本篇概述

基于IUnknown，我們可以開發服務或功能的對外介面，介面實作的本質是函式地址的傳導與函式的呼叫，因此IUnknown在M核和A核系統服務開發框架基礎代碼中顯得尤為重要，
組件對外公布的是介面，一個組件可以實作多個介面，也就是說可以對外公布多個介面，組件本身就提供對自己查詢的一個介面，讓客戶去詢問組件，問它是否支持某個介面，在經過多次的這種詢問之后，客戶對于組件的認識將越來越清晰，這是COM組件物件模型的核心內容，詳情請參考com編程，
本篇將講述iunknown.c和iunknown.h的核心內容，基于介面的底層代碼探索介面實作的核心技術，

代碼框架

iunknown
├── 宏定義
│ └── DEFAULT_VERSION
│ └── INHERIT_IUNKNOWN
│ └── INHERIT_IUNKNOWNENTRY
│ └── DEFAULT_IUNKNOWN_IMPL
│ └── IUNKNOWN_ENTRY_BEGIN
│ └── IUNKNOWN_ENTRY_END
│ └── GET_IUNKNOWN
├── 結構體
│ └── IUnknown
│ └── IUnknownEntry
├── 介面功能函式
│ └── IUNKNOWN_QueryInterface
│ └── IUNKNOWN_AddRef
│ └── IUNKNOWN_Release

iunknown.h

iunknown.h中包含了有關介面物件以及類的相關宏定義、介面與介面入口的結構體以及介面功能函式的部分宣告，通過iunknown.h中的相關代碼以及注釋我們可以發現：介面的結構與呼叫思路采用COM編程的方案，詳情請見：

宏定義部分

/**
 * @brief Defines the default IUnknown IUnknown. You can customize the version.
 * 概述：定義默認IUnknown型號，你可以自定義這個型號
 * The <b>IUnknown</b> interface of the default version can be called only in the current process.
 * Inter-process communication is not supported. \n
 * IUnknown默認型號介面只能在當前行程被呼叫
 * 跨行程間通信不被支持
 * @since 1.0
 * @version 1.0
 */
#define DEFAULT_VERSION 0x20 //16進制數轉10進制為32

/**
 * @brief Defines the macro for inheriting the <b>IUnknown</b> interface.
 * 概述：定義為了繼承IUnknown介面的宏
 * When developing a subclass of the <b>IUnknown</b> class, you can use this macro to inherit the
 * structures of the <b>IUnknown</b> interface. \n
 * 在開發一個IUnknown的子類時，你可以使用這個宏來繼承IUnknown介面的框架
 */
#define INHERIT_IUNKNOWN                                                   \
    int (*QueryInterface)(IUnknown *iUnknown, int version, void **target); \
    int (*AddRef)(IUnknown *iUnknown);                                     \
    int (*Release)(IUnknown *iUnknown)

/**
 * @brief Defines the macro for inheriting the classes that implement the <b>IUnknown</b>
 * interface.
 * 概述：定義macro（宏）用來繼承實作IUnknown介面的類
 * When developing a subclass of a class that implements the <b>IUnknown</b> interface, you can use
 * this macro to inherit the structures of the <b>IUnknown</b> implementation class. \n
 * 當開發一個實作IUnknown介面的子類時，你可以使用這個macro來繼承IUnknown實作類的框架
 */
#define INHERIT_IUNKNOWNENTRY(T) \
    uint16 ver;                   \
    int16 ref;                   \
    T iUnknown

/**
 * @brief Defines the default marco for initializing the <b>IUnknown</b> interface.
 * 概述：定義默認的marco（宏）用來初始化IUnknown介面
 * When creating a subclass object of the <b>IUnknown</b> interface, you can use this macro to
 * initialize members of the <b>IUnknown</b> interface to their default values. \n
 * 當創建一個IUnknown介面的子類時，你可以使用這個macro（宏）來初始化IUnknown介面成員的默認值
 */
#define DEFAULT_IUNKNOWN_IMPL                  \
    .QueryInterface = IUNKNOWN_QueryInterface, \
    .AddRef = IUNKNOWN_AddRef,                 \
    .Release = IUNKNOWN_Release

/**
 * @brief Defines the macro for initializing the classes that implement the <b>IUnknown</b>
 * interface.
 * 概述：定義一個宏用于初始化實作IUnknown介面的類
 * When creating a subclass object of a class that implements the <b>IUnknown</b> interface, you
 * can use this macro to initialize members of the <b>IUnknown</b> implementation class to their
 * default values. \n
 * You need to add the initialization of the customized member variable. \n
 * 當創建一個類的用于實作IUnknown介面的子類物件時，你可以使用這個macro（宏）來初始化IUnknown實作類成員的默認值
 */
#define IUNKNOWN_ENTRY_BEGIN(version)   \
    .ver = (version),                   \
    .ref = 1,                           \
    .iUnknown = {                       \
        DEFAULT_IUNKNOWN_IMPL

/**
 * @brief IUnknown Defines the end macro for initializing the <b>IUnknown</b> implementation
 * object.
 * 概述：IUnknown定義一個結尾標記宏用于初始化IUnknown實作物件
 * This macro is used when a subclass object of the <b>IUnknown</b> implementation class is
 * initialized. \n
 * 這個宏在子類物件的IUnknown實作類被初始化時呼叫
 */
#define IUNKNOWN_ENTRY_END }

#define DEFAULT_IUNKNOWN_ENTRY_BEGIN IUNKNOWN_ENTRY_BEGIN(DEFAULT_VERSION)

#define DEFAULT_IUNKNOWN_ENTRY_END IUNKNOWN_ENTRY_END

/**
 * @brief Obtains the pointer of the <b>IUnknown</b> interface object from the subclass object T
 *  (generic macro) of the <b>IUnknown</b> implementation class.
 * 概述：從子類物件T中獲取IUnknown介面物件的指標
 * Use this macro when registering <b>IUnknown</b> interfaces with Samgr so that you can obtain
 * the interfaces from the subclass objects of different <b>IUnknown</b> implementation classes. \n
 * 當使用Samgr注冊IUnknown介面時使用這個宏，這樣你就能從子類物件的不同IUnknown實作類中獲取介面
 * @since 1.0
 * @version 1.0
 */
#define GET_IUNKNOWN(T) (IUnknown *)(&((T).iUnknown))

結構體部分

通過宏定義和結構體我們可以發現：IUnknownEntry 是對IUnknown的一個繼承，IUnknown介面是一個框架，在實作IUnknown介面類時需要在這個框架的基礎上展開，詳細代碼如下：

/**
 * @brief Defines the <b>IUnknown</b> class.
 * 概述：定義IUnknown類
 * You need to inherit this structure when developing a subclass of the <b>IUnknown</b> interface. \n
 * 當開發一個IUnknown子類介面時，你需要繼承這個框架
 */
struct IUnknown {
    /**
     * Queries the subclass object of the <b>IUnknown</b> interface of a specified version
     * (downcasting).
     * 查詢指定版本IUnknown介面的子類物件
     */
    int (*QueryInterface)(IUnknown *iUnknown, int version, void **target);
    /** Adds the reference count. 增加一個參考計數*/
    int (*AddRef)(IUnknown *iUnknown);
    /** Release the reference to an <b>IUnknown</b> interface. 釋放一個IUnknown介面參考*/
    int (*Release)(IUnknown *iUnknown);
};

/**
 * @brief Defines the <b>IUnknown</b> implementation .
 * 概述：定義一個IUnknown實作類入口
 * You need to inherit this structure when developing a subclass of the <b>IUnknown</b>
 * implementation class. \n
 * 你需要繼承這個類當開發一個IUnknown實作類的子類時
 * Each <b>IUnknown</b> interface must correspond to one or more <b>IUnknown</b> implementation
 * classes. \n
 * 每一個IUnknown介面必須對應一個或者更多的IUnknown實作類
 */
typedef struct IUnknownEntry {
    /** Version information of <b>IUnknown</b> interface. IUnknown介面的版本資訊 */
    uint16 ver;
    /** Reference count of <b>IUnknown</b> interface. IUnknown介面的參考計數 */
    int16 ref;
    /**
     * Implementation of <b>IUnknown</b> interface, which is related to the specific definition
     * implementation.
     * IUnknown介面的實作，與特定的實作定義相關
     */
    IUnknown iUnknown;
} IUnknownEntry;

函式宣告部分

函式宣告部分結合函式的定義一起講解，故此處先做省略，

iunknown.c

iunknown介面實作框架基于COM編程中的：QueryInterface、AddRefRelease，在此我們詳細分析這三處功能函式的代碼：

IUNKNOWN_QueryInterface

函式宣告部分及注釋：

/**
 * @brief Queries the <b>IUnknown</b> interfaces of a specified version (downcasting).
 * 概述：查詢指定版本的IUnknown介面
 * 
 * After obtaining the <b>IUnknown</b> interface object, the function caller uses
 * <b>QueryInterface</b> to convert the object to the required subclass type. \n
 * 在獲取了IUnknown介面物件之后，函式呼叫QueryInterface來轉變物件為需要的子型別
 * The system converts {@link DEFAULT_VERSION} into the subclass type required by the caller.
 * 系統將{@link DEFAULT_VERSION}轉換為呼叫者需要的子型別別
 * If the type conversion requirements cannot be met, you need to re-implement this function. \n
 * 如果不能滿足型別轉換要求，則需要重新實作該函式，
 * 
 * @param iUnknown Indicates the pointer to the <b>IUnknown</b> interface.
 * @param version Indicates the version of the <b>IUnknown</b> interface object to be converted.
 * @param target Indicates the <b>IUnknown</b> subclass type required by the caller. This is an
 * output parameter.
 * target代表呼叫者需要的IUnknown子型別別，這是一個輸出引數
 * 
 * @return Returns <b>EC_SUCCESS</b> if the conversion is successful; returns other error codes
 * if the conversion fails.
 *
 * @since 1.0
 * @version 1.0
 */
int IUNKNOWN_QueryInterface(IUnknown *iUnknown, int ver, void **target);

函式體部分及注釋：

//查詢指定版本的IUnknown介面
int IUNKNOWN_QueryInterface(IUnknown *iUnknown, int ver, void **target)
{
    if (iUnknown == NULL || target == NULL) {
        return EC_INVALID;
    }
    //先獲取一個物件，然后判斷IUnknown入口的型別是否正確
    IUnknownEntry *entry = GET_OBJECT(iUnknown, IUnknownEntry, iUnknown);
    if ((entry->ver & (uint16)ver) != ver) {
        return EC_INVALID;
    }

    //接著檢查型別資訊
    if (ver == OLD_VERSION &&
        entry->ver != OLD_VERSION &&
        (entry->ver & (uint16)DEFAULT_VERSION) != DEFAULT_VERSION) {
        return EC_INVALID;
    }

    //賦值呼叫者需要的IUnknown子型別別介面值
    *target = iUnknown;
    //將該IUnknown介面結構體中增加一個參考
    iUnknown->AddRef(iUnknown);
    return EC_SUCCESS;
}

IUNKNOWN_AddRef

函式宣告部分及注釋:

/**
 * @brief Increments the reference count in this <b>IUnknown</b> interface.
 * 概述：在IUnknown介面中增加參考計數
 * 
 * This function is called in <b>QueryInterface</b>. Do not call this function in the
 * <b>IUnknown<b> interface. \n
 * 這個函式在QueryInterface中呼叫，不要在IUnknown介面中呼叫這個函式
 * 
 * When the <b>QueryInterface</b> function is re-implemented, you need to call this function
 * in the new <b>QueryInterface</b>.
 * 當QueryInterface函式被重新實作時，你需要在新的QueryInterface中呼叫這個函式
 * 
 * The system does not provide a lock to protect functions. Therefore, you need to re-implement
 * functions if multiple developers are using them. \n
 * 系統不提供鎖保護功能，因此，如果多個開發人員正在使用這些函式，則需要重新實作它們
 * 
 * @param iUnknown Indicates the pointer to the <b>IUnknown<b> interface object.
 * 引數iUnknown：代表介面物件的iUnknown 的指標，
 * @return Indicates the number of objects that reference the <b>IUnknown<b> interface.
 * 回傳值：參考IUnknown介面的節點數
 * @since 1.0
 * @version 1.0
 */
int IUNKNOWN_AddRef(IUnknown *iUnknown);

函式體部分代碼及注釋：

int IUNKNOWN_AddRef(IUnknown *iUnknown)
{
    //檢查傳入介面地址是否為空
    if (iUnknown == NULL) {
        return EC_INVALID;
    }

    //定義一個IUnknown入口，將iUnknown指標向下轉為T成員變數地址
    IUnknownEntry *entry = GET_OBJECT(iUnknown, IUnknownEntry, iUnknown);
    //給iUnknown入口中的參考計數+1
    entry->ref++;
    //回傳實作后的iUnknown參考數
    return entry->ref;
}

IUNKNOWN_Release

函式宣告部分及注釋:

/**
 * @brief Releases a reference to an <b>IUnknown</b> interface that is no longer used.
 * 概述：釋放一個對不再使用的IUnknown介面的參考，
 * In the default implementation provided by the system, if the reference count is <b>0</b>,
 * the memory of the <b>IUnknown</b> interface object and implementation object is not released. \n
 * 在系統默認實作中，如果參考數為0，記憶體中IUnknown介面物件和實作物件不被釋放
 * 
 * If the memory of the <b>IUnknown</b> interface object and implementation object is dynamically
 * allocated, this function needs to be re-implemented. \n
 * 如果記憶體IUnknown介面物件和應用物件被動態分配，這個函式需要被重新實作
 * If the reference count is <b>0</b>, the memory of the <b>IUnknown</b> interface object and
 * implementation object is released. \n
 * 如果參考數為0，記憶體中的IUnknown介面物件與應用物件需要被釋放
 *
 * @param iUnknown Indicates the pointer to the <b>IUnknown<b> interface object.
 * @return Indicates the number of <b>IUnknown<b> interface objects that are referenced after the
 * current reference is released.
 *
 * @since 1.0
 * @version 1.0
 */
int IUNKNOWN_Release(IUnknown *iUnknown);

函式體部分代碼及注釋：

//釋放一個對不再使用的IUnknown介面的參考
int IUNKNOWN_Release(IUnknown *iUnknown)
{
    //判斷傳入iUnknown是否為空
    if (iUnknown == NULL) {
        return EC_INVALID;
    }

    //獲取物件后，再對IUnknown入口介面進行參考數的操作
    IUnknownEntry *entry = GET_OBJECT(iUnknown, IUnknownEntry, iUnknown);
    int ref = entry->ref - 1;
    if (ref < 0) {
        // The iUnknown is already freed, there is some exception.iUnknown已經被釋放，有一些例外;
    } else {
        if (ref == 0) {
            // Nobody reference to the iUnknown, should delete it.沒有人參考iUnknown，應該洗掉它，
            // But iUnknown may be global variable, so the default version don`t delete it.
            // 但是iUnknown可能是全域變數，所以默認版本不會洗掉它，
        } else {
            entry->ref = ref;
        }
    }
    return ref;
}

本篇小結

在細致地解讀了介面部分代碼之后，相信對開發服務或功能的對外介面的實質有了更清晰的認識，以及介面在服務于功能中所處的地位有了更深刻的理解，這有助于我們去進一步解讀系統服務開發框架，