backpress-logic-android-13.java

// 1.
class NativePreImeInputStage ... {
    ...

    @Override
    protected int onProcess(QueuedInputEvent q) {
        ...
        // If the new back dispatch is enabled, intercept KEYCODE_BACK before it reaches the
        // view tree or IME, and invoke the appropriate {@link OnBackInvokedCallback}.
        if (isBack(event) ... ) {
            OnBackInvokedCallback topCallback = getOnBackInvokedDispatcher().getTopCallback(); // check below code
            if (event.getAction() == KeyEvent.ACTION_UP) {
                if (topCallback != null) {
                    topCallback.onBackInvoked();  // <-- Called here
                    return FINISH_HANDLED;
                }
            } 
        }
        ...
    }
  ...
}

// 2.
class WindowOnBackInvokedDispatcher ... {
    ...
    
    // This method is only available in WindowOnBackInvokedDispatcher & it returns the
    // latest `onBackInvokedCallback` registered through `registerOnBackInvokedCallback`.
    public OnBackInvokedCallback getTopCallback() {
        if (mAllCallbacks.isEmpty()) {
            return null;
        }
        for (Integer priority : mOnBackInvokedCallbacks.descendingKeySet()) {
            ArrayList<OnBackInvokedCallback> callbacks = mOnBackInvokedCallbacks.get(priority);
            if (!callbacks.isEmpty()) {
                return callbacks.get(callbacks.size() - 1);
            }
        }
        return null;
    }
    
    ...
}