Android NDK纯C++开发(2)

继续了解使用native_app_glue来编写纯C++的Android NDK开发。

下面从一个"最简单“的可运行的程序来了解native_app_glue程序的基本组成。

1. 源码main.cpp:

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1. // main.cpp 
2. #include <android_native_app_glue.h> 
3.  
4.  
5. /**
6. * This is the main entry point of a native application that is using
7. * android_native_app_glue.  It runs in its own thread, with its own
8. * event loop for receiving input events and doing other things.
9. */ 
10. void android_main(struct android_app* state) { 
11. // Make sure glue isn't stripped. 
12. app_dummy(); 
13. } 

// main.cpp#include <android_native_app_glue.h>/** * This is the main entry point of a native application that is using * android_native_app_glue.  It runs in its own thread, with its own * event loop for receiving input events and doing other things. */void android_main(struct android_app* state) {// Make sure glue isn't stripped.app_dummy();}

首先，需要包含头文件android_native_app_glue.h，然后，必须实现一个android_main()函数，否则是无法运行的(编译可能没有问题)。另外，在android_main里面必须至少调用app_dummy()，这个函数的作用是为了保证glue没有被优化掉，参考android_native_app_glue.h中的说明:

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1. /**
2. * Dummy function you can call to ensure glue code isn't stripped.
3. */ 
4. void app_dummy(); 

/** * Dummy function you can call to ensure glue code isn't stripped. */void app_dummy();

2. Android.mk

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1. # Android.mk 
2. LOCAL_PATH := $(call my-dir) 
3.  
4. include $(CLEAR_VARS) 
5.  
6. LOCAL_MODULE    := test 
7. LOCAL_SRC_FILES := main.cpp 
8. LOCAL_LDLIBS    := -landroid 
9. LOCAL_STATIC_LIBRARIES := android_native_app_glue 
10.  
11. include $(BUILD_SHARED_LIBRARY) 
12.  
13. $(call import-module,android/native_app_glue) 

# Android.mkLOCAL_PATH := $(call my-dir)include $(CLEAR_VARS)LOCAL_MODULE    := testLOCAL_SRC_FILES := main.cppLOCAL_LDLIBS    := -landroidLOCAL_STATIC_LIBRARIES := android_native_app_glueinclude $(BUILD_SHARED_LIBRARY)$(call import-module,android/native_app_glue)

这里可见和一般的NDK程序的Android.mk的区别在于:

(1) 需要增加LOCAL_LDLIBS=-landroid

(2) 增加LOCAL_STATIC_LIBRARIES=android_native_app_glue

(3) 增加$(call import-module,android/native_app_glue)

这就是和一般的NDK程序的makefile的区别，另外，这里的LOCAL_MODULE=test，下面会用到。

3. AndroidManifest.xml

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1. <?xml version="1.0" encoding="utf-8"?> 
2. <!-- BEGIN_INCLUDE(manifest) --> 
3. <manifest xmlns:android="http://schemas.android.com/apk/res/android" 
4.         package="com.example.native_activity" 
5.         android:versionCode="1" 
6.         android:versionName="1.0"> 
7.  
8.     <!-- This is the platform API where NativeActivity was introduced. --> 
9.     <uses-sdk android:minSdkVersion="14" /> 
10.  
11.     <!-- This .apk has no Java code itself, so set hasCode tofalse. --> 
12.     <application android:label="@string/app_name" android:hasCode="false"> 
13.  
14.         <!-- Our activity is the built-in NativeActivity framework class. 
15.              This will take care of integrating with our NDK code. --> 
16.         <activity android:name="android.app.NativeActivity" 
17.                 android:label="@string/app_name" 
18.                 android:configChanges="orientation|keyboardHidden"> 
19.             <!-- Tell NativeActivity the name of or .so --> 
20.             <meta-data android:name="android.app.lib_name" 
21.                     android:value="test" /> 
22.             <intent-filter> 
23.                 <action android:name="android.intent.action.MAIN" /> 
24.                 <category android:name="android.intent.category.LAUNCHER" /> 
25.             </intent-filter> 
26.         </activity> 
27.     </application> 
28.  
29. </manifest>  
30. <!-- END_INCLUDE(manifest) --> 

<?xml version="1.0" encoding="utf-8"?><!-- BEGIN_INCLUDE(manifest) --><manifest xmlns:android="http://schemas.android.com/apk/res/android"        package="com.example.native_activity"        android:versionCode="1"        android:versionName="1.0">    <!-- This is the platform API where NativeActivity was introduced. -->    <uses-sdk android:minSdkVersion="14" />    <!-- This .apk has no Java code itself, so set hasCode to false. -->    <application android:label="@string/app_name" android:hasCode="false">        <!-- Our activity is the built-in NativeActivity framework class.             This will take care of integrating with our NDK code. -->        <activity android:name="android.app.NativeActivity"                android:label="@string/app_name"                android:configChanges="orientation|keyboardHidden">            <!-- Tell NativeActivity the name of or .so -->            <meta-data android:name="android.app.lib_name"                    android:value="test" />            <intent-filter>                <action android:name="android.intent.action.MAIN" />                <category android:name="android.intent.category.LAUNCHER" />            </intent-filter>        </activity>    </application></manifest> <!-- END_INCLUDE(manifest) -->

基本上和一般的AndroidManifest文件也差不多，需要注意的是：

(1) Activity的名称必须为NativeActivity

一般的程序Activity就是自己新建的Activity的名称，这里必须为NativeActivity

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1. <activity android:name="android.app.NativeActivity" 
2.                 android:label="@string/app_name" 
3.                 android:configChanges="orientation|keyboardHidden"> 

<activity android:name="android.app.NativeActivity"                android:label="@string/app_name"                android:configChanges="orientation|keyboardHidden">

(2) 增加对JNI中库的引用

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1. <!-- Tell NativeActivity the name of or .so --> 
2.            <meta-data android:name="android.app.lib_name" 
3.                    android:value="test" /> 

 <!-- Tell NativeActivity the name of or .so -->            <meta-data android:name="android.app.lib_name"                    android:value="test" />

一般的程序不需要这一部分，但是对于纯C++开发，需要指定其meta data，这里的test就是上面的Android.mk中的module的名称，要保持一致。

总结：上面的程序是一个”最简单“的纯C++的Android应用的例子，编译之后就可以运行了，如果上面的某一个地方错误，就会导致无法编译或运行。其中重点要理解的是android_main和android_dummy函数，另外，上面的程序尽管可以编译运行，但是并不具有实际意义，因为它不能响应任何事件，会导致无法响应。下面讨论更一般的情况，了解一般的程序的组成。

重点理解：

android_main和android_dummy的作用

纯C++程序的基本组成