全而产生的bug。结合之前的information flow的思路,会让我们的软件设计更加清楚。

demo code

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