Game Logic In C++

We'll implement the rules of the game in C++ as well. The general philosophy of Slint is that merely the user interface is implemented in the .slint language and the business logic in your favorite programming language. The game rules shall enforce that at most two tiles have their curtain open. If the tiles match, then we consider them solved and they remain open. Otherwise we wait for a little while, so the player can memorize the location of the icons, and then close them again.

We'll modify the .slint markup in the memory.slint file to signal to the C++ code when the user clicks on a tile. Two changes to MainWindow are needed: We need to add a way for the MainWindow to call to the C++ code that it should check if a pair of tiles has been solved. And we need to add a property that C++ code can toggle to disable further tile interaction, to prevent the player from opening more tiles than allowed. No cheating allowed! First, we paste the callback and property declarations into MainWindow:

    MainWindow := Window {
        width: 326px;
        height: 326px;

        callback check_if_pair_solved(); // Added
        property <bool> disable_tiles; // Added

        property <[TileData]> memory_tiles: [
           { image: @image-url("icons/at.png") },

The last change to the .slint markup is to act when the MemoryTile signals that it was clicked on. We add the following handler in MainWindow:

        for tile[i] in memory_tiles : MemoryTile {
            x: mod(i, 4) * 74px;
            y: floor(i / 4) * 74px;
            width: 64px;
            height: 64px;
            icon: tile.image;
            open_curtain: tile.image_visible || tile.solved;
            // propagate the solved status from the model to the tile
            solved: tile.solved;
            clicked => {
                // old: tile.image_visible = !tile.image_visible;
                // new:
                if (!root.disable_tiles) {
                    tile.image_visible = !tile.image_visible;

On the C++ side, we can now add an handler to the check_if_pair_solved callback, that will check if two tiles are opened. If they match, the solved property is set to true in the model. If they don't match, start a timer that will close them after one second. While the timer is running, we disable every tile so one cannot click anything during this time.

Insert this code before the main_window->run() call:

    auto tiles_model = std::make_shared<slint::VectorModel<TileData>>(new_tiles);

            [main_window_weak = slint::ComponentWeakHandle(main_window)] {
                auto main_window = *main_window_weak.lock();
                auto tiles_model = main_window->get_memory_tiles();
                int first_visible_index = -1;
                TileData first_visible_tile;
                for (int i = 0; i < tiles_model->row_count(); ++i) {
                    auto tile = *tiles_model->row_data(i);
                    if (!tile.image_visible || tile.solved)
                    if (first_visible_index == -1) {
                        first_visible_index = i;
                        first_visible_tile = tile;
                    bool is_pair_solved = tile == first_visible_tile;
                    if (is_pair_solved) {
                        first_visible_tile.solved = true;
                        tile.solved = true;
                        tiles_model->set_row_data(i, tile);

                        [=]() mutable {
                            first_visible_tile.image_visible = false;
                            tile.image_visible = false;
                            tiles_model->set_row_data(i, tile);


Notice that we take a weak pointer of our main_window. This is very important because capturing a copy of the main_window itself within the callback handler would result in a circular ownership. The MainWindow owns the callback handler, which itself owns a reference to the MainWindow, which must be weak instead of strong to avoid a memory leak.

These were the last changes and running the result gives us a window on the screen that allows us to play the game by the rules.