Android 1.0的时代，没有提供对于C/C++开发本地Android代码的支持，尽管Android系统本身使用了大量的C++做底层开发。
Android第一次支持本地开发是在Android 1.5版本，对应Android API level 3。这一版本，有了正式的Android NDK的支持，可以通过jni写so库的方式，供Android应用来调用。
Android 1.6增加了对于OpenGL ES 1.x的支持 Android 2.0开始支持OpenGL ES 2.0
Android 2.3是一个重要的版本，这一版本增加了完全用C++写本地应用的接口。
我们看看这一版提供了什么API头文件： * native_activity.h * looper.h * input.h * keycodes.h * sensor.h * rect.h * window.h * native_window.h * native_window_jni.h * configuration.h * asset_manager.h * storage_manager.h * obb.h
直至今天，Android 7.0，API level 24的时代，上面这些仍然构成了我们这系列教程的主要内容。
同时，Android 2.3还开始支持EGL接口。
Android 4.0开始支持OpenMAX AL库。
Android 4.3开始支持Open GL ES 3.0版
Android 6.0开始支持trace库
Android 7.0开始支持Vulkan, Camera, Choreographer和Multinework库，同时对Open GL ES 3.2的支持
首先，写native应用需要一个本地应用的框架的支持。 java应用需要写一个manifest.xml，我们也入乡随俗需要写一个，我们先看一个NDK sample的例子：
<?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 .apk has no Java code itself, so set hasCode to false. --> <application android:allowBackup="false" android:fullBackupContent="false" android:icon="@mipmap/ic_launcher" 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 our .so --> <meta-data android:name="android.app.lib_name" android:value="native-activity" /> <intent-filter> <action android:name="android.intent.action.MAIN" /> <category android:name="android.intent.category.LAUNCHER" /> </intent-filter> </activity> </application></manifest>因为我们没有写Java代码，所以需要将android:hasCode值设为false.
可以使用NDK中的android_native_app_glue定义的类来对本地API进行封装，我们来看一下，先认识一下后面我们会介绍的几个类：
struct android_app { // The application can place a pointer to its own state object // here if it likes. void* userData; // Fill this in with the function to PRocess main app commands (APP_CMD_*) void (*onAppCmd)(struct android_app* app, int32_t cmd); // Fill this in with the function to process input events. At this point // the event has already been pre-dispatched, and it will be finished upon // return. Return 1 if you have handled the event, 0 for any default // dispatching. int32_t (*onInputEvent)(struct android_app* app, AInputEvent* event); // The ANativeActivity object instance that this app is running in. ANativeActivity* activity; // The current configuration the app is running in. AConfiguration* config; // This is the last instance's saved state, as provided at creation time. // It is NULL if there was no state. You can use this as you need; the // memory will remain around until you call android_app_exec_cmd() for // APP_CMD_RESUME, at which point it will be freed and savedState set to NULL. // These variables should only be changed when processing a APP_CMD_SAVE_STATE, // at which point they will be initialized to NULL and you can malloc your // state and place the information here. In that case the memory will be // freed for you later. void* savedState; size_t savedStateSize; // The ALooper associated with the app's thread. ALooper* looper; // When non-NULL, this is the input queue from which the app will // receive user input events. AInputQueue* inputQueue; // When non-NULL, this is the window surface that the app can draw in. ANativeWindow* window; // Current content rectangle of the window; this is the area where the // window's content should be placed to be seen by the user. ARect contentRect; // Current state of the app's activity. May be either APP_CMD_START, // APP_CMD_RESUME, APP_CMD_PAUSE, or APP_CMD_STOP; see below. int activityState; // This is non-zero when the application's NativeActivity is being // destroyed and waiting for the app thread to complete. int destroyRequested; // ------------------------------------------------- // Below are "private" implementation of the glue code. pthread_mutex_t mutex; pthread_cond_t cond; int msgread; int msgwrite; pthread_t thread; struct android_poll_source cmdPollSource; struct android_poll_source inputPollSource; int running; int stateSaved; int destroyed; int redrawNeeded; AInputQueue* pendingInputQueue; ANativeWindow* pendingWindow; ARect pendingContentRect;};我们看下这个结构中都用到了什么： * ANativeActivity: 它是本地应用中对应于android.app.NativeActivity的代理类，定义于android/native_activity.h * AConfiguration: 处理配置项。定义于android/configuration.h * ALooper: 对应于Java中的Looper,用于处理消息队列，每个线程只能有一个. 定义于android/looper.h * AInputQueue: 输入事件的队列，定义于android/input.h中 * ANativeWindow: 处理窗口的类，定义于android/native_window.h中 * ARect：代表一个矩形，定义于android/rect.h中
native应用比起Java应用来，跟Android版本的相关性更高一些。 所以，这些API都是根据平台版本号分成不同的目录的。 下面介绍的android下面的API，对应于Android源码中的frameworks/native/include/中。 在NDK中，以r13b为例，Android 7.0，也就是API 24的头文件位于：android-ndk/r13b/platforms/android-24/arch-arm64/usr/include/中。 我们首先通过一张图来看看Android为我们提供了哪些API:
主要包括下面的头文件： * android/native_activity.h * ANativeActivity结构：对应android.app.NativeActivity * ANativeActivityCallback结构，处理回调 * android/native_window.h * ANativeWindow结构 * ANativeWindow_Buffer结构 * android/rect.h * ARect结构：有left, top, right, bottom四个属性的矩阵的抽象 * android/native_window_jni.h: 辅助类 * android/window.h: 辅助类
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