How to use

Overview
The wrapper classes are placed in the org.apertusvr package. For each ApertusVR entity type there's a corresponding Java wrapper class. Also there are a wrapper classes for the manager classes (e.g.  ape::EventManager, ape::SceneManager)and ape::Event and there are several supporting classes (e.g. apeColor, apeVector3). To be able to cast the interfaces, we ha a builder Java interface (apeBuilder) too.
Event connection
Just like using the C++ interface, the user can connect to various events (fired by ApertusVR) from Java code. To do that, we need to implement the apeEventCallback function:
class MyEventCallback implements apeEventCallback {
    @Override
    public void onEvent(apeEvent event) {
        // do sth with event
    }
}
We also have to create an instance of the MyEventCallback class, e.g.
MyEventCallback eventCallback = new MyEventCallback();
After instantiation, the connection works simply with the usage of apeManager's connectEvent(...) function. E.g., to be able to listen NODE events, just type:
apeEventManager.connectEvent(apeEvent.Group.NODE, eventCallback);
To get an insight of what actually happens here, let us discuss the details! When the user connects an event, the apeEventManager (which is a static class) puts this eventCallback into a map:
private static Map<apeEvent.Group, LinkedList<apeEventCallback>> mEventMap
The app developer also creates a FrameCallback  class, which implements Android's built in Choreographer.FrameCallback. Like in the following code block:
class FrameCallback implements Choreographer.FrameCallback {
        @Override
        public void doFrame(long frameTimeNanos) {
            choreographer.postFrameCallback(this);
            
            if (apeSystem.isRunning()) {
                ApertusJNI.processEventDoubleQueue();
            }
        }
    }
This will parse all the events fired in the C++ side, during the last frame. The ApertusJNI member function called processEventDoubleQueue() is a JNI-function, which calls back to Java with apeEventManager's fireEvent() function, which will call the onEvent(apeEvent) function of our apeEventCallback implementation, if its instance is connected to the event's group (so the implementation object can be found under the apeEvent.Group.NODE key in the map).
Entity wrappers
As it was written before, for each entity class in ApertusVR we have a corresponding Java class. The member functions and the class hierarchies on the Java side are the same, as they are on the C++ side. Therefore the API does not differ much, but there are tiny differences between the two. We will go through everything you need, to use ApertusVR entities from Java.
Getting access to entities
In C++, knowing the entity's name, we ask the ape::ISceneManager to give us a weak pointer. Then we use the lock() function to access it. After calling lock(), we can also cast the resulted shared_ptr to the proper type (ape::Entity::Type), if we know what the entity's type should be.
When using ApertusVR from Java, we just use the interface's constructor to get access to an entity, then we check with the isValid() function, that we can actually lock it in C++.
E.g. assume, that we want to access an apeFileGeometry entity we know about, with the name of "FooBar", and our goal is to query its OwnerID. The procedure goes something like on the following code example:
String fooBarName = "FooBar";
apeFileGeometry fooBarGeometry = new apeFileGeometry(fooBarName);
​
if (fooBarGeometry.isValid()) {
    String ownerID = fooBarGeometry.getOwnerID();
}
The following figure summarizes the procedure executed in the background:
This is the way in 90% of the cases. However, what if we don't know already the type of the entity, and only its name is given. In this case, we can use the apeSceneManager interface:
String fooBarName = "FooBar";
apeEntity fooBarEntity = apeSceneManager.getEntity(fooBarName);
​
if (fooBarEntity != null && fooBarEntity.isValid()) {
    apeEntity.Type fooBarType = fooBarEntity.getType();
    
    if (fooBarType == apeEntity.Type.GEOMETRY_FILE) {
        apeFileGeometry fooBarGeometry = new apeFileGeometry(fooBarName);
        // ...
    }
}
Creating an entity
The next question is, how can we create entities in Java? Easy. Just use the apeSceneManager interface!:
String fooBarName = "FooBar";
apeEntity.Type fooBarType = apeEntity.Type.GEOMETRY_FILE;
boolean fooBarIsReplicate = false;
String fooBarOwner = "FooBarOwner";
​
apeEntity fooBarEntity = apeSceneManager.createEntity(
    fooBarName, fooBarType, fooBarIsReplicate, fooBarOwner);
 Okay, then how do we cast it to our needs? We wanted apeFileGeometry interface, not apeEntity.
This is where the apeBuilder interface come in! The apeEntity abstract class has a member function called cast:
public <apeType extends apeEntity> apeType cast(apeBuilder<apeType> builder)
Providing a proper builder class (which implements the apeBuilder interface), we can cast our apeEntity to any given type extended from it. In the example, we just type:
apeFileGeometry fooBarGeometry =  Objects.requireNonNull(
        apeSceneManager.createEntity(
            fooBarName,
            fooBarType,
            fooBarIsReplicate,
            fooBarOwner))
            .cast(new apeFileGeometry.FileGeometryBuilder());
Or an alternative way is to instance an other apeFileGeometry entity  with the apeEntity's name you got from apeSceneManager (like you saw it in the former sub-subsection).
Deleting entities
Deleting an entity in Java is the same as in C++, an example is presented below:
String entityName = "FooBar";
apeSceneManager.deleteEntity(entityName);
Writing an application
Now you know how to query events and handle them with entity usage, we can discuss how you can set up ApertusVR for your android application with the support of the JNI-plugin.
On Android, we build our applications from Activity classes, and each activity has its own lifecycle. To learn about activity lifecycles, see the Android guide on activity lifecycles. In code it appears as five function we have to override:
protected void onCreate(Bundle savedInstanceState) {
    // ...
}
​
protected void onStart() {
    // ...
}
​
protected void onResume() {
    // ...
}
​
protected void onPause() {
    // ...
}
​
protected void onDestroy() {
    // ...
}
When the activity is launched (and the onCreate(...) function is called) by Android , we have to do three things:
1.Get an instance from Android choreographer, and store it for later usages.
2. Start the ApertusVR system and optionally connect to a room.
3.Initialize the plug-ins for our application (more about it in the Plugins section).
The 1st is done by typing choreographer = Choreographer.getInstance(). Not a big deal. The 2nd needs further explanation, and we will go through the 3rd in the next section.
Start ApertusVR
Before doing anything with ApertusVR, we have to call its C++ start method in the ape::Systemnamespace:
void Start(const char* configFolderPath, bool isBlocking, std::function<void()> userThreadFunction = std::function<void()>());
When we do that, we have to tell ApertusVR where it can find the apeCore.json file (configFolderPath), which contains necessary informations. On Android it is stored in the assets folder of the given application. Therefore, when calling this method through the JNI-interface and the apeSystem wrapper class, we type:
String configFolderPath = /* some path in the assets folder */;
​
if (!apeSystem.isRunning()) {
    apeSystem.start(configFolderPath, getAssets());
}
As you see, we have to provide the Android AssetManager (getAssets()) for the start function. This will call the JNI-function startApertusVR(...), which will call the actual ape::System::Start(...)function. 
For reading the asset folder from C++, the Android build has a shared library called apeAAssetOpen. The apeCoreConfig module, uses this shared library to open the asset folder. The JNI-method startApertusVR(...) also initializes apeAAssetOpen library.
Connect to room
ApertusVR users on Android-Java can set the room they want to connect pre-runtime in the apeCore.json, or runtime with the help of the apeSceneNetwork wrapper. So basically everything goes like in the C++ case:
/* start ApertusVR */
​
String roomName = /* existing room name */
apeSceneNetwork.connectToRoom(roomName);
Plugins
In the ApertusVR C++ API plug-ins are implementations of the ape::IPlugin interface. Developing on Android and Java, they have a different form. Our Java plug-ins are actually Android implementations of the AndroidLifecycleObserver interface.
For being able to partition our application, Android provides us an interface called LifecycleObserver. Classes implementing this interface (called lifecycle-aware components) can be attached to the activity's lifecycle as something which constantly observes its lifecycle, and respond to the events occouring during the activity's lifetime. To learn more about lifecycle-aware components, again please read the Android guide on the given topic.
As activities can select their lifecycle-aware components, it works kind of like a plug-in mechanism. Therefore the ApertusVR Android API contains an interface called apePlugin, which only presents a LifecycleObserver interface as it was a plug-in(ish thing):
apePlugin.java
package org.apertusvr;
​
import /* ... */
​
public interface apePlugin extends LifecycleObserver {
​
    @OnLifecycleEvent(Lifecycle.Event.ON_CREATE)
    void onCreate();
​
    @OnLifecycleEvent(Lifecycle.Event.ON_START)
    void onStart();
​
    @OnLifecycleEvent(Lifecycle.Event.ON_RESUME)
    void onResume();
​
    @OnLifecycleEvent(Lifecycle.Event.ON_PAUSE)
    void onPause();
​
    @OnLifecycleEvent(Lifecycle.Event.ON_DESTROY)
    void onDestroy();
}
So, in order to create a plug-in which works on Android and can respond to ApertusVR events from Java code, we need to implement this interface (or equivalently implement the "raw" LifecycleObserver interface):
public final class apeExamplePlugin implements apePlugin {
​
    public apeExamplePlugin(Context context, Lifecycle lifecycle, /* ... */) {
        // constructing the plugin
    }
    
    @Override
    void onCreate() {
        // ...
    }
​
    @Override
    void onStart() {
        // ...
    }
​
    @Override
    void onResume() {
        // ...
    }
​
    @Override
    void onPause() {
        // ...
    }
​
    @Override
    void onDestroy() {
        // ...
    }
​
}
Then in our activity, when we want to start to use this example plug-in (e.g., when Android creates the activity), we create an instance of the plug-in, then attach it to the activity's lifecycle:
public class ExampleActivity extends AppCompatActivity {
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        
        // ...
     
        /* instance the example plug-in */
        
        apeExamplePlugin examplePlugin = new apeExamplePlugin(
                this, getLifecycle(), /* ... */);
        
        getLifecycle().addObserver(examplePlugin);
    }
}
Not every Activity subclass supports lifecycle-aware components. AppCompatActivity specially supports it.
You can use configuration files for the plug-in if needed. Place the configuration file in the application's asset folder, then just query the path for the configuration file from the apeCoreCinfig.getConfigFolderPath(). 
Plugins on Android - Previous
Java Native Interface
Extending the API
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Outline
Overview
Event connection
Entity wrappers
Writing an application
Plugins