BInvoker

BInvoker is a class in Haiku OS that facilitates message invocation. It allows you to send messages to a target object, such as a BHandler, and execute a specific function on that object. This mechanism enables seamless communication between different components of an application.

It serves as an intermediary between the graphical user interface (GUI) elements and the underlying application logic. BInvoker allows you to associate actions with GUI elements such as buttons, menu items, and list items, and execute these actions when the associated GUI elements are activated.

This class is defined at location:

• Header file: headers/os/app/Invoker.h
• src file: src/kits/app/Invoker.cpp

Typical Use Cases:

• Handling user interface events such as button clicks, menu selections, and slider movements.
• Invoking specific actions on target objects without subclassing BHandler.

Key Components of BInvoker:

• Target: The object on which the message will be invoked.
• Message: The message to be sent to the target object.
• Invoker: The object responsible for invoking the message on the target.

How Does BInvoker Work?

The key to understanding BInvoker is its ability to invoke messages on a target object. Here's how it works:

1. Target Object: You specify a target object (usually a BHandler) on which you want to invoke a message.
2. Message: You define a message (usually a BMessage) that you want to send to the target object.
3. Invocation: You use BInvoker to invoke the message on the target object.

Example Usage of BInvoker:

Let's say you have a BButton object and you want to perform an action when the button is clicked. You can achieve this using BInvoker:

#include <Invoker.h>
#include <Button.h>
#include <Message.h>
#include <Handler.h>
#include <stdio.h>

class MyHandler : public BHandler {
public:
    MyHandler() {}

    void MessageReceived(BMessage* message) {
        switch (message->what) {
            case 'BTN_CLICKED':
                printf("Button clicked!\n");
                break;
            default:
                BHandler::MessageReceived(message);
                break;
        }
    }
};

int main() {
    BButton* button = new BButton("MyButton", "Click Me",
                                  new BMessage('BTN_CLICKED'));

    MyHandler* handler = new MyHandler();

    BInvoker* invoker = new BInvoker(new BMessageRunner(handler));
    button->SetTarget(invoker);

    // Add the button to a window and display the window
    // ...

    return 0;
}

Explanation:

1 Creating A Button:

BButton* button = new BButton("MyButton", "Click Me",
                              new BMessage('BTN_CLICKED'));

• We create a BButton object with a message (BTN_CLICKED) attached to it.

2 Create a Handler:

class MyHandler : public BHandler {
public:
    MyHandler() {}

    void MessageReceived(BMessage* message) {
        switch (message->what) {
            case 'BTN_CLICKED':
                printf("Button clicked!\n");
                break;
            default:
                BHandler::MessageReceived(message);
                break;
        }
    }
};

• We create a MyHandler object to handle the message.

3 Creating an Invoker:

BInvoker* invoker = new BInvoker(new BMessageRunner(handler));

• We create a BInvoker object and pass a BMessageRunner object to it.

4 Setting the Target:

button->SetTarget(invoker);

• We set the BInvoker object as the target for the button.