Android N 插拔USB行为分析

        在Anrdoid L的版本上，如果用户USB连接PC设备后，选择MTP模式，然后移除连接，再次重新连接之后，设备还是以MTP模式连接的。而在Android N上，如果用户按照相同的操作，设备却是以充电模式连接的，这是为什么呢？如果更改才能回到以前的逻辑呢？

    private static final String USB_STATE_MATCH = "DEVPATH=/devices/virtual/android_usb/android0";
    private final UEventObserver mUEventObserver = new UEventObserver() {
        @Override
        public void onUEvent(UEventObserver.UEvent event) {
            if (DEBUG) Slog.v(TAG, "USB UEVENT: " + event.toString());

            String state = event.get("USB_STATE");
            String accessory = event.get("ACCESSORY");
            if (state != null) {
                mHandler.updateState(state);
            } else if ("START".equals(accessory)) {
                if (DEBUG) Slog.d(TAG, "got accessory start");
                startAccessoryMode();
            }
        }
    };

   mUEventObserver.startObserving(USB_STATE_MATCH);

         首先UsbDeviceManager中注册了UEvent接收内核中USB拔插的消息，收到消息后会调用mHandler.updateState(state);

        public void updateState(String state) {
            int connected, configured;

            if ("DISCONNECTED".equals(state)) {
                connected = 0;
                configured = 0;
            } else if ("CONNECTED".equals(state)) {
                connected = 1;
                configured = 0;
            } else if ("CONFIGURED".equals(state)) {
                connected = 1;
                configured = 1;
            } else {
                Slog.e(TAG, "unknown state " + state);
                return;
            }
            removeMessages(MSG_UPDATE_STATE);
            Message msg = Message.obtain(this, MSG_UPDATE_STATE);
            msg.arg1 = connected;
            msg.arg2 = configured;
            // debounce disconnects to avoid problems bringing up USB tethering
            sendMessageDelayed(msg, (connected == 0) ? UPDATE_DELAY : 0);
        }

                case MSG_UPDATE_STATE:
                    mConnected = (msg.arg1 == 1);
                    mConfigured = (msg.arg2 == 1);
                    if (!mConnected) {
                        // When a disconnect occurs, relock access to sensitive user data
                        mUsbDataUnlocked = false;
                    }
                    updateUsbNotification();
                    updateAdbNotification();
                    if (UsbManager.containsFunction(mCurrentFunctions,
                            UsbManager.USB_FUNCTION_ACCESSORY)) {
                        updateCurrentAccessory();
                    } else if (!mConnected) {
                        // restore defaults when USB is disconnected
                        setEnabledFunctions(null, false);//设置usb功能为空，也就是充电模式
                    }
                    if (mBootCompleted) {
                        updateUsbStateBroadcastIfNeeded();
                        updateUsbFunctions();
                    }
                    break;

        可以看出当拔出USB时，会更新mConnected的值，最终如果值为false的话会调用

setEnabledFunctions(null, false);//设置usb功能为空，也就是充电模式

        那要想改这块的逻辑，是不是把这段代码注释了就可以了？我试了，是不行的，虽然persist.sys.usb.config和sys.usb.config的值是mtp,adb，但是实际mtp功能是不能用的，到底为什么呢？答案在下面的代码：

                    if (!mConnected) {
                        // When a disconnect occurs, relock access to sensitive user data
                        mUsbDataUnlocked = false;
                    }

        private void updateUsbStateBroadcastIfNeeded() {
            // send a sticky broadcast containing current USB state
            Intent intent = new Intent(UsbManager.ACTION_USB_STATE);
            intent.addFlags(Intent.FLAG_RECEIVER_REPLACE_PENDING
                    | Intent.FLAG_RECEIVER_FOREGROUND);
            intent.putExtra(UsbManager.USB_CONNECTED, mConnected);
            intent.putExtra(UsbManager.USB_HOST_CONNECTED, mHostConnected);
            intent.putExtra(UsbManager.USB_CONFIGURED, mConfigured);
            intent.putExtra(UsbManager.USB_DATA_UNLOCKED, isUsbTransferAllowed() && mUsbDataUnlocked);//奥秘就在这里

            if (mCurrentFunctions != null) {
                String[] functions = mCurrentFunctions.split(",");
                for (int i = 0; i < functions.length; i++) {
                    final String function = functions[i];
                    if (UsbManager.USB_FUNCTION_NONE.equals(function)) {
                        continue;
                    }
                    intent.putExtra(function, true);
                }
            }

            // send broadcast intent only if the USB state has changed
            if (!isUsbStateChanged(intent)) {
                if (DEBUG) {
                    Slog.d(TAG, "skip broadcasting " + intent + " extras: " + intent.getExtras());
                }
                return;
            }

            if (DEBUG) Slog.d(TAG, "broadcasting " + intent + " extras: " + intent.getExtras());
            mContext.sendStickyBroadcastAsUser(intent, UserHandle.ALL);
            mBroadcastedIntent = intent;
        }

 intent.putExtra(UsbManager.USB_DATA_UNLOCKED, isUsbTransferAllowed() && mUsbDataUnlocked);//奥秘就在这里

        把这段代码里面的mUsbDataUnlocked改为true就可以达到Android L的效果。具体为什么，我没有去看。