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简述
当你在Android系统中使用不同的硬件设备（例如摄像头、传感器、音频设备等）时，你需要与硬件抽象层（HAL）进行通信。 HAL是一个中间层，它充当了硬件和应用程序之间的桥梁。但是，由于硬件设备的不同，HAL接口在不同的硬件之间也会有所不同。这就是Android HAL接口定义语言（HIDL）的用武之地。
HIDL允许开发人员为每个硬件设备定义自己的接口。这些接口定义了硬件设备的功能和特性，包括输入参数、返回值和异常。使用这些接口，应用程序可以直接与硬件设备通信，而无需了解特定的硬件细节。在Android中，HIDL被广泛用于与HAL进行通信。
在Android Project Treble被提出，在android O中被全面的推送，设计 HIDL 这个机制的目的，主要目的是把框架（framework）与 HAL 进行隔离，使得框架部分可以直接被覆盖、更新，而不需要重新对 HAL 进行编译。
HIDL 实际上是用于进行进程间通信（Inter-process Communication，IPC）的。进程间的通信可以称为 Binder 化（Binderized）。对于必须连接到进程的库，也可以使用 passthough 模式（但在Java中不支持）。
官方介绍
编写代码
在Aosp代码目录中创建目录 hardware/interfaces/stksensor/1.0(有关1.0版本相关的，可以查看官网的版本管理概念)，
1、新建文件IStksensor.hal
package android,hardware.stksensor@1.0;improt IStksensorCallBack;interface IStksensor {setCallBack(IStksensorCallBack callback);write(string data) generates (bool res);init() generates (MyResult result);
};
2、创建UDT(用户自定义类型)，types.hal
package android.hardware.stksensor@1.0;enum ResultCode : int32_t {UNKNOWN = -1,ERROR = 0,OK = 1,};struct MyResult{ResultCode resultCode;string msg;
};
3、新建文件回调文件IStksensorCallBack.hal
package android.hardware.stksensor@1.0;interface IStksensorCallBack {passData(uint32_t data) generates (MyResult result);
};使用hidl-gen生成相关文件
在终端执行以下命令，设置临时变量
PACKAGE=android.hardware.stksensor@1.0
LOC=hardware/interfaces/stksensor/1.0/default
在AOSP源码根目录执行hidl-gen生成default目录里的c++文件
hidl-gen -o $LOC -Lc++-impl -randroid.hardware:hardware/interfaces -randroid.hidl:system/libhidl/transport $PACKAGE
在AOSP源码根目录执行hidl-gen生成default目录里的Android.bp文件
hidl-gen -o $LOC -Landroidbp-impl -randroid.hardware:hardware/interfaces -randroid.hidl:system/libhidl/transport $PACKAGE
在AOSP源码根目录执行./hardware/interfaces/update-makefiles.sh会生成hardware/interfaces/stksensor/1.0/目录里的Android.bp文件
实现接口代码
修改hardware/interfaces/stksensor/1.0/default/Stksensor.cpp，这边实现单纯是打印日志
// FIXME: your file license if you have one#include "Stksensor.h"
#include <log/log.h>namespace android::hardware::stksensor::implementation {// Methods from ::android::hardware::stksensor::V1_0::IStksensor follow.
Return<void> Stksensor::setCallBack(const sp<::android::hardware::stksensor::V1_0::IStksensorCallBack>& callback) {// TODO implementALOGE("stksensor service have called the funtion of setCallBack");return Void();
}Return<::android::hardware::stksensor::V1_0::ResultCode> Stksensor::write(const hidl_string& data) {// TODO implementALOGE("stksensor service have called the funtion of write");return ::android::hardware::stksensor::V1_0::ResultCode {};
}Return<void> Stksensor::init(init_cb _hidl_cb) {// TODO implementALOGE("stksensor service have called the funtion of init");return Void();
}// Methods from ::android::hidl::base::V1_0::IBase follow.//IStksensor* HIDL_FETCH_IStksensor(const char* /* name */) {//return new Stksensor();
//}
//
} // namespace android::hardware::stksensor::implementation添加启动服务
1、在hardware/interfaces/stksensor/1.0/default/创建service.cpp
#define LOG_TAG "android.hardware.stksensor@1.0-service"
#include <hidl/HidlTransportSupport.h>
#include <log/log.h>using android::hardware::configureRpcThreadpool;
using android::hardware::joinRpcThreadpool;
using android::hardwara::stksensor::V1_0::IStksensor;
using android::hardware::stksensor::V1_0::implementation::Stksensor;
using namespace android;int main() {configureRpcThreadpool(1, true);sp<IStksensor> service = new Stksensor();status_t status = service->registerAsService("stksensor"); if (status != OK) {ALOGE("Error registering stksensor as service: %d", status);return status;}ALOGE("Ok to registering stksensor as service");joinRpcThreadpool();return 1;
}2、在hardware/interfaces/stksensor/1.0/default/创建android.hardware.stksensor@1.0-service.rc启动脚本
service stksensor-hal-1-0 /vendor/bin/hw/android.hardware.stksensor@1.0-serviceclass haluser systemgroup system
3、在hardware/interfaces/stksensor/1.0/default/创建android.hardware.stksensor@1.0-service.xml的vintf
<manifest version="1.0" type="device"><hal format="hidl"><name>android.hardware.stksensor</name><transport>hwbinder</transport><version>1.0</version><interface><name>IStksensor</name><instance>stksensor</instance></interface></hal>
</manifest>
修改编译脚本
1、再修改hardware/interfaces/stksensor/1.0/default/Android.bp
// FIXME: your file license if you have onecc_library_shared {// FIXME: this should only be -impl for a passthrough hal.// In most cases, to convert this to a binderized implementation, you should:// - change '-impl' to '-service' here and make it a cc_binary instead of a// cc_library_shared.// - add a *.rc file for this module.// - delete HIDL_FETCH_I* functions.// - call configureRpcThreadpool and registerAsService on the instance.// You may also want to append '-impl/-service' with a specific identifier like// '-vendor' or '-<hardware identifier>' etc to distinguish it.name: "android.hardware.stksensor@1.0-impl",relative_install_path: "hw",// FIXME: this should be 'vendor: true' for modules that will eventually be// on AOSP.proprietary: true,srcs: ["Stksensor.cpp","StksensorCallBack.cpp",],shared_libs: ["libhidlbase","libutils","android.hardware.stksensor@1.0",],
}cc_binary {name: "android.hardware.stksensor@1.0-service",relative_install_path: "hw",defaults: ["hidl_defaults"],proprietary: true,init_rc: ["android.hardware.stksensor@1.0-service.rc"],vintf_fragments: ["android.hardware.stksensor@1.0-service.xml"],srcs: ["SerialPort.cpp","service.cpp",],shared_libs: ["libbase","liblog","libdl","libutils","libhardware","libhidlbase","libhidltransport","android.hardware.stksensor@1.0",],
}2、在device/$your_company/$SOC/device.mk添加
PRODUCT_PACKAGES += \android.hardware.stksensor@1.0-service \android.hardware.stksensor@1.0-impl
现在stksensor目录结构
stksensor/
└── 1.0├── Android.bp├── default│ ├── Android.bp│ ├── android.hardware.stksensor@1.0-service.rc│ ├── android.hardware.stksensor@1.0-service.xml│ ├── service.cpp│ ├── StksensorCallBack.cpp│ ├── StksensorCallBack.h│ ├── Stksensor.cpp│ └── Stksensor.h├── IStksensorCallBack.hal├── IStksensor.hal└── types.hal
注意：会报一些avc权限 ，需要自己添加