Native Activities and Applications:
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I. Overview
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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.