Question

I am wondering when exactly the NFC Service is started and stopped. The source code for android 4.0.3 seems to state that the polling is dependent on a single constant (located in NfcService.java)

/** minimum screen state that enables NFC polling (discovery) */
static final int POLLING_MODE = SCREEN_STATE_ON_UNLOCKED;

I would interpret this as "the screen light is on, therefore the nfc service is active". BUT when the screen is locked, a NFC Tag wont be recognized, altough the screen is lit.

所以,我很好奇:当锁定屏出现时,NFC服务是否已经停止使用,或者它仍在运行,但没有处理标记?

Answer 1

实际上,我不认为NFC Service已被禁用。当屏幕的值较低时, SCREEN_STATE_ON_UNLOCKED 设备会停止询问 NFC 标签。您可以从此代码中看到 :

    // configure NFC-C polling
    if (mScreenState >= POLLING_MODE) {
        if (force || !mNfcPollingEnabled) {
            Log.d(TAG, "NFC-C ON");
            mNfcPollingEnabled = true;
            mDeviceHost.enableDiscovery();
        }
    } else {
        if (force || mNfcPollingEnabled) {
            Log.d(TAG, "NFC-C OFF");
            mNfcPollingEnabled = false;
            mDeviceHost.disableDiscovery();
        }
    }

NFC-EE路由被启用的电磁屏状态高于 SCREEN_STATE_ON_Locked :

    // configure NFC-EE routing
    if (mScreenState >= SCREEN_STATE_ON_LOCKED &&
            mEeRoutingState == ROUTE_ON_WHEN_SCREEN_ON) {
        if (force || !mNfceeRouteEnabled) {
            Log.d(TAG, "NFC-EE ON");
            mNfceeRouteEnabled = true;
            mDeviceHost.doSelectSecureElement();
        }
    } else {
        if (force ||  mNfceeRouteEnabled) {
            Log.d(TAG, "NFC-EE OFF");
            mNfceeRouteEnabled = false;
            mDeviceHost.doDeselectSecureElement();
        }
    }

此服务本身在本类其他部分启动并停止。

Answer 2

见相关http://forum.xda-developers.com/showthread.php?t=1712024&page=14

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