Android R(11)实现HIDL接口--ICustomHardware(三)

1.实现接口 ICustomHardware

1.1使用hidl-gen自动生成实现类

  使用hidl-gen直接生成ICustomHardware接口的实现类CustomHardware.cpp，命令如下

flagstaff@flagstaff-pc:~/aosp_r.lns$ hidl-gen -L c++-impl -o test/flagstaffTest/hardware/interfaces/custom_hardware/1.0/default -r  flagstaff.hardware:test/flagstaffTest/hardware/interfaces flagstaff.hardware.custom_hardware@1.0

  成功执行后，目录结构如下

flagstaff@flagstaff-pc:~/aosp_r.lns/test/flagstaffTest/hardware/interfaces/custom_hardware/1.0$ tree
.
├── Android.bp
├── default
│   ├── CustomHardware.cpp
│   └── CustomHardware.h
└── ICustomHardware.hal

1.2 CustomHardware.h

  使用hidl-gen确实为我们省略了不少的工作量。

#pragma once
#include 
#include 
#include 
#include 

namespace flagstaff::hardware::custom_hardware::implementation {

using ::android::hardware::hidl_array;
using ::android::hardware::hidl_memory;
using ::android::hardware::hidl_string;
using ::android::hardware::hidl_vec;
using ::android::hardware::Return;
using ::android::hardware::Void;
using ::android::sp;

struct CustomHardware : public V1_0::ICustomHardware {
private:
    const std::string mName;
public:
    CustomHardware(const std::string& name);
    Return<bool> enableHardware(bool data) override;
    const std::string& getName();
};
}

命名空间:
  CustomHardware的命名空间为如下，该命名空间由ICustomHardware.hal中的package转化而来，规则很简单，就是将packge中的.用::来替换，对于实现类后跟implementation。

//file:ICustomHardware.hal
package flagstaff.hardware.custom_hardware@1.0;
//CustomHardware.h
namespace flagstaff::hardware::custom_hardware::implementation

  HIDL中的后端大都是cpp来写的，这里实在没看懂为什么HIDL中喜欢用package加点的方式。
类定义:
  下面是CustomHardware的类继承关系图，顶层的父类为RefBase，可见CustomHardware是支持强弱指针的。

返回值–Return:
  Return实际上是一个模板类，方便调用者捕获远程调用状态的。并且如果Client本次远程调用失败且不调用Return.isOk检查的话，HIDL框架则会强制kill掉当前进程。其定义如下

//file:system\libhidl\base\include\hidl\Status.h
class Status final {
public:
    ...
    bool isOk() const { return mException == EX_NONE; }
};

class return_status {
private:
    Status mStatus {};
public:
    bool isOk() const {
        mCheckedStatus = true;
        return mStatus.isOk();
    }
};

template<typename T> class Return : public details::return_status {
...
};

  因为Return重载了符号，所以使用时直接当作原类型即可，如此处为boolean类型。

1.3 CustomHardware.cpp

  测试用，所以在方法enableHardware中打印值来验证是否调用成功即可，代码如下

#include "CustomHardware.h"
#include 

namespace flagstaff::hardware::custom_hardware::implementation {
CustomHardware::CustomHardware(const std::string& name):mName(name) {

}

const std::string& CustomHardware::getName(){
    return mName;
}

Return<bool> CustomHardware::enableHardware(bool data) {
    LOG(ERROR)<<"[service]"<<mName<<"[data]"<<data;
    return bool {false};
}
}

  mName用于验证单接口多实例功能，此处可以直接忽略。可见最终接口enableHardware被调用时会打印入参和被调用的实例名称。

2.HIDL实例的注册

  前面提过，远程调用模型就是C/S模型，对于调用者也就是Client端，其也可是一个常驻的后台服务也可以是单次调用命令。但是实现端其必须是一个常驻后台的server，下面则是HIDL server的一种写法。

#include 
#include 
#include 

#include 

using flagstaff::hardware::custom_hardware::implementation::CustomHardware;
using flagstaff::hardware::custom_hardware::V1_0::ICustomHardware;

int main(int /* argc */, char** /* argv */) {
	::android::hardware::configureRpcThreadpool(5, true);

	android::sp<ICustomHardware> instance = new CustomHardware("default");
	if (instance->registerAsService() != ::android::OK) {
		LOG(ERROR)<<"Failed to register ICustomHardware default instance";
		return -1;
	}
	android::sp<ICustomHardware> instance1 = new CustomHardware("custom");
	if (instance->registerAsService("custom") != ::android::OK) {
		LOG(ERROR)<<"Failed to register ICustomHardware custome instance";
		return -1;
	}
    LOG(INFO)<<"The instance of CustomHardware is registered to hwmanager in success.";
	::android::hardware::joinRpcThreadpool();
	return 1;	// joinRpcThreadpool shouldn't exit
}

configureRpcThreadpool
  用于设置server的线程池数量，每一次client端调用，Server都会对应一个线程去处理client的调用请求。如果client的请求数量小于等于此处设置的5，那么这5次调用实际上就是并行的，即使此处的接口没有设置oneway关键字，所以实现接口的时候要考虑并发。
  本次的server则是为ICustomHardware接口注册了两个实例，并且其命名分别为default/custom，其中registerAsService无入参的时候默认为defalut，其代码如下

//file:out\soong\.intermediates\test\flagstaffTest\hardware\interfaces\custom_hardware\1.0\[email protected]_genc++_headers\gen\flagstaff\hardware\custom_hardware\1.0\ICustomHardware.h
namespace flagstaff {
namespace hardware {
namespace custom_hardware {
namespace V1_0 {

struct ICustomHardware : public ::android::hidl::base::V1_0::IBase {
...
/**
* Registers a service with the service manager. For Trebilized devices, the service
* must also be in the VINTF manifest.
*/
__attribute__ ((warn_unused_result))::android::status_t registerAsService(const std::string &serviceName="default");
...
};

joinRpcThreadpool
  该函数实际上等同于thread->join()，并且binder线程是在死循环中轮询client是否发送消息过来的，所以并不会返回，正常情况下server也就不会退出。

3.Android.bp

  Android团队推荐使用Android.bp来编写编译规则，因此此处也使用Android.bp。

cc_binary {
    name: "[email protected]",
    defaults: ["hidl_defaults"],
    vendor: true,
    relative_install_path: "hw",
    srcs: [
        "service.cpp",
        "src/*.cpp",
    ],
    init_rc: ["[email protected]"],
    local_include_dirs: [
        "include",
    ],
    shared_libs: [
        "libbinder",
        "libhidlbase",
        "libbase",
        "libutils",
        "libcutils",
        "liblog",
        "[email protected]",
    ],
    cppflags:[
        "-Wno-unused-parameter",
        "-DLOG_TAG=\"flagstaff.hardware.custom_hardware\"",
    ],
    vintf_fragments: ["[email protected]"],
}

  对于其中关键字的解释AOSP自带的文档已经解释的很详细了，所以就不赘诉了，注意只有编译后才会生成。

flagstaff@flagstaff-pc:~/aosp_r.lns/out/soong/docs$ ls cc.html
cc.html

  额外说明下，如果熟悉Android.mk的写法，想学习Android.bp的写法，可以使用AOSP自带的工具androidmk来将Android.mk转化为Android.bp来对比学习，下面是其简略使用说明

flagstaff@flagstaff-pc:~/aosp_r.lns$ androidmk
usage: androidmk [flags] <inputFile>

androidmk parses <inputFile> as an Android.mk file and attempts to output an analogous Android.bp file (to standard out)

  另外如果有问题，欢迎留言或者email([email protected])我，技术升级在于交流~~