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1Native Activities and Applications:
2===
3
4I. Overview
5===========
6The Android SDK provides a helper class, `NativeActivity`, that allows you to write a completely
7native activity. With a native activity, it is possible to write a completely native application.
8NativeActivity handles the communication between the Android framework and your
9native code, so you do not have to subclass it or call its methods. All you need to do is declare
10your application to be native in your `AndroidManifest.xml` file and begin creating your native
11application.
12
13Native activities do not change the fact that Android applications still run in their own virtual
14machine, sandboxed from other applications. Because of this, you can still access Android framework
15APIs through the JNI. There are, however, native interfaces to access things such as sensors, input
16events, and assets that you can use. For more information about what is supported, see the
17docs/STABLE-APIS.HTML.
18
19If you are developing a native activity, you should still create your projects with Eclipse or the
20"android create project" command. You still build and package native applications with the usual
21Android build tools, so the build system can only build Android projects that have the correct
22structure. Using the android tool or Eclipse helps ensure that.
23
24The Android NDK provides you with two choices to implement your native activity:
25
26  - The native_activity.h header defines the native version of the NativeActivity class. It
27    contains the callback interface and data structures that you need to create your native
28    activity. Because the main thread of your application handles the callbacks, your callback
29    implementations must not be blocking. If they block, you might receive ANR (Application Not
30    Responding) errors because your main thread will be unresponsive until the callback returns.
31    Read the comments in the
32    `<ndk_root>/platforms/android-9/arch-arm/usr/include/android/native_activity.h` file for
33    more information.
34
35  - The android_native_app_glue.h file defines a static helper library built on top of the
36    native_activity.h interface. It spawns another thread to handle things such as callbacks or
37    input events. This prevents any callbacks from blocking your main thread and adds some
38    flexibility in how you implement the callbacks, so you might find this programming model a bit
39    easier to implement. The `<ndk_root>/sources/android/native_app_glue/android_native_app_glue.c`
40    source is also available to you, so you can modify the implementation if you need. Read the
41    comments in the `<ndk_root>/sources/android/native_app_glue/android_native_app_glue.h` file
42    for more information.
43
44II. Using the native-activity.h interface:
45==========================================
46
47You can use the native-activity.h interface to implement a completely native activity. If you use
48this interface you must ensure that your callback implementations do not block the main UI thread.
49For more information on how to use this interface, see
50`<ndk_root>/platforms/android-9/arch-arm/usr/include/android/native_activity.h`.
51
52You might find it easier to use the native_app_glue static helper library that handles the
53callbacks in an event loop in another thread. See the native-activity sample application for more
54information on how to use this static library.
55
56To implement a native activity with the native-activity.h interface:
57
58  1. Create a project with the "android create project" command or from Eclipse. Create a jni/
59     directory in the project's root directory. This directory stores all of your native code.
60
61  2. Declare your native activity in the AndroidManifest.xml file. An example is shown below:
62
63          <manifest xmlns:android="http://schemas.android.com/apk/res/android"
64            package="com.example.native_activity"
65            android:versionCode="1"
66            android:versionName="1.0">
67
68            <uses-sdk android:minSdkVersion="9" />
69
70            <application android:label="@string/app_name" android:hasCode="false">
71
72              <activity android:name="android.app.NativeActivity"
73                android:label="@string/app_name"
74                android:configChanges="orientation|keyboardHidden">
75
76              <meta-data android:name="android.app.lib_name"
77                android:value="native-activity" />
78                <intent-filter>
79                  <action android:name="android.intent.action.MAIN" />
80                  <category android:name="android.intent.category.LAUNCHER" />
81                </intent-filter>
82              </activity>
83            </application>
84          </manifest>
85
86    The main things to note are:
87
88      - The android:name attribute of the activity tag must be set to android.app.NativeActivity.
89        It is possible to subclass the NativeActivity, however, so if you do, specify the name of
90        that class instead.
91      - The android:name attribute of the meta-data tag must be in the form of android.app.lib_name
92        where lib_name is the name of the module without the lib prefix and .so suffix.
93
94  3. Create a file for your native activity and implement the ANativeActivity_onCreate() function,
95     which is called when your native activity starts. This function receives a pointer to an
96     ANativeActivity structure, which contains function pointers to the various callback
97     implementations that you need to write. Set the applicable callback function pointers in
98     ANativeActivity->callbacks to the implementations of your callbacks.
99
100  4. Set the ANativeActivity->instance field to the address of any instance specific data that
101     you want to use.
102
103  5. Implement any other things that you want your activity to do upon starting.
104
105  6. Implement the rest of the callbacks that you set in ANativeActivity->callbacks. For more
106     information on when the callbacks are called, see the SDK documentation for Activity
107     Lifecycles. Remember that your callback implementations must not be blocking, or you might get
108     ANR (Application Not Responding) errors because the main UI thread is waiting for the callbacks
109     to return.
110
111  7. Develop the rest of your application.
112
113  8. Create an Android.mk file in the jni/ directory of your project to describe your native module
114     to the build system. An Android.mk file is essentially a snippet of a GNU Make file. For
115     example:
116
117          LOCAL_PATH := $(call my-dir)
118          include $(CLEAR_VARS)
119          LOCAL_MODULE    := my_native_module
120          LOCAL_SRC_FILES := my_native_code.c
121          include $(BUILD_SHARED_LIBRARY)
122
123     For more information on how to create an Android.mk file and what the variables mean,
124     see the <ndk_root>/docs/ANDROID-MK.TXT file.
125
126  9. Once you have an Android.mk file, compile your native code using the "ndk-build" command.
127
128          cd path/to/project
129          <ndk_root>/ndk-build
130
131 10. Build and install your Android project as usual, using Ant or Eclipse. The build automatically
132     packages your native code into the .apk file if it is present in the jni/ directory.
133