    Native Activities and Applications:
    -----------------------------------

    I. Overview
    ===========
    The Android SDK provides a helper class, NativeActivity, that allows you to write a completely
    native activity. With a native activity, it is possible to write a completely native application.
    NativeActivity handles the communication between the Android framework and your
    native code, so you do not have to subclass it or call its methods. All you need to do is declare
    your application to be native in your AndroidManifest.xml file and begin creating your native
    application.

    Native activities do not change the fact that Android applications still run in their own virtual
    machine, sandboxed from other applications. Because of this, you can still access Android framework
    APIs through the JNI. There are, however, native interfaces to access things such as sensors, input
    events, and assets that you can use. For more information about what is supported, see the
    <ndk_root>/docs/STABLE-APIS.HTML.

    If you are developing a native activity, you should still create your projects with Eclipse or the
    "android create project" command. You still build and package native applications with the usual
    Android build tools, so the build system can only build Android projects that have the correct
    structure. Using the android tool or Eclipse helps ensure that.

    The Android NDK provides you with two choices to implement your native activity:

      - The native_activity.h header defines the native version of the NativeActivity class. It
        contains the callback interface and data structures that you need to create your native
        activity. Because the main thread of your application handles the callbacks, your callback
        implementations must not be blocking. If they block, you might receive ANR (Application Not
        Responding) errors because your main thread will be unresponsive until the callback returns.
        Read the comments in the
        <ndk_root>/platforms/android-9/arch-arm/usr/include/android/native_activity.h file for
        more information.

      - The android_native_app_glue.h file defines a static helper library built on top of the
        native_activity.h interface. It spawns another thread to handle things such as callbacks or
        input events. This prevents any callbacks from blocking your main thread and adds some
        flexibility in how you implement the callbacks, so you might find this programming model a bit
        easier to implement. The <ndk_root>/sources/android/native_app_glue/android_native_app_glue.c
        source is also available to you, so you can modify the implementation if you need. Read the
        comments in the <ndk_root>/sources/android/native_app_glue/android_native_app_glue.h file
        for more information.

    II. Using the native-activity.h interface:
    ==========================================
    You can use the native-activity.h interface to implement a completely native activity. If you use
    this interface you must ensure that your callback implementations do not block the main UI thread.
    For more information on how to use this interface, see
    <ndk_root>/platforms/android-9/arch-arm/usr/include/android/native_activity.h.

    You might find it easier to use the native_app_glue static helper library that handles the
    callbacks in an event loop in another thread. See the native-activity sample application for more
    information on how to use this static library.

    To implement a native activity with the native-activity.h interface:

      1/ Create a project with the "android create project" command or from Eclipse. Create a jni/
         directory in the project's root directory. This directory stores all of your native code.

      2/ Declare your native activity in the AndroidManifest.xml file. An example is shown below:

          <manifest xmlns:android="http://schemas.android.com/apk/res/android"
            package="com.example.native_activity"
            android:versionCode="1"
            android:versionName="1.0">

            <uses-sdk android:minSdkVersion="9" />

            <application android:label="@string/app_name" android:hasCode="false">

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

              <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>

        The main things to note are:

          - The android:name attribute of the activity tag must be set to android.app.NativeActivity.
            It is possible to subclass the NativeActivity, however, so if you do, specify the name of
            that class instead.
          - The android:name attribute of the meta-data tag must be in the form of android.app.lib_name
            where lib_name is the name of the module without the lib prefix and .so suffix.

      3/ Create a file for your native activity and implement the ANativeActivity_onCreate() function,
         which is called when your native activity starts. This function receives a pointer to an
         ANativeActivity structure, which contains function pointers to the various callback
         implementations that you need to write. Set the applicable callback function pointers in
         ANativeActivity->callbacks to the implementations of your callbacks.

      4/ Set the ANativeActivity->instance field to the address of any instance specific data that
         you want to use.

      5/ Implement any other things that you want your activity to do upon starting.

      6/ Implement the rest of the callbacks that you set in ANativeActivity->callbacks. For more
         information on when the callbacks are called, see the SDK documentation for Activity
         Lifecycles. Remember that your callback implementations must not be blocking, or you might get
         ANR (Application Not Responding) errors because the main UI thread is waiting for the callbacks
         to return.

      7/ Develop the rest of your application.

      8/ Create an Android.mk file in the jni/ directory of your project to describe your native module
         to the build system. An Android.mk file is essentially a snippet of a GNU Make file. For
         example:

           LOCAL_PATH := $(call my-dir)
           include $(CLEAR_VARS)
           LOCAL_MODULE    := my_native_module
           LOCAL_SRC_FILES := my_native_code.c
           include $(BUILD_SHARED_LIBRARY)

         For more information on how to create an Android.mk file and what the variables mean,
         see the <ndk_root>/docs/ANDROID-MK.TXT file.

      9/ Once you have an Android.mk file, compile your native code using the "ndk-build" command.

           cd path/to/project
           <ndk_root>/ndk-build

     10/ Build and install your Android project as usual, using Ant or Eclipse. The build automatically
         packages your native code into the .apk file if it is present in the jni/ directory.