【架构分析】Fuchsia FIDL IPC 详解

目录

概述

Fuchsia IPC Architecture Overview

FIDL IPC 详解

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概述

Fuchsia使用Fuchsia Interface Definition Language (FIDL) 作为进程IPC的接口描述语言，本文旨在详细分析基于FIDL的IPC原理

 

Fuchsia FIDL IPC Architecture Overview

Fuchsia IPC Overview

• 通过FIDL描述IPC接口Interface
• Client 进程使用InterfacePtr来调用Interface中的接口获得服务
• Service进程实现Interface中的接口来提供服务
• Client和Service通过Message Loop被通知channel上有数据从而进行回调处理（关于Message Loop请参考我的另外一篇文章 https://blog.csdn.net/HaoBBNuanMM/article/details/86774697）
• 为了描述更具体清晰，本文使用view_provider.fidl 作为Interface的具体例子来分析IPC的原理和过程

[Discoverable]
interface ViewProvider {
    // Creates a new View under the control of the ViewProvider.
    //
    // |token| is one half of the shared eventpair which will bind the new View
    // to its associated ViewHolder.  The ViewProvider will use |token| to
    // create its internal View representation.  The caller is expected to use
    // its half to create corresponding ViewHolder object.
    //
    // |incoming_services| allows clients to request services from the
    // ViewProvider implementation.  |outgoing_services| allows clients to
    // provide services of their own to the ViewProvider implementation.
    //
    // Clients can embed a ViewHolder (and by proxy the paired View) into their
    // scene graph by using |Node.AddChild()|.  The ViewHolder cannot itself
    // have any children. A ViewProvider implementation can nest scene objects
    // within its View by using |View.AddChild()|.  The View itself
    // cannot be a child of anything.
    //
    // Modules can use these mechanisms to establish a distributed,
    // inter-process scene graph.
    CreateView(handle token,
               request? incoming_services,
               fuchsia.sys.ServiceProvider? outgoing_services);
};

FIDL IPC 详解

FIDL IPC 类图

FIDL IPC中涉及的Class大致可以分为4种

1. Client端使用的Class
2. Service端使用的Class
3. FIDL自动代码生成的Class
4. Client和Service都会使用到的Class（包括访问Zircon微内核channel的Class）

Binding过程

理解FIDL IPC通信一定要理解Binding的作用，如上图所示，Service端包括FIDL Interface实现类在内的几个核心数据结构共同完成了将底层channel绑定到FIDL Interface实现类的过程

IPC 数据与控制流

完成了Binding过程后，如上图所示Client通过FIDL Interface的调用就可以IPC到Service端并且派发给对应的实现类来完成需要的功能了

下面依然以FIDL view_provider.fidl 中定义的interface ViewProvider作为具体的例子说明Client与Service的IPC时序

Client通过FIDL接口ViewProvider发起CreateView IPC的调用时序

 

Service通过FIDL接口ViewProvider接收到CreateView IPC的调用时序

 

  ViewProviderService view_provider(startup_context.get());

  // Add our ViewProvider service to the outgoing services.
  startup_context->outgoing().AddPublicService(
      [&view_provider](fidl::InterfaceRequest request) {
        view_provider.HandleViewProviderRequest(std::move(request));
      });

其中 FIDL Interface ViewProvider 的Service端bouncing ball应用程序main函数中，通过AddPublicService对外暴露了ViewProvider service，当有新的Client连接ViewProvider service的时候，就会先调用AddPublicService中作为参数的InterfaceRequestHandler函数（上面的代码中是以Lamba函数的形式实现），而该函数中最重要的信息就是参数fidl::InterfaceRequest request，它代表了链接Client的channel

  void HandleViewProviderRequest(fidl::InterfaceRequest request) {
    bindings_.AddBinding(this, std::move(request));
  }

ViewProvider service在收到Client的request 后通过Binding 将该request对应的channel绑定到了ViewProvider service的实例即this 对象上，这样就完成了Client channel与Service之间建立IPC链接的过程，为后续Client通过FIDL ViewProvider Interface IPC调用service奠定了通信基础；此处使用bindings的原因是可以有很多个Client的request连接到Service

 

Tips

• Fuchsia FIDL文件的位置: fuchsia/garnet/public/fidl/
• 编译FIDL后自动生成的代码位置:fuchsia/out/arm64/fidling/gen/fuchsia//