Method onPressed

src/fl/ui/button.h:94–98

    }

    int onPressed(function<void()> callback) FL_NOEXCEPT {
        int id = mPressCallbacks.add(callback);
        mListener.addToEngineEventsOnce();
        return id;
    }

    int onReleased(function<void()> callback) FL_NOEXCEPT {
        int id = mReleaseCallbacks.add(callback);