Keyboard: The Journey of a Keystroke

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# The Journey of a Keystroke

When the user presses a key, a series of events is launched to handle that action. Let's follow the bits through the system after the user presses a key on the keyboard.

## Step 1: The Input Server Add-on

When the user presses a key on the keyboard, the entity responsible for recognizing this action is the Input Server add-on for the keyboard device on which the key was pressed. The add-on uses the current keymap (which it has obtained by calling `get_key_map()`) to map the keycode to the character.

The add-on then constructs a `B_KEY_DOWN` or `B_UNMAPPED_KEY_DOWN` BMessage, containing both the raw keycode and the actual characters generated by the key, and sends the message to the Application Server by calling the `BInputServerDevice::EnqueueMessage()` function.

## Step 2: The Application Server

The Aplication Server receives the message from the Input Server add-on and decides what to do with it. If the key pressed is something the Application Server traps, like a workspace change keystroke, it gets handled internally.

Otherwise, the Application server locates the active window and passes the BMessage along to its `BWindow::DispatchMessage()` function.

## Step 3: The Active Window

The `BWindow::DispatchMessage()` function receives the BMessage from the Application Server, and intercepts some keys (like menu shortcuts and the Command-W close window sequence). If it's not a key the BWindow class (or derived class) wants to intercept, DispatchMessage passes the message along to the currently focused view's `BView::MessageReceived()` function.

## Step 4: The Focused View

The message finally arrives at the view that's currently focused in the active window. This view's `MessageReceived()` function processes the keypress in whatever way it wants to.

Likewise, when the key is released, a `B_KEY_UP` or `B_UNMAPPED_KEY_UP` event is sent through this same route.

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