从Objective-C转战C++ Android平台开发实践（C++/Java）

浏览数：55 / 时间：2015年06月11日

 

是否使用虚拟方法

  1. 最好在不用“virtual”关键字的情况下声明所有cpp成员方法
  2. 但是在写CPP头文件时，请检查有没有父类的方法被当前的工作覆盖。如果有，请确保将这些方法改为虚拟方法。
  3. 如果从父类继承了一个虚拟方法，确保这个方法可以继承“virtual”（虚拟）关键字

public/protected/private方法介绍

  1. 默认情况下，将所有成员方法声明为“public”（公共）
  2. 如果以下任意条件满足，方法必须为“private”:该方法在.m文件被声明;该方法位于“private”范畴

public/protected/private成员变量

  1. 声明所有成员变量为“protected”，没有其他选择。

两阶段构造

如何

  • 第一阶段：在构造器初始化列表中设置所有成员变量的默认值。但不要在构造器中编写任何逻辑init（初始化）。
  • 第二阶段：在“CMyClass* init(...)”函数中编写逻辑init（初始化）。如果初始化失败会返回NULL。

为什么

  • 我们决定在C++中不再使用捕获异常机制（try-catch exception mechanism）。这是为了减少足迹和二进制大小。因此在C++构造中发生的任何异常都不会被报告给调用者。

时间

  • 两阶段构造并不是在每个类中都一定要进行，只是在那些存在初始化逻辑步骤的类中进行。换言之，在构造器中编写逻辑初始化是禁止的，这种情况可能会返回错误。

调用者须知

  • 如何你调用的类有“bool init(...)”函数，请在构造之后立即调用该函数。
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#define CCX_BREAK_IF(cond) if(cond) break;
#define CCX_SAFE_DELETE(p)  if(p) {delete (p); (p) = NULL;}
 
// declaration
class CCar
{
public:
    CCar();
    bool    init();
    virtual ~CCar();
 
protected:
    CEngine* m_pEngine;
    bool     m_bStarted;
    bool     m_bLocked;
};
 
// 1st-phase construction
// only set the default value & alloc memory
CCar::CCar()
:m_pEngine(new CEngine)
,m_bStarted(false)
,m_bLocked(true)
{
    printf("CCar constructor\n");
}
 
// 2st-phase construction
// do logical initialization
bool CCar::init()
{
    bool bRet = false;
 
    do
    {
        m_bLocked = false;
 
        CCX_BREAK_IF( !m_pEngine );        // defensive
        CCX_BREAK_IF( !m_pEngine->start() );    // logic
 
        // success
        bRet = true;
 
    } while(0);
 
    printf("CCar init\n");
    return bRet;
}
 
// destruction
CCar::~CCar()
{
    if (m_pEngine)
    {
        delete m_pEngine;
        m_pEngine = NULL;
    }
 
    printf("CCar destructor\n");
}
 
// invoker
int _tmain(int argc, _TCHAR* argv[])
{
    // in heap
    CCar* pCar = new CCar;
    if (!pCar->init())
    {
        CCX_SAFE_DELETE(pCar);
    }
 
    // in stack
    CCar car;
    if (!car.init())
    {
        // do sth.
    }
 
    return 0;
}

下载样本代码请参见附件“TwoPhaseConstruction.zip”。该项目已经在Win32环境+VS2008测试过。

objc属性

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/** CCX_PROPERTY_READONLY is used to declare a protected variable.
    We can use get method to read the variable.
    @param varType : the type of variable.
    @param varName : variable name.
    @param funName : "get + funName" is the name of the get method.
    @warning : The get method is a public virtual function, you should override it first.
            The variables and methods declared after CCX_PROPERTY_READONLY are all public.
            If you need protected or private, please declare.
*/
#define CCX_PROPERTY_READONLY(varType, varName, funName)\
    protected: varType varName;\
    public: virtual varType get##funName(void);
 
/** CCX_PROPERTY is used to declare a protected variable.
    We can use get method to read the variable, and use the set method to change the variable.
    @param varType : the type of variable.
    @param varName : variable name.
    @param funName : "get + funName" is the name of the get method.
                     "set + funName" is the name of the set method.
    @warning : The get and set methods are public virtual functions, you should override them first.
            The variables and methods declared after CCX_PROPERTY are all public.
            If you need protected or private, please declare.
*/
#define CCX_PROPERTY(varType, varName, funName)\
    protected: varType varName;\
    public: virtual varType get##funName(void);\
    public: virtual void set##funName(varType var);
 
/** CCX_SYNTHESIZE_READONLY is used to declare a protected variable.
    We can use get method to read the variable.
    @param varType : the type of variable.
    @param varName : variable name.
    @param funName : "get + funName" is the name of the get method.
    @warning : The get method is a public inline function.
            The variables and methods declared after CCX_SYNTHESIZE_READONLY are all public.
            If you need protected or private, please declare.
*/
#define CCX_SYNTHESIZE_READONLY(varType, varName, funName)\
    protected: varType varName;\
    public: inline varType get##funName(void){ return varName; }
 
/** CCX_SYNTHESIZE is used to declare a protected variable.
    We can use get method to read the variable, and use the set method to change the variable.
    @param varType : the type of variable.
    @param varName : variable name.
    @param funName : "get + funName" is the name of the get method.
                     "set + funName" is the name of the set method.
    @warning : The get and set methods are public  inline functions.
            The variables and methods declared after CCX_SYNTHESIZE are all public.
            If you need protected or private, please declare.
*/
#define CCX_SYNTHESIZE(varType, varName, funName)\
    protected: varType varName;\
    public: inline varType get##funName(void){ return varName; }\
    public: inline void set##funName(varType var){ varName = var; }

id

Objc中一些函数会返回“ID“，在转换成CPP后就会返回“bool”。在Objc中，你可以像“[[MyClass alloc] init] autorelease]”一样编写代码，无需在意初始化是否失败返回NULL。在这种情况下“[nil autorelease]”不会使程序崩溃。但是在CPP中，返回“bool”是为了防止开发人员编写“pClass = (new MyClass())->init()->foo()”。如果初始化失败返回NULL，在CPP中“null->fool()”会崩溃然后跳出程序。另一方面，如果“foo()”返回值不是“MyClass*”，例如返回“bool”，那调用者就会失去“new MyClass”的指针，然后无法从堆栈（heap）中删除指针。这就会很危险。

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@interface CTest
-(id) foo();

以上代码必须转换为

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class CTest
{
    bool foo();
}

 

 

目的

在Cocos2dx场景中点击按钮，即可向本地平台Java弹出对话框发送信息。详见本文。

指令

你需要对项目执行几次include（包含）指令，本人已创建一个在线Repo库，根据开发环境种类分成了几个部分。请确保include（包含）了所有C++和Java的文件。在线Repo连接如下：EasyNDK。

从C++包含

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#include "NDKHelper.h"
 
// The button click method of Cocos2dx
void HelloWorld::menuCloseCallback(CCObject* pSender)
{
    // Register a selector in a global space
    // So that when our native environment will call the method with the string
    // It can respond to the selector
    // Note : Group name is there for ease of removing the selectors
    NDKHelper::AddSelector("HelloWorldSelectors",
                           "SampleSelector",
                           callfuncND_selector(HelloWorld::SampleSelector),
                           this);
 
    // Making parameters for message to be passed to native language
    // For the ease of use, i am sending the method to be called name from C++
    CCDictionary* prms = CCDictionary::create();
    prms->setObject(CCString::create("SampleSelector"), "to_be_called");
 
    // Finally call the native method in current environment
    SendMessageWithParams(string("SampleSelector"), prms);
}
 
// A selector that will respond to us, when native language will call it
void HelloWorld::SampleSelector(CCNode *sender, void *data)
{
    CCLog("Called from native environment");
}
 
// Destructor to remove all the selectors which are grouped as HelloWorldSelectors
HelloWorld::~HelloWorld()
{
    // Remove the associated selector group from the global space,
    // Because we are destroying this instance
    NDKHelper::RemoveSelectorsInGroup("HelloWorldSelectors");
}

从Java包含:

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/** Called when the activity is first created. */
public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
 
    // In the main Activity, assigning that activity as a receiver for C++ messages
    AndroidNDKHelper.SetNDKReciever(this);
}
 
// Implement the method to be called for a message from C++
// Be sure to name the method to be of the same string as you will pass from C++
// Like we passed "SampleSelector" from C++, that is why created this method
public void SampleSelector(JSONObject prms)
{
    Log.v("SampleSelector", "purchase something called");
    Log.v("SampleSelector", "Passed params are : " + prms.toString());
 
    String CPPFunctionToBeCalled = null;
    try
    {
        CPPFunctionToBeCalled = prms.getString("to_be_called");
    }
    catch (JSONException e)
    {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
 
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setMessage("This is a sample popup on Android").
    setTitle("Hello World!").
    setNeutralButton("OK", null).show();
 
    // Send C++ a message with paramerts
    // C++ will recieve this message, only if the selector list will have a method
    // with the string we are passing
    AndroidNDKHelper.SendMessageWithParameters(CPPFunctionToBeCalled, null);
}

注意 若连接其他SDK，你可以参考相关SDK的Java指南并分别实施消息传递机制从Cocos2d-x进行调用。本人通过这种方法已经实现了AppCircle、Flurry以及其他很多SDK。 拥有完整源码的样本项目可从网上下载：Sample Android Project。 祝编程愉快！

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