windows mobile 开发：让GPS一直在待机模式下也能运行

最近，遇到一个需求，就是每 30 秒更新一次 GPS 位置，在测试过程中，发现在系统待机后，更新 GPS 位置就不能正常运行了，搜索后，发现如下的解决方案，实际应用了之后，有效，赞！！！

http://stackoverflow.com/questions/873816/keep-windows-mobile-app-running-in-standby-mode

    public const int PPN_UNATTENDEDMODE = 0x0003;
    public const int POWER_NAME = 0x00000001;
    public const int POWER_FORCE = 0x00001000;

    [DllImport("coredll.dll")]
    public static extern bool PowerPolicyNotify(int dwMessage, bool dwData);

    [DllImport("coredll.dll", SetLastError = true)]
    public static extern IntPtr SetPowerRequirement(string pvDevice, CedevicePowerStateState deviceState, uint deviceFlags, string pvSystemState, ulong stateFlags);

    [DllImport("coredll.dll", SetLastError = true)]
    public static extern int ReleasePowerRequirement(IntPtr hPowerReq);

    public enum CedevicePowerStateState : int
    {
        PwrDeviceUnspecified = -1,
        D0 = 0,
        D1,
        D2,
        D3,
        D4,
    }

    // Keep the GPS and device alive:
    PowerPolicyNotify(PPN_UNATTENDEDMODE, true)
    IntPtr gpsPowerHandle = SetPowerRequirement("gpd0:", CedevicePowerStateState.D0, POWER_NAME | POWER_FORCE, null, 0);

    // Call before exiting your app:
    ReleasePowerRequirement(gpsPowerHandle);
    PowerPolicyNotify(PPN_UNATTENDEDMODE, false);