ndk/samples/sample_na.jd

page.title=Sample: native-activity
@jd:body

<div id="qv-wrapper">
    <div id="qv">
      <h2>On this page</h2>

      <ol>
        <li><a href="#am">AndroidManifest.xml</a></li>
        <li><a href="#anm">Android.mk</a></li>
        <li><a href="#apm">Application.mk</a></li>
        <li><a href="#mac">main.c</a></li>
          </ol>
        </li>
      </ol>
    </div>
  </div>

<p>The native-activity sample resides under the NDK installation root, in
{@code samples/native-activity}. It is a very simple example of a purely native
application, with no Java source code. In the absence of any Java source, the
Java compiler still creates an executable stub for the virtual machine to run.
The stub serves as a wrapper for the actual, native program, which is located in the {@code .so}
file.</p>

<p>The app itself simply renders a color onto the entire screen, and
then changes the color partly in response to movement that it detects.</p>

<h2 id="am">AndroidManifest.xml</h2>

<p>An app with only native code must not specify an Android API level lower than 9, which introduced
the <a href="{@docRoot}ndk/guides/concepts.html#naa">{@code NativeActivity}</a> framework class.</p>

<pre class="no-pretty-print">
&lt;uses-sdk android:minSdkVersion="9" /&gt;
</pre>

<p>The following line declares {@code android:hasCode} as {@code false}, as this app has only
native code&ndash;no Java.
</p>

<pre class="no-pretty-print">
&lt;application android:label="@string/app_name"
android:hasCode="false"&gt;
</pre>

<p>The next line declares the {@code NativeActivity} class.</p>

<pre class="no-pretty-print">
&lt;activity android:name="android.app.NativeActivity"
</pre>

<p>Finally, the manifest specifies {@code android:value} as the name of the shared library to be
built, minus the initial {@code lib} and the {@code .so} extension. This value must be the same as
the name of {@code LOCAL_MODULE} in {@code Android.mk}.</p>

<pre class="no-pretty-print">
&lt;meta-data android:name="android.app.lib_name"
        android:value="native-activity" /&gt;
</pre>

<h2 id="anm">Android.mk</h2>
<p>This file begins by providing the name of the shared library to generate.</p>

<pre class="no-pretty-print">
LOCAL_MODULE    := native-activity
</pre>

<p>Next, it declares the name of the native source-code file.</p>

<pre class="no-pretty-print">
LOCAL_SRC_FILES := main.c
</pre>

<p>Next, it lists the external libraries for the build system to use in building the binary. The
{@code -l} (link-against) option precedes each library name.</p>

<ul>
<li>{@code log} is a logging library.</li>
<li>{@code android} encompasses the standard Android support APIs for NDK. For more information about
the APIs that Android and the NDK support, see  <a href="stable_apis.html">Android NDK Native
APIs</a>.</li>
<li>{@code EGL} corresponds to the platform-specific portion of the graphics API.</li>
<li>{@code GLESv1_CM} corresponds to OpenGL ES, the version of OpenGL for Android. This library
depends on EGL.</li>
</ul>

<p>For each library:</p>

<ul>
<li>The actual file name starts with {@code lib}, and ends with the
{@code .so} extension. For example, the actual file name for the
{@code log} library is {@code liblog.so}.</li>
<li>The library resides in the following directory, NDK root:
{@code &lt;ndk&gt;/platforms/android-&lt;sdk_version&gt;/arch-&lt;abi&gt;/usr/lib/}.</li>
</ul>

<pre class="no-pretty-print">
LOCAL_LDLIBS    := -llog -landroid -lEGL -lGLESv1_CM
</pre>

<p>The next line provides the name of the static library, {@code android_native_app_glue}, which the
application uses to manage {@code NativeActivity} lifecycle events and touch input.</p>

<pre class="no-pretty-print">
LOCAL_STATIC_LIBRARIES := android_native_app_glue
</pre>

<p>The final line tells the build system to build this static library.
The {@code ndk-build} script places the built library
({@code libandroid_native_app_glue.a}) into the {@code obj} directory
generated during the build process. For more information about the {@code android_native_app_glue}
library, see its {@code android_native_app_glue.h} header and corresponding {@code .c}source file.
</p>


<pre class="no-pretty-print">
$(call import-module,android/native_app_glue)
</pre>

<p>For more information about the {@code Android.mk} file, see
<a href="{@docRoot}ndk/guides/android_mk.html">Android.mk</a>.</p>


<h2 id="apm">Application.mk</h2>

<p>This line defines the minimum level of Android API Level support.</p>

<pre class="no-pretty-print">
APP_PLATFORM := android-10
</pre>

<p>Because there is no ABI definition, the build system defaults to building only for
{@code armeabi}.</p>

<h2 id="mac">main.c</h2>
<p>This file essentially contains the entire progam.</p>

<p>The following includes correspond to the libraries, both shared and static,
enumerated in {@code Android.mk}.</p>

<pre class="no-pretty-print">
#include &lt;EGL/egl.h&gt;
#include &lt;GLES/gl.h&gt;


#include &lt;android/sensor.h&gt;
#include &lt;android/log.h&gt;
#include &lt;android_native_app_glue&gt;
</pre>

<p>The {@code android_native_app_glue} library calls the following function,
passing it a predefined state structure. It also serves as a wrapper that
simplifies handling of {@code NativeActivity} callbacks.</p>

<pre class="no-pretty-print">
void android_main(struct android_app* state) {
</pre>

<p>Next, the program handles events queued by the glue library. The event
handler follows the state structure.</p>

<pre class="no-pretty-print">
struct engine engine;


// Suppress link-time optimization that removes unreferenced code
// to make sure glue isn't stripped.
app_dummy();


memset(&amp;engine, 0, sizeof(engine));
state-&gt;userData = &amp;engine;
state-&gt;onAppCmd = engine_handle_cmd;
state-&gt;onInputEvent = engine_handle_input;
engine.app = state;
</pre>

<p>The application prepares to start monitoring the sensors, using the
APIs in {@code sensor.h}.</p>

<pre class="no-pretty-print">
    engine.sensorManager = ASensorManager_getInstance();
    engine.accelerometerSensor =
                    ASensorManager_getDefaultSensor(engine.sensorManager,
                        ASENSOR_TYPE_ACCELEROMETER);
    engine.sensorEventQueue =
                    ASensorManager_createEventQueue(engine.sensorManager,
                        state-&gt;looper, LOOPER_ID_USER, NULL, NULL);
</pre>

<p>Next, a loop begins, in which the application polls the system for
messages (sensor events). It sends messages to
{@code android_native_app_glue}, which checks to see whether they match
any {@code onAppCmd} events defined in {@code android_main}. When a
match occurs, the message is sent to the handler for execution.</p>

<pre class="no-pretty-print">
while (1) {
        // Read all pending events.
        int ident;
        int events;
        struct android_poll_source* source;


        // If not animating, we will block forever waiting for events.
        // If animating, we loop until all events are read, then continue
        // to draw the next frame of animation.
        while ((ident=ALooper_pollAll(engine.animating ? 0 : -1, NULL,
                &amp;events,
                (void**)&amp;source)) &gt;= 0) {


            // Process this event.
            if (source != NULL) {
                source-&gt;process(state, source);
            }


            // If a sensor has data, process it now.
            if (ident == LOOPER_ID_USER) {
                if (engine.accelerometerSensor != NULL) {
                    ASensorEvent event;
                    while (ASensorEventQueue_getEvents(engine.sensorEventQueue,
                            &amp;event, 1) &gt; 0) {
                        LOGI("accelerometer: x=%f y=%f z=%f",
                                event.acceleration.x, event.acceleration.y,
                                event.acceleration.z);
                    }
                }
            }


        // Check if we are exiting.
        if (state-&gt;destroyRequested != 0) {
            engine_term_display(&amp;engine);
            return;
        }
    }
</pre>

<p>Once the queue is empty, and the program exits the polling loop, the
program calls OpenGL to draw the screen.</p>
<pre class="no-pretty-print">
    if (engine.animating) {
        // Done with events; draw next animation frame.
        engine.state.angle += .01f;
        if (engine.state.angle &gt; 1) {
            engine.state.angle = 0;
        }


        // Drawing is throttled to the screen update rate, so there
        // is no need to do timing here.
        engine_draw_frame(&amp;engine);
    }
}
</pre>