Question

我尝试这样做：连接到USB设备并获得打开（或失败）的连接。我根据我发现的例子和解释做了逻辑，但是我在等待许可授予方面遇到了问题。首先，我尝试了一个好的＆＃34;使用wait（）+ notifyAll（）的方式，比我尝试使用检查的直接循环，但是两次等待方法（waitConnection（））都阻塞了我给它的超时，并且只有在收到消息之后。所以我尝试了这两个版本。

等待/ notifyAll的：

public UsbConnector startConnection(Context context) {

BroadcastReceiver receiver = new BroadcastReceiver() {
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        if (ACTION_USB_PERMISSION.equals(action)) {
            synchronized (syncObj) {
                if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) {
                    UsbDevice device = intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);
                    if (device != null) {
                        if (device.getVendorId() == vendorId && device.getProductId() == productId) {
                            connection = usbManager.openDevice(device);
                            connectedDevice = device;
                        }
                    }
                }
                syncObj.notyfyAll();
            }
        }
    }
};

try {
    usbManager = (UsbManager) context.getSystemService(Context.USB_SERVICE);
    for (final UsbDevice device : usbManager.getDeviceList().values()) {
        if (device.getVendorId() == this.vendorId && device.getProductId() == this.productId) {

            context.registerReceiver(receiver, new IntentFilter(ACTION_USB_PERMISSION));
            usbManager.requestPermission(device,
                    PendingIntent.getBroadcast(context, 0, new Intent(ACTION_USB_PERMISSION), 0));
            break;
        }
    }
} catch (Exception e) {
}
return this;
}

public UsbDeviceConnection waitConnection(int timeout) {
try {
    Thread.sleep(10, 0);
    syncObj.wait(timeout);
} catch (InterruptedException e) {
}
return getConnection();
}

直接循环

public UsbConnector startConnection(Context context) {

BroadcastReceiver receiver = new BroadcastReceiver() {
    public void onReceive(Context context, Intent intent) {
        String action = intent.getAction();
        if (ACTION_USB_PERMISSION.equals(action)) {
            synchronized (this) {
                if (intent.getBooleanExtra(UsbManager.EXTRA_PERMISSION_GRANTED, false)) {
                    UsbDevice device = intent.getParcelableExtra(UsbManager.EXTRA_DEVICE);
                    if (device != null) {
                        if (device.getVendorId() == vendorId && device.getProductId() == productId) {
                            connection = usbManager.openDevice(device);
                            connectedDevice = device;
                        }
                    }
                }
                permissionRequested = false;
            }
        }
    }
};

try {
    permissionRequested = false;
    usbManager = (UsbManager) context.getSystemService(Context.USB_SERVICE);
    for (final UsbDevice device : usbManager.getDeviceList().values()) {
        if (device.getVendorId() == this.vendorId && device.getProductId() == this.productId) {

            permissionRequested = true;
            context.registerReceiver(receiver, new IntentFilter(ACTION_USB_PERMISSION));
            usbManager.requestPermission(device,
                    PendingIntent.getBroadcast(context, 0, new Intent(ACTION_USB_PERMISSION), 0));
            break;
        }
    }
} catch (Exception e) {
}
return this;
}

public UsbDeviceConnection waitConnection(int timeout) {
int waited = timeout;
while (permissionRequested && waited > 0) {
    try {
        Thread.sleep(10, 0);
    } catch (InterruptedException e) {
    }
    waited -= 10;
}

return getConnection();
}

所以在这两种情况下，根据日志，waitConnection（）方法（在startConnection（）之后立即被消费者调用）似乎阻止执行（我给它超时10秒，它被阻止了10秒），只有在它完成后，BroadcastReceiver才会收到消息。似乎requestPermission（）不是异步（我认为是），但在这种情况下，startConnection（）如何在接收到消息之前立即退出？我如何等待BroadcastReceiver获取消息？假设我不使用waitConnection（）方法，我的消费者应该如何知道它何时可以开始检查连接可用性？

Answer 1

“并且只有在完成后，BroadcastReceiver才会收到消息”

默认情况下，onReceived回调在主线程上调用。听起来你也在主线程上调用waitConnection()。由于waitConnection()阻塞，主线程在waitConnection()返回之前无法处理任何其他消息。这意味着在onReceived超时之前不会调用waitConnection()。

阻止主线程通常是一个坏主意。 Read here

相反，您可以onReceive启动一项新活动，然后在获得USB许可后执行您需要执行的操作。这对你来说可能是也可能不是最好的解决方案，但无论如何，这里的关键是永远不要阻止主线程。

Android：连接USB设备，等待许可

1 个答案: