Slint Memory Game Tutorial (node)

# Introduction

This tutorial will introduce you to the Slint UI framework in a playful way by implementing a little memory game. We're going to combine the `.slint` language for the graphics with the game rules implemented in JavaScript.

The game consists of a grid of 16 rectangular tiles. Clicking on a tile uncovers an icon underneath. We know that there are 8 different icons in total, so each tile has a sibling somewhere in the grid with the same icon. The objective is to locate all icon pairs. You can uncover two tiles at the same time. If they aren't the same, the icons will be obscured again. If you uncover two tiles with the same icon, then they remain visible - they're solved.

This is how the game looks like in action:

# Getting Started

In this tutorial, we use JavaScript as the host programming language. We also support other programming languages like Rust or C++.

You'll need a development environment with Node.js 16 and npm installed. More recent versions of NodeJS are currently not supported, for details check Issue #961. Since Slint is implemented in the Rust programming language, you also need to install a Rust compiler (1.70 or newer). You can easily install a Rust compiler following the instruction from the Rust website. You will also need some additional platform-specific dependencies, see https://github.com/slint-ui/slint/blob/master/docs/building.md#prerequisites

We're going to use `slint-ui` as `npm` dependency.

In a new directory, we create a new `package.json` file.

    {
        "name": "memory",
        "version": "1.3.0",
        "main": "main.js",
        "type": "module",
        "dependencies": {
            "slint-ui": "^1.0.0"
        },
        "scripts": {
            "start": "node ."
        }
    }

This should look familiar to people familiar with NodeJS. We see that this package.json references a `main.js`, which we will add later. We must then create, in the same directory, the `memory.slint` file. Let's just fill it with a hello world for now:

    // memory.slint
    export component MainWindow inherits Window {
        Text {
            text: "hello world";
            color: green;
        }
    }

What's still missing is the `main.js`:

    // main.js
    import * as slint from "slint-ui";

    let ui = slint.loadFile("./memory.slint");
    let mainWindow = new ui.MainWindow();
    mainWindow.run();

To recap, we now have a directory with a `package.json`, `memory.slint`, and `main.js`.

We can now compile and run the program:

    npm install
    npm start

and a window will appear with the green "Hello World" greeting.

Feel free to use your favorite IDE for this purpose.

# Memory Tile

With the skeleton in place, let's look at the first element of the game, the memory tile. It will be the visual building block that consists of an underlying filled rectangle background, the icon image. Later we'll add a covering rectangle that acts as a curtain. The background rectangle is declared to be 64 logical pixels wide and tall, and it's filled with a soothing tone of blue. Note how lengths in the `.slint` language have a unit, here the `px` suffix. That makes the code easier to read and the compiler can detect when your are accidentally mixing values with different units attached to them.

We copy the following code into the `memory.slint` file:

    component MemoryTile inherits Rectangle {
        width: 64px;
        height: 64px;
        background: #3960D5;

        Image {
            source: @image-url("icons/bus.png");
            width: parent.width;
            height: parent.height;
        }
    }

    export component MainWindow inherits Window {
        MemoryTile {}
    }

Note that we export the MainWindow component. This is necessary so that we can later access it from our business logic.

Inside the Rectangle we place an Image element that loads an icon with the @image-url() macro. The path is relative to the folder in which the `memory.slint` is located. This icon and others we're going to use later need to be installed first. You can download a Zip archive that we have prepared.

If you are on Linux or macOS, download and extract it with the following two commands:

    curl -O https://slint.dev/blog/memory-game-tutorial/icons.zip
    unzip icons.zip

If you are on Windows, use the following commands:

    powershell curl -Uri https://slint.dev/blog/memory-game-tutorial/icons.zip -Outfile icons.zip
    powershell Expand-Archive -Path icons.zip -DestinationPath .

This should unpack an `icons` directory containing a bunch of icons.

We running the program with `npm start` and it gives us a window on the screen that shows the icon of a bus on a blue background.

# Polishing the Tile

Next, let's add a curtain like cover that opens up when clicking. We achieve this by declaring two rectangles below the Image, so that they are drawn afterwards and thus on top of the image. The TouchArea element declares a transparent rectangular region that allows reacting to user input such as a mouse click or tap. We use that to forward a callback to the MainWindow that the tile was clicked on. In the MainWindow we react by flipping a custom open_curtain property. That in turn is used in property bindings for the animated width and x properties. Let's look at the two states a bit more in detail:

open_curtain value: | false | true
--- | --- | ---
Left curtain rectangle | Fill the left half by setting the width width to half the parent's width | Width of zero makes the rectangle invisible
Right curtain rectangle | Fill the right half by setting x and width to half of the parent's width | width of zero makes the rectangle invisible. x is moved to the right, to slide the curtain open when animated

In order to make our tile extensible, the hard-coded icon name is replaced with an icon property that can be set from the outside when instantiating the element. For the final polish, we add a solved property that we use to animate the color to a shade of green when we've found a pair, later. We replace the code inside the `memory.slint` file with the following:

    component MemoryTile inherits Rectangle {
        callback clicked;
        in property <bool> open_curtain;
        in property <bool> solved;
        in property <image> icon;

        height: 64px;
        width: 64px;
        background: solved ? #34CE57 : #3960D5;
        animate background { duration: 800ms; }

        Image {
            source: icon;
            width: parent.width;
            height: parent.height;
        }

        // Left curtain
        Rectangle {
            background: #193076;
            x: 0px;
            width: open_curtain ? 0px : (parent.width / 2);
            height: parent.height;
            animate width { duration: 250ms; easing: ease-in; }
        }

        // Right curtain
        Rectangle {
            background: #193076;
            x: open_curtain ? parent.width : (parent.width / 2);
            width: open_curtain ? 0px : (parent.width / 2);
            height: parent.height;
            animate width { duration: 250ms; easing: ease-in; }
            animate x { duration: 250ms; easing: ease-in; }
        }

        TouchArea {
            clicked => {
                // Delegate to the user of this element
                root.clicked();
            }
        }
    }

    export component MainWindow inherits Window {
        MemoryTile {
            icon: @image-url("icons/bus.png");
            clicked => {
                self.open_curtain = !self.open_curtain;
            }
        }
    }

Note the use of `root` and `self` in the code. `root` refers to the outermost element in the component, that's the MemoryTile in this case. `self` refers to the current element.

Running this gives us a window on the screen with a rectangle that opens up to show us the bus icon, when clicking on it. Subsequent clicks will close and open the curtain again.

# From One To Multiple Tiles

After modeling a single tile, let's create a grid of them. For the grid to be our game board, we need two features:

1. A data model: This shall be an array where each element describes the tile data structure, such as the url of the image, whether the image shall be visible and if this tile has been solved. We modify the model from JS code.
2. A way of creating many instances of the tiles, with the above `.slint` markup code.

In Slint we can declare an array of structures using brackets, to create a model. We can use the for loop to create many instances of the same element. In `.slint` the for loop is declarative and automatically updates when the model changes. We instantiate all the different MemoryTile elements and place them on a grid based on their index with a little bit of spacing between the tiles.

First, we copy the tile data structure definition and paste it at top inside the `memory.slint` file:

    // Added:
    struct TileData {
        image: image,
        image_visible: bool,
        solved: bool,
    }

    component MemoryTile inherits Rectangle {

Next, we replace the export component MainWindow inherits Window { ... } section at the bottom of the `memory.slint` file with the following snippet:

    export component MainWindow inherits Window {
        width: 326px;
        height: 326px;

        in property <[TileData]> memory_tiles: [
            { image: @image-url("icons/at.png") },
            { image: @image-url("icons/balance-scale.png") },
            { image: @image-url("icons/bicycle.png") },
            { image: @image-url("icons/bus.png") },
            { image: @image-url("icons/cloud.png") },
            { image: @image-url("icons/cogs.png") },
            { image: @image-url("icons/motorcycle.png") },
            { image: @image-url("icons/video.png") },
        ];
        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 => {
                tile.image_visible = !tile.image_visible;
            }
        }
    }

The `for tile[i] in memory_tiles:` syntax declares a variable `tile` which contains the data of one element from the `memory_tiles` array, and a variable `i` which is the index of the tile. We use the `i` index to calculate the position of tile based on its row and column, using the modulo and integer division to create a 4 by 4 grid.

Running this gives us a window that shows 8 tiles, which can be opened individually.

# Creating The Tiles From JavaScript

What we'll do is take the list of tiles declared in the .slint language, duplicate it, and shuffle it. We'll do so by accessing the `memory_tiles` property through the JavaScript code - in our case `memory_tiles`. Since `memory_tiles` is an array in the `.slint` language, it's represented as a JavaScript `Array`. We can't modify the model generated by the .slint, but we can extract the tiles from it, and put it in a `slint.ArrayModel` which implements the `Model` interface. `ArrayModel` allows us to make modifications and we can use it to replace the static generated model.

We modify the main function like so:

    // main.js
    import * as slint from "slint-ui";
    let ui = slint.loadFile("./memory.slint");
    let mainWindow = new ui.MainWindow();

    let initial_tiles = mainWindow.memory_tiles;
    let tiles = initial_tiles.concat(initial_tiles.map((tile) => Object.assign({}, tile)));

    for (let i = tiles.length - 1; i > 0; i--) {
        const j = Math.floor(Math.random() * i);
        [tiles[i], tiles[j]] = [tiles[j], tiles[i]];
    }

    let model = new slint.ArrayModel(tiles);
    mainWindow.memory_tiles = model;

    mainWindow.run();

Running this gives us a window on the screen that now shows a 4 by 4 grid of rectangles, which can show or obscure the icons when clicking. There's only one last aspect missing now, the rules for the game.

# Game Logic In JavaScript

We'll implement the rules of the game in JavaScript 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 JavaScript 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 JavaScript code that it should check if a pair of tiles has been solved. And we need to add a property that JavaScript 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:

        export component MainWindow inherits Window {
            width: 326px;
            height: 326px;

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

            in-out 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;
                        root.check_if_pair_solved();
                    }
                }
            }

On the JavaScript 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 can't click anything during this time.

Insert this code before the `mainWindow.run()` call:

    let model = new slint.ArrayModel(tiles);
    mainWindow.memory_tiles = model;

    mainWindow.check_if_pair_solved = function () {
        let flipped_tiles = [];
        tiles.forEach((tile, index) => {
            if (tile.image_visible && !tile.solved) {
                flipped_tiles.push({
                    index,
                    tile
                });
            }
        });

        if (flipped_tiles.length == 2) {
            let {
                tile: tile1,
                index: tile1_index
            } = flipped_tiles[0];

            let {
                tile: tile2,
                index: tile2_index
            } = flipped_tiles[1];

            let is_pair_solved = tile1.image.path === tile2.image.path;
            if (is_pair_solved) {
                tile1.solved = true;
                model.setRowData(tile1_index, tile1);
                tile2.solved = true;
                model.setRowData(tile2_index, tile2);
            } else {
                mainWindow.disable_tiles = true;
                setTimeout(() => {
                    mainWindow.disable_tiles = false;
                    tile1.image_visible = false;
                    model.setRowData(tile1_index, tile1);
                    tile2.image_visible = false;
                    model.setRowData(tile2_index, tile2);
                }, 1000)

            }
        }
    };

    mainWindow.run();

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.

# Ideas For The Reader

The game is visually a little bare. Here are some ideas how you could make further changes to enhance it:

- The tiles could have rounded corners, to look a little less sharp. The border-radius property of Rectangle can be used to achieve that.

- In real world memory games, the back of the tiles often have some common graphic. You could add an image with the help of another Image element. Note that you may have to use Rectangle's clip property element around it to ensure that the image is clipped away when the curtain effect opens.

Let us know in the comments on Github Discussions how you polished your code, or feel free to ask questions about how to implement something.

# Conclusion

In this tutorial, we have demonstrated how to combine some built-in Slint elements with JavaScript code to build a little game. There are many more features that we haven't talked about, such as layouts, widgets, or styling.

We recommend the following links to continue:

- Examples: In the Slint repository we have collected a few demos and examples. These are a great starting point to learn how to use many Slint features.
  - Todo Example: This is one of the examples that implements a classic use-case.
  - Memory Puzzle: This is a slightly more polished version of the code in this example. And you can play the wasm version in your browser.
- Slint API Docs: The reference documentation for the NodeJS library.