我正在开发与平板电脑连接时可与USB设备通信的应用程序。为了避免用户接受android访问设备,我设置了一个意图过滤器:
<intent-filter>
<action android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED" />
<action android:name="android.hardware.usb.action.USB" />
</intent-filter>
<meta-data android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED" android:resource="@xml/usb_device_filter" />
我的问题是,有了这个意图过滤器,每次插入设备并启动另一个活动时,都会调用“ onCreate”方法,而没有意图过滤器,则只会调用一次。这是“ onCreate”方法:
@Override
protected void onCreate(Bundle savedInstanceState) {
try {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_msp430_hid); //define activity layout
setVersionToTitle();
btnSend = (Button) findViewById(R.id.btnSend); //Send button
btnSend.setOnClickListener(this); //Listener for Send Button
btnSend.setEnabled(true);
btnSelectHIDDevice = (Button) findViewById(R.id.btnSelectHIDDevice); //Select HID Device button
btnSelectHIDDevice.setOnClickListener(this); //Listener for Select HID Device button
btnClear = (Button) findViewById(R.id.btnClear); //Clear button
btnClear.setOnClickListener(this); //Listener for Clear button
edtxtHidInput = (EditText) findViewById(R.id.edtxtHidInput); //User editable text area for sending information to attached device
log_txt = (EditText) findViewById(R.id.log_txt); //Text area for displaying information
mLog("Initialized\nPlease select your USB HID device");
getWindow().setSoftInputMode(WindowManager.LayoutParams.SOFT_INPUT_STATE_ALWAYS_HIDDEN);
mPermissionIntent = PendingIntent.getBroadcast(this, 0, new Intent(ACTION_USB_PERMISSION), 0); //Get USB permission intent for broadcast
IntentFilter filter = new IntentFilter(ACTION_USB_PERMISSION);
filter.addAction(UsbManager.ACTION_USB_DEVICE_ATTACHED);
filter.addAction(UsbManager.ACTION_USB_DEVICE_DETACHED);
registerReceiver(mUsbReceiver, filter); //Register broadcast receiver
edtxtHidInput.setText("Enter Text Here");
uiHandler.postDelayed(runnable, 100); //Start runnable after 100ms
} catch (Exception e) {
Log.e("Init", "Initialization error", e);
}
}
广播接收器:
private final BroadcastReceiver mUsbReceiver = new BroadcastReceiver() {
public void onReceive(Context context, Intent intent) {
String action = intent.getAction();
if (ACTION_USB_PERMISSION.equals(action)) {
synchronized (this) {
setDevice(intent);
}
}
//device attached
if (UsbManager.ACTION_USB_DEVICE_ATTACHED.equals(action)) {
synchronized (this) {
setDevice(intent); //Connect to the selected device
}
if (device == null) {
mLog("device connected");
}
}
//device detached
if (UsbManager.ACTION_USB_DEVICE_DETACHED.equals(action)) {
if (device != null) {
device = null;
//btnSend.setEnabled(false);
}
mLog("device disconnected");
}
}
简单地连接到设备的“ setDevice”方法:
private void setDevice(Intent intent) {
device = (UsbDevice) intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);
if (device != null && intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) {
mLog("Selected device VID:" + Integer.toHexString(device.getVendorId()) + " PID:" + Integer.toHexString(device.getProductId()));
connection = mUsbManager.openDevice(device); //Connect to device
intf = device.getInterface(0);
为什么呢
显然,这是默认行为。android:launchMode
在这里检查。
默认的launchMode为standard
,这意味着每次您连接USB设备时,都会创建一个新的Activity实例。如果设置了singleTop
,onNewIntent()
则会被调用,并且如果您的活动位于堆栈的顶部,则会重新使用您的活动。否则,它将创建一个新的。
在大多数情况下,您也可以使用singleTask
,singleInstance
但Google不鼓励这样做。但是有时候这可能是解决问题的正确方法。
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