Android GPS架构分析（二）

Android GPS架构分析

Daniel Wood 20101222

转载时请注明出处和作者

文章出处：http://danielwood.cublog.cn

作者：Daniel Wood

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initialize函数

LocationManagerService.java[frameworks\base\services\java\com\android\server]

private void initialize() {
        // Create a wake lock, needs to be done before calling loadProviders() below
        PowerManager powerManager = (PowerManager) mContext.getSystemService(Context.POWER_SERVICE);
        mWakeLock = powerManager.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, WAKELOCK_KEY);
        // Load providers
        loadProviders();

      ...


initialize函数中最重要的就是loadProviders函数了，该函数调用loadProvidersLocked，然后loadProvidersLocked函数又调用_loadProvidersLocked函数。为什么要这么折腾呢?

先来看一部分的_loadProvidersLocked函数：

private void _loadProvidersLocked() {
        // Attempt to load "real" providers first
        if (GpsLocationProvider.isSupported()) {
            // Create a gps location provider
            GpsLocationProvider gpsProvider = new GpsLocationProvider(mContext, this);
            mGpsStatusProvider = gpsProvider.getGpsStatusProvider();
            mNetInitiatedListener = gpsProvider.getNetInitiatedListener();
            addProvider(gpsProvider);
            mGpsLocationProvider = gpsProvider;
        }

        ...

注意这个if语句，狠重要，因为在这个语句中得到了HAL层的GPS接口GpsInterface。就是通过调用GpsLocationProvider的isSupported()函数才调用到gps.cpp[hardware/libhardware_legacy/gps]中的gps_get_interface()。这个isSupported函数才是第一个吃螃蟹的人。（而不是JNI层的init函数，这个下面会提到)。

GpsLocationProvider.cpp [frameworks\base\location\java\com\android\internal\location]

public static boolean isSupported() {
        return native_is_supported();
    }

然而isSupported只有一句话，果然是高手，一击必中。然后就调用native方法，也就是JNI层定义的方法。native_is_supported函数对于JNI层是android_location_GpsLocationProvider_is_supported方法。

android_location_GpsLocationProvider.cpp [frameworks\base\core\jni]

static jboolean android_location_GpsLocationProvider_is_supported(JNIEnv* env, jclass clazz) {
    if (!sGpsInterface)
        sGpsInterface = gps_get_interface();
    return (sGpsInterface != NULL);
}

前面已经提到JNI起到承上启下的作用，gps_get_interface函数属于HAL层的调用，在文件gps.cpp中。

gps.cpp [hardware\libhardware_legacy\gps]

const GpsInterface*
gps_get_interface()
{
    if (sGpsInterface == NULL)
         gps_find_hardware();
    return sGpsInterface;
}

然后通过gps_find_hardware函数去得到gps接口，下面只模拟器中的gpsinterface。

static void
gps_find_hardware( void )
{
#ifdef HAVE_QEMU_GPS_HARDWARE
    if (qemu_check()) {
        sGpsInterface = gps_get_qemu_interface();
        if (sGpsInterface) {
            LOGD("using QEMU GPS Hardware emulation\n");
            return;
        }
    }
#endif
#ifdef HAVE_GPS_HARDWARE
    sGpsInterface = gps_get_hardware_interface();
#endif
    if (!sGpsInterface)
        LOGD("no GPS hardware on this device\n");
}

gps_qemu.c [hardware\libhardware_legacy\gps]

const GpsInterface* gps_get_qemu_interface()
{
    return &qemuGpsInterface;
}

qemuGpsInterface的整体实现就在文件gps_qemu.c中。

static const GpsInterface qemuGpsInterface = {
    qemu_gps_init,
    qemu_gps_start,
    qemu_gps_stop,
    qemu_gps_cleanup,
    qemu_gps_inject_time,
    qemu_gps_inject_location,
    qemu_gps_delete_aiding_data,
    qemu_gps_set_position_mode,
    qemu_gps_get_extension,
};