The android_native_app_glue library is built on top of the native_activity.h interface. Therefore, readers are recommended to read the Creating a native activity with the native_activity.h interface recipe before going through this one.

The following steps create a simple Android NDK application based on the android_native_app_glue library:

This recipe demonstrates how the android_native_app_glue library is used to create a native activity.

The following steps should be followed to use the android_native_app_glue library:

In our example, we implement a simple function named handle_activity_lifecycle_events and point the android_app->onAppCmd function pointer to it. This function simply prints the cmd value and the user data passed along with the android_app data structure. cmd is defined in android_native_app_glue.h as an enum. For example, when the app starts, the cmd values are 10, 11, 0, 1, and 6, which correspond to APP_CMD_START, APP_CMD_RESUME, APP_CMD_INPUT_CHANGED, APP_CMD_INIT_WINDOW, and APP_CMD_GAINED_FOCUS respectively.

android_native_app_glue Library Internals: The source code of the android_native_app_glue library can be found under the sources/android/native_app_glue folder of Android NDK. It only consists of two files, namely android_native_app_glue.c and android_native_app_glue.h. Let's first describe the flow of the code and then discuss some important aspects in detail.

android_native_app_glue is built on top of the native_activity.h interface. As shown in the following code (extracted from sources/android/native_app_glue/android_native_app_glue.c). It implements the ANativeActivity_onCreate function, where it registers the callback functions and calls the android_app_create function. Note that the returned android_app instance is pointed by the instance field of the native activity, which can be passed to various callback functions:

void ANativeActivity_onCreate(ANativeActivity* activity,
        void* savedState, size_t savedStateSize) {
    LOGV("Creating: %p
", activity);
    activity->callbacks->onDestroy = onDestroy;
    activity->callbacks->onStart = onStart;
    activity->callbacks->onResume = onResume;
    … …
    activity->callbacks->onNativeWindowCreated = onNativeWindowCreated;
    activity->callbacks->onNativeWindowDestroyed = onNativeWindowDestroyed;
    activity->callbacks->onInputQueueCreated = onInputQueueCreated;
    activity->callbacks->onInputQueueDestroyed = onInputQueueDestroyed;
    activity->instance = android_app_create(activity, savedState, savedStateSize);
}

The android_app_create function (shown in the following code snippet) initializes an instance of the android_app data structure, which is defined in android_native_app_glue.h. This function creates a unidirectional pipe for inter-thread communication. After that, it spawns a new thread (let's call it background thread thereafter) to run the android_app_entry function with the initialized android_app data as the input argument. The main thread will wait for the background thread to start and then return:

static struct android_app* android_app_create(ANativeActivity* activity, void* savedState, size_t savedStateSize) {
    struct android_app* android_app = (struct android_app*)malloc(sizeof(struct android_app));
    memset(android_app, 0, sizeof(struct android_app));
    android_app->activity = activity;

    pthread_mutex_init(&android_app->mutex, NULL);
    pthread_cond_init(&android_app->cond, NULL);
  ……
    int msgpipe[2];
    if (pipe(msgpipe)) {
        LOGE("could not create pipe: %s", strerror(errno));
        return NULL;
    }
    android_app->msgread = msgpipe[0];
    android_app->msgwrite = msgpipe[1];

    pthread_attr_t attr; 
    pthread_attr_init(&attr);
    pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_DETACHED);
    pthread_create(&android_app->thread, &attr, android_app_entry, android_app);
    // Wait for thread to start.
    pthread_mutex_lock(&android_app->mutex);
    while (!android_app->running) {
        pthread_cond_wait(&android_app->cond, &android_app->mutex);
    }
    pthread_mutex_unlock(&android_app->mutex);
    return android_app;
}

The background thread starts with the android_app_entry function (as shown in the following code snippet), where a looper is created. Two event queues will be attached to the looper. The activity lifecycle events queue is attached to the android_app_entry function. When the activity's input queue is created, the input queue is attached (to the android_app_pre_exec_cmd function of android_native_app_glue.c). After attaching the activity lifecycle event queue, the background thread signals the main thread it is already running. It then calls a function named android_main with the android_app data. android_main is the function we need to implement, as shown in our sample code. It must run in a loop until the activity exits:

static void* android_app_entry(void* param) {
    struct android_app* android_app = (struct android_app*)param;
  … …
  //Attach life cycle event queue with identifier LOOPER_ID_MAIN
    android_app->cmdPollSource.id = LOOPER_ID_MAIN;
    android_app->cmdPollSource.app = android_app;
    android_app->cmdPollSource.process = process_cmd;
    android_app->inputPollSource.id = LOOPER_ID_INPUT;
    android_app->inputPollSource.app = android_app;
    android_app->inputPollSource.process = process_input;
    ALooper* looper = ALooper_prepare(ALOOPER_PREPARE_ALLOW_NON_CALLBACKS);
    ALooper_addFd(looper, android_app->msgread, LOOPER_ID_MAIN, ALOOPER_EVENT_INPUT, NULL, &android_app->cmdPollSource);
    android_app->looper = looper;

    pthread_mutex_lock(&android_app->mutex);
    android_app->running = 1;
    pthread_cond_broadcast(&android_app->cond);
    pthread_mutex_unlock(&android_app->mutex);
    android_main(android_app);
    android_app_destroy(android_app);
    return NULL;
}

The following diagram indicates how the main and background thread work together to create the multi-threaded native activity:

We use the activity lifecycle event queue as an example. The main thread invokes the callback functions, which simply writes to the write end of the pipe, while true loop implemented in the android_main function will poll for events. Once an event is detected, the function calls the event handler, which reads the exact command from the read end of the pipe and handles it. The android_native_app_glue library implements all the main thread stuff and part of the background thread stuff for us. We only need to supply the polling loop and the event handler as illustrated in our sample code.

Pipe: The main thread creates a unidirectional pipe in the android_app_create function by calling the pipe method. This method accepts an array of two integers. After the function is returned, the first integer will be set as the file descriptor referring to the read end of the pipe, while the second integer will be set as the file descriptor referring to the write end of the pipe.

A pipe is usually used for Inter-process Communication (IPC), but here it is used for communication between the main UI thread and the background thread created at android_app_entry. When an activity lifecycle event occurs, the main thread will execute the corresponding callback function registered at ANativeActivity_onCreate. The callback function simply writes a command to the write end of the pipe and then waits for a signal from the background thread. The background thread is supposed to poll for events continuously and once it detects a lifecycle event, it will read the exact event from the read end of the pipe, signal the main thread to unblock and handle the events. Because the signal is sent right after receiving the command and before actual processing of the events, the main thread can return from the callback function quickly without worrying about the possible long processing of the events.

Different operating systems have different implementations for the pipe. The pipe implemented by Android system is "half-duplex", where communication is unidirectional. That is, one file descriptor can only write, and the other file descriptor can only read. Pipes in some operating system is "full-duplex", where the two file descriptors can both read and write.

Looper is an event tracking facility, which allows us to attach one or more event queues for an event loop of a thread. Each event queue has an associated file descriptor. An event is data available on a file descriptor. In order to use a looper, we need to include the android/looper.h header file.

The library attaches two event queues for the event loop to be created by us in the background thread, including the activity lifecycle event queue and the input event queue. The following steps should be performed in order to use a looper: