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  3. Android 服务

Android 服务

Starting with Qt 5.7, you can create Android services using Qt. A service is a component that runs in background, so, it has no user interface. It is useful to perform long-term operations such as logging GPS, waiting for social media notifications, and so on. A service will continue to run even if the application that started it exits.

Assemble the Service

To get started, create an Android package directory as instructed in Qt Creator：把应用程序部署到 Android 设备 . This directory contains the AndroidManifest.xml file. Inside the package directory, create a src directory, where all your Java packages and classes will be created.

创建服务类

You can create a service by extending the class QtService or Android: Service to your Java class. Depending on whether you want to use Qt features in your service or call native C++ functions from Java, you need to extend either QtService or Service . Let's start with a simple service, as follows: 

import android.content.Context;
import android.content.Intent;
import android.util.Log;
import org.qtproject.qt5.android.bindings.QtService;
public class QtAndroidService extends QtService
{
    private static final String TAG = "QtAndroidService";
    @Override
    public void onCreate() {
        super.onCreate();
        Log.i(TAG, "Creating Service");
    }
    @Override
    public void onDestroy() {
        super.onDestroy();
        Log.i(TAG, "Destroying Service");
    }
    @Override
    public int onStartCommand(Intent intent, int flags, int startId) {
        int ret = super.onStartCommand(intent, flags, startId);
        // Do some work
        return ret;
    }
}

Start the Service

Android allows starting services on demand or at boot time. You can do both using Qt as well.

Start a Service On Demand

You can start the service in the following ways:

  • Directly from C++ using QAndroidIntent and QAndroidJniObject , by creating a service Intent and calling the app's main activity method startService() : 
    QAndroidIntent serviceIntent(QtAndroid::androidActivity().object(),
                                    "org/qtproject/example/qtandroidservice/QtAndroidService");
QAndroidJniObject result = QtAndroid::androidActivity().callObjectMethod(
            "startService",
            "(Landroid/content/Intent;)Landroid/content/ComponentName;",
            serviceIntent.handle().object());
  • Start the service by calling a Java method. The easiest way is to create a static method in your service class: 
    public static void startQtAndroidService(Context context) {
        context.startService(new Intent(context, QtAndroidService.class));
}

    The you can call it from C++ using the following JNI call: 

    QAndroidJniObject::callStaticMethod<void>(
    "org/qtproject/example/qtandroidservice/QtAndroidService",
    "startQtAndroidService",
    "(Landroid/content/Context;)V",
    QtAndroid::androidActivity().object());

Start a Service At Boot Time

To run a service at boot time, you need a BroadcastReceiver .

Create a custom Java class: 

public class QtBootServiceBroadcastReceiver extends BroadcastReceiver {
    @Override
    public void onReceive(Context context, Intent intent) {
        Intent startServiceIntent = new Intent(context, QtAndroidService.class);
        context.startService(startServiceIntent);
    }
}

Add the following uses-permission in the body of the <manifest> section in the AndroidManifest.xml 文件： 

<uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED" />

Also, add the receiver definition in the body of the <application> section: 

<receiver android:name=".QtBootServiceBroadcastReceiver" android:exported="true">
    <intent-filter>
        <action android:name="android.intent.action.BOOT_COMPLETED" />
    </intent-filter>
</receiver>

注意： Android 8.0 introduced some limitations on running background services, which means using a nomal Service class might not work. For more information, see Android's recommendation to use either Foreground services or JobIntentService .

Manage the Service in AndroidMnifest.xml

For the service to be usable in an Android app, you must declare it in the AndroidManifest.xml file. Let's start with adding the service section:

  • When extending Service , just declare the service section as a normal Android service. Add the following inside the <application> section: 
    <service android:name=".QtAndroidService" android:exported="true">
<!-- Background running -->
<meta-data android:name="android.app.background_running" android:value="true"/>
<!-- Background running -->
</service>

    This way the service will start in the same process as QtActivity , which allows you to use native C++ calls from Java code. You can run it in a separate process but that way you cannot use any native calls for communication because the Qt libraries are not loaded for that process. To run on separate process, add this to the service tag: 

    android:process=":qt_service"
  • When extending QtService , you need to declare other items for loading all the necessary libs required for Qt, mainly the same items as in <activity> section for QtActivity . Add the following: 
    <service android:process=":qt_service" android:name=".QtAndroidService" android:exported="true">
<meta-data android:name="android.app.lib_name" android:value="service"/>
<meta-data android:name="android.app.qt_sources_resource_id" android:resource="@array/qt_sources"/>
<meta-data android:name="android.app.repository" android:value="default"/>
<meta-data android:name="android.app.qt_libs_resource_id" android:resource="@array/qt_libs"/>
<meta-data android:name="android.app.bundled_libs_resource_id" android:resource="@array/bundled_libs"/>
<!-- Deploy Qt libs as part of package -->
<meta-data android:name="android.app.bundle_local_qt_libs" android:value="-- %%BUNDLE_LOCAL_QT_LIBS%% --"/>
<!-- Run with local libs -->
<meta-data android:name="android.app.use_local_qt_libs" android:value="-- %%USE_LOCAL_QT_LIBS%% --"/>
<meta-data android:name="android.app.libs_prefix" android:value="/data/local/tmp/qt/"/>
<meta-data android:name="android.app.load_local_libs_resource_id" android:resource="@array/load_local_libs"/>
<meta-data android:name="android.app.load_local_jars" android:value="-- %%INSERT_LOCAL_JARS%% --"/>
<meta-data android:name="android.app.static_init_classes" android:value="-- %%INSERT_INIT_CLASSES%% --"/>
<!-- Run with local libs -->
<!-- Background running -->
<meta-data android:name="android.app.background_running" android:value="true"/>
<!-- Background running -->
</service>

注意： Make sure to define the following to run the service in the background: 

<meta-data android:name="android.app.background_running" android:value="true"/>

There are a few variations on how to declare services. Some of them are already used in the previous manifest snippet. Depending on your use case, run the service either in the same process as QtActivity or in a separate process.

Service in the Same Process as QtActivity

To run a service in the same process as QtActivity, declare the service header as follows: 

<service android:name=".QtAndroidService" android:exported="true">

Service in Separate Process

To run a service in a dedicated process, declare the service header as follows: 

<service android:process=":qt_service" android:name=".QtAndroidService" android:exported="true">

Qt loads the .so file defined in android.app.lib_name meta-data , and calls the main() function with all the arguments set in android.app.arguments meta-data . When running in a separate process, you can start the service using either the same lib file as the main activity or a separate lib file.

Use the Same .so Lib File

Using the same .so lib file as the main activity means the service will use the same entry point with an extra argument to distinguish it from the main activity. You can handle your application's execution in the main() function according the arguments provided. Add the following argument declaration to your service body: 

<!-- Application arguments -->
<meta-data android:name="android.app.arguments" android:value="-service"/>
<!-- Application arguments -->

Then make sure the service android.app.lib_name is the same as the main activity, add the following: 

<meta-data android:name="android.app.lib_name" android:value="-- %%INSERT_APP_LIB_NAME%% --"/>

When using the same .so lib file, your application's main() function is executed two times, one to start the main activity and the second time to start the service. Thus, you have to handle each execution according to the provided argument. One way to acheive that is as follows: 

if (argc <= 1) {
    // code to handle main activity execution
} else if (argc > 1 && strcmp(argv[1], "-service") == 0) {
    qDebug() << "Service starting with from the same .so file";
    QAndroidService app(argc, argv);
    return app.exec();
} else {
    qWarning() << "Unrecognized command line argument";
    return -1;
}
Use a Separate .so Lib File

In this case, you need to have a sub-project with a lib template that provides a different executable for the service. A sample project .pro is: 

TEMPLATE = lib
TARGET = service
CONFIG += dll
QT += core androidextras
SOURCES += \
    service_main.cpp
HEADERS += servicemessenger.h

在 service_main.cpp you could have the following: 

#include <QDebug>
#include <QAndroidService>
int main(int argc, char *argv[])
{
    qWarning() << "Service starting from a separate .so file";
    QAndroidService app(argc, argv);
    return app.exec();
}

定义 android.app.lib_name for the service in the AndroidManifest.xml : 

<meta-data android:name="android.app.lib_name" android:value="service"/>

与服务通信

Qt for Android offers a variety of inter-process communication (IPC) methods to communicate with Android Services. Depending on the structure of your project, you can use either native C++ calls from Java Service or Android BroadcastReceiver.

从 Java 服务调用本机 C++

This can work with services running in the same process as QtActivity and even if Service is extended. For more information, see Calling QML/C++ Functions from Java Code .

使用 Android BroadcastReceiver

Android BroadcastReceiver enables exchanging messages between the Android system, apps, activities and services. Similarly to other Android features, Qt can use broadcast receivers to exchange messages between QtActivity and your service. Let's start with logic to send a message from your service. Add the following in your service implementation, which calls sendBroadcast() : 

@Override
public int onStartCommand(Intent intent, int flags, int startId) {
    int ret = super.onStartCommand(intent, flags, startId);
    Intent sendToUiIntent = new Intent();
    sendToUiIntent.setAction(ActivityUtils.BROADCAST_CUSTOM_ACTION);
    sendToUiIntent.putExtra("message", "simple_string");
    Log.i(TAG, "Service sending broadcast");
    sendBroadcast(sendToUiIntent);
    return ret;
}

Then, you need to create and register the broadcast receiver from the Qt's main activity. The easiest way is to create a custom class with a method and implement all that logic in Java. In the following example, the service sends a message "simple_string" to Qt by calling the native method sendToQt() : 

public class ServiceBroadcastUtils {
    private static native void sendToQt(String message);
    private static final String TAG = "ActivityUtils";
    public static final String BROADCAST_CUSTOM_ACTION = "org.qtproject.example.qtandroidservice.broadcast.custom";
    public void registerServiceBroadcastReceiver(Context context) {
        IntentFilter intentFilter = new IntentFilter();
        intentFilter.addAction(BROADCAST_CUSTOM_ACTION);
        context.registerReceiver(serviceMessageReceiver, intentFilter);
        Log.i(TAG, "Registered broadcast receiver");
    }
    private BroadcastReceiver serviceMessageReceiver = new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            Log.i(TAG, "In OnReceive()");
            if (BROADCAST_CUSTOM_ACTION.equals(intent.getAction())) {
                String message = intent.getStringExtra("message");
                sendToQt(data);
                Log.i(TAG, "Service sent back message to C++: " + message);
            }
        }
    };
}

For more information on working with native calls, see Calling QML/C++ Functions from Java Code .

To make use of all that, start your service as shown in Start the Service , an then register the broadcast receiver by calling the method registerServiceBroadcastReceiver() : 

QAndroidJniEnvironment env;
jclass javaClass = env.findClass("org/qtproject/example/qtandroidservice/ActivityUtils");
QAndroidJniObject classObject(javaClass);
classObject.callMethod<void>("registerServiceBroadcastReceiver",
                             "(Landroid/content/Context;)V",
                             QtAndroid::androidContext().object());

Using Qt Remote Objects

Qt Remote Objects offers an easy way to share APIs between Qt processes. The main concept is to server in the service process, and have a replica in the Qt application, then those two parts are able to exchange data between each other, using signals and slots.

Prepare the replica

Let's consider a service example with separate .so lib file. Define a .rep file which defines our communication class: 

class ServiceMessenger {
    SLOT(void ping(const QString &message));
    SIGNAL(pong(const QString &message));
}

The define the class in the service sub-project as servicemessenger.h : 

#include "rep_servicemessenger_source.h"
class ServiceMessenger : public ServiceMessengerSource {
public slots:
    void ping(const QString &name) override {
        emit pong("Hello " + name);
    }
};

Then, add the .rep file to both the main application and service .pro files, in the main application: 

QT += remoteobjects
REPC_REPLICA += servicemessenger.rep

And in the service sub-project: 

QT += remoteobjects
REPC_SOURCE += servicemessenger.rep

Connect the source and replica

Define the Qt Remote Objects source node in the service sub-project's main() 函数： 

#include "servicemessenger.h"
#include <QDebug>
#include <QAndroidService>
int main(int argc, char *argv[])
{
    qWarning() << "QtAndroidService starting from separate .so";
    QAndroidService app(argc, argv);
    QRemoteObjectHost srcNode(QUrl(QStringLiteral("local:replica")));
    ServiceMessenger serviceMessenger;
    srcNode.enableRemoting(&serviceMessenger);
    return app.exec();
}

Then, in the application's main() function, connect to source node: 

QRemoteObjectNode repNode;
repNode.connectToNode(QUrl(QStringLiteral("local:replica")));
QSharedPointer<ServiceMessengerReplica> rep(repNode.acquire<ServiceMessengerReplica>());
bool res = rep->waitForSource();
Q_ASSERT(res);
QObject::connect(rep.data(), &ServiceMessengerReplica::pong, [](const QString &message){
    qDebug() << "Service sent: " << message;
});
rep->ping("Qt and Android are friends!");

This example sends a message from the main application's process to the service. The service replies with the same message, which is printed on the debug logcat.

注意： The same method could be used when using the same .so lib file. For more information, see Use the same .so Lib File .

使用 QAndroidBinder

While using Qt Remote Objects for communication is a cross-platform solution, QAndroidBinder Class Reference} is Android specific. QAndroidBinder is a convenience class that implements the most important methods in Android: Binder . It allows sending QByteArray or QVariant objcets between processes.

注意： Qt for Android has a limitation forcing the execution of only one service at a time when running multiple services in one process. Thus, it is recommended to run each service in its own process. For more information, see QTBUG-78009 .

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