1 Native Activities and Applications: 2 === 3 4 I. Overview 5 =========== 6 The Android SDK provides a helper class, `NativeActivity`, that allows you to write a completely 7 native activity. With a native activity, it is possible to write a completely native application. 8 NativeActivity handles the communication between the Android framework and your 9 native code, so you do not have to subclass it or call its methods. All you need to do is declare 10 your application to be native in your `AndroidManifest.xml` file and begin creating your native 11 application. 12 13 Native activities do not change the fact that Android applications still run in their own virtual 14 machine, sandboxed from other applications. Because of this, you can still access Android framework 15 APIs through the JNI. There are, however, native interfaces to access things such as sensors, input 16 events, and assets that you can use. For more information about what is supported, see the 17 docs/STABLE-APIS.HTML. 18 19 If 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 21 Android build tools, so the build system can only build Android projects that have the correct 22 structure. Using the android tool or Eclipse helps ensure that. 23 24 The 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 44 II. Using the native-activity.h interface: 45 ========================================== 46 47 You can use the native-activity.h interface to implement a completely native activity. If you use 48 this interface you must ensure that your callback implementations do not block the main UI thread. 49 For more information on how to use this interface, see 50 `<ndk_root>/platforms/android-9/arch-arm/usr/include/android/native_activity.h`. 51 52 You might find it easier to use the native_app_glue static helper library that handles the 53 callbacks in an event loop in another thread. See the native-activity sample application for more 54 information on how to use this static library. 55 56 To 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
