Initial port of the C++ version of the memory tutorial

A couple of things still need to be done though, including more sharing with
the Rust version, cmake syntax highlighting, externalizing the code.

docs/tutorial/cpp/src/SUMMARY.md

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+# Summary
+
+- [Introduction](./introduction.md)
+- [Getting Started](./getting_started.md)
+- [Memory Tile](./memory_tile.md)
+- [Polishing the Tile](./polishing_the_tile.md)
+- [From One To Multiple Tiles](./from_one_to_multiple_tiles.md)
+- [Creating The Tiles From C++](./creating_the_tiles_from_cpp.md)
+- [Game Logic In C++](./game_logic_in_cpp.md)
+- [Ideas For The Reader](./ideas_for_the_reader.md)
+- [Conclusion](./conclusion.md)

docs/tutorial/cpp/src/conclusion.md

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+# Conclusion
+
+In this tutorial, we have demonstrated how to combine some built-in SixtyFPS elements with C++ code to build a little
+game. There are many more features that we have not talked about, such as layouts, widgets, or styling. Have a look
+at the [examples](https://github.com/sixtyfpsui/sixtyfps/tree/master/examples) in the SixtyFPS repo to
+see how these look like and can be used, such as the [todo example](https://github.com/sixtyfpsui/sixtyfps/tree/master/examples/todo).
+
+A slightly more polished version of this memory puzzle game is [available in the SixtyFPS repository](
+https://github.com/sixtyfpsui/sixtyfps/tree/master/examples/memory). And you can <a href="https://sixtyfps.io/demos/memory/" target="_blank">play the wasm version</a> in your browser.
+

docs/tutorial/cpp/src/creating_the_tiles_from_cpp.md

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+# Creating The Tiles From C++
+
+What we'll do is take the list of tiles declared in the .60 language, duplicate it, and shuffle it.
+We'll do so by accessing the `memory_tiles` property through the Rust code. For each top-level property,
+a getter and a setter function is generated - in our case `get_memory_tiles` and `set_memory_tiles`.
+Since `memory_tiles` is an array in the `.60` language, it is represented as a [`std::shared_ptr<sixtyfps::Model>`](https://sixtyfps.io/docs/cpp/api/classsixtyfps_1_1model).
+We can't modify the model generated by the .60, but we can extract the tiles from it, and put it
+in a [`sixtyfps::VectorModel`](https://sixtyfps.io/docs/cpp/api/classsixtyfps_1_1vectormodel) which inherits from `Model`.
+`VectorModel` allows us to make modifications and we can use it to replace the static generated model.
+
+We modify the main function like so:
+
+```cpp
+ // main.cpp
+#include "memory.h"
+#include &lt;random> // Added
+
+int main()
+{
+    auto main_window = MainWindow::create();
+    auto old_tiles = main_window->get_memory_tiles();
+    std::vector&lt;TileData> new_tiles;
+    new_tiles.reserve(old_tiles->row_count() * 2);
+    for (int i = 0; i &lt; old_tiles->row_count(); ++i) {
+        new_tiles.push_back(old_tiles->row_data(i));
+        new_tiles.push_back(old_tiles->row_data(i));
+    }
+    std::default_random_engine rng{};
+    std::shuffle(new_tiles.begin(), new_tiles.end(), rng);
+    auto tiles_model = std::make_shared&lt;
+        sixtyfps::VectorModel&lt;TileData>>(new_tiles);
+    main_window->set_memory_tiles(tiles_model);
+
+    main_window->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.
+
+<video autoplay loop muted playsinline src="https://sixtyfps.io/blog/memory-game-tutorial/creating-the-tiles-from-rust.mp4"></video>
+

docs/tutorial/cpp/src/from_one_to_multiple_tiles.md

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+# 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 Rust code.
+1. A way of creating many instances of the tiles, with the above `.60` markup code.
+    
+In SixtyFPS 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 `.60` 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.60` file:
+
+```60
+{{#include ../../rust/src/main_multiple_tiles.rs:tile_data}}
+```
+
+Next, we replace the *`MainWindow` := { ... }* section at the bottom of the `memory.60` file with the following snippet:
+
+```60
+{{#include ../../rust/src/main_multiple_tiles.rs:main_window}}
+```
+
+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.
+
+<video autoplay loop muted playsinline src="https://sixtyfps.io/blog/memory-game-tutorial/from-one-to-multiple-tiles.mp4"></video>

docs/tutorial/cpp/src/game_logic_in_cpp.md

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+# Game Logic In C++
+
+We'll implement the rules of the game in C++ as well. The general philosophy of SixtyFPS is that merely the user
+interface is implemented in the `.60` 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 `.60` markup in the `memory.60` 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`:
+
+
+```60
+...
+MainWindow := Window {
+    callback check_if_pair_solved(); // Added
+    property <bool> disable_tiles; // Added
+
+    width: 326px;
+    height: 326px;
+
+    property <[TileData]> memory_tiles: [
+       { image: img!"icons/at.png" },
+...
+```
+
+The last change to the `.60` markup is to act when the `MemoryTile` signals that it was clicked on. We add the following handler:
+
+```60
+...
+MainWindow := Window {
+    ...
+    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 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:
+
+```cpp
+    // ...
+    main_window->on_check_if_pair_solved([main_window_weak =
+            sixtyfps::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)
+                continue;
+            if (first_visible_index == -1) {
+                first_visible_index = i;
+                first_visible_tile = tile;
+                continue;
+            }
+            bool is_pair_solved = tile == first_visible_tile;
+            if (is_pair_solved) {
+                first_visible_tile.solved = true;
+                tiles_model->set_row_data(first_visible_index,
+                                          first_visible_tile);
+                tile.solved = true;
+                tiles_model->set_row_data(i, tile);
+                return;
+            }
+            main_window->set_disable_tiles(true);
+
+            sixtyfps::Timer::single_shot(std::chrono::seconds(1),
+                [=]() mutable {
+                    main_window->set_disable_tiles(false);
+                    first_visible_tile.image_visible = false;
+                    tiles_model->set_row_data(
+                        first_visible_index,
+                        first_visible_tile);
+                    tile.image_visible = false;
+                    tiles_model->set_row_data(i, tile);
+                });
+        }
+    });
+
+    main_window->run();
+```
+
+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.

docs/tutorial/cpp/src/getting_started.md

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+# Getting Started
+
+In this tutorial, we use C++ as the host programming language. We also support other programming languages like
+[Rust](https://sixtyfps.io/docs/rust/sixtyfps/) or [JavaScript](https://sixtyfps.io/docs/node/).
+
+You will need a development environment that can compile C++17 with CMake 3.16.
+We do not provide binaries of SixtyFPS yet, so we will use the CMake integration that will automatically build
+the tools and library from source. Since it is implemented in the Rust programming language, this means that
+you also need to install a Rust compiler (1.48). You can easily install a Rust compiler
+following the instruction from [the Rust website](https://www.rust-lang.org/learn/get-started).
+We are going to use cmake's builtin FetchContent module to fetch the source code of SixtyFPS.
+
+In a new directory, we create a new `CMakeLists.txt` file.
+
+```cmake
+# CMakeLists.txt
+cmake_minimum_required(VERSION 3.16)
+project(memory LANGUAGES CXX)
+
+include(FetchContent)
+FetchContent_Declare(
+    SixtyFPS
+    GIT_REPOSITORY https://github.com/sixtyfpsui/sixtyfps.git
+    GIT_TAG v0.0.6
+    SOURCE_SUBDIR api/sixtyfps-cpp
+)
+FetchContent_MakeAvailable(SixtyFPS)
+
+add_executable(memory_game main.cpp)
+target_link_libraries(memory_game PRIVATE SixtyFPS::SixtyFPS)
+sixtyfps_target_60_sources(memory_game memory.60)
+```
+
+This should look familliar to people familliar with CMake. We see that this CMakeLists.txt
+references a `main.cpp`, which we will add later, and it also has a line
+`sixtyfps_target_60_sources(memory_game memory.60)`, which is a SixtyFPS function used to
+add the `memory.60` file to the target. We must then create, in the same directory,
+the `memory.60` file. Let's just fill it with a hello world for now:
+
+```60
+// memory.60
+MainWindow := Window {
+    Text {
+        text: "hello world";
+        color: green;
+    }
+}
+```
+
+What's still missing is the `main.cpp`:
+
+```cpp
+// main.cpp
+
+#include "memory.h" // generated header from memory.60
+
+int main() {
+    auto main_window = MainWindow::create();
+    main_window->run();
+}
+```
+
+To recap, we now have a directory with a `CMakeLists.txt`, `memory.60` and `main.cpp`.
+
+We can now compile and run this program:
+
+```sh
+cmake -GNinja .
+cmake --build .
+./memory_game
+```
+
+and a window will appear with the green "Hello World" greeting.
+
+![Screenshot of initial tutorial app showing Hello World](https://sixtyfps.io/blog/memory-game-tutorial/getting-started.png "Hello World")
+
+Feel free to use your favorite IDE for this purpose, or use out-of-tree build, or Ninja, ...
+We just keep it simple here for the purpose of this blog.
+
+*Note*: When configuring with CMake, the FetchContent module will fetch the source code of SixtyFPS via git.
+this may take some time. When building for the first time, the first thing that need to be build
+is the SixtyFPS runtime and compiler, this can take a few minutes.

docs/tutorial/cpp/src/icons

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+../../../../examples/memory/icons

docs/tutorial/cpp/src/ideas_for_the_reader.md

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+# 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](https://sixtyfps.io/docs/rust/sixtyfps/docs/builtin_elements/index.html#rectangle)
+  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](https://sixtyfps.io/docs/rust/sixtyfps/docs/builtin_elements/index.html#image)*
+  element. Note that you may have to use *Rectangle*'s *[clip](https://sixtyfps.io/docs/rust/sixtyfps/docs/builtin_elements/index.html#properties-1) 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.
+

docs/tutorial/cpp/src/introduction.md

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+# Introduction
+
+This tutorial will introduce you to the SixtyFPS UI framework in a playful way by implementing a little memory game. We are going to combine the `.60` language for the graphics with the game rules implemented in C++.
+
+Before we start, here's a sneak preview of how the game is going to look when finished:
+
+<video autoplay loop muted playsinline src="https://sixtyfps.io/blog/memory-game-tutorial/memory_clip.mp4"
+        class="img-fluid img-thumbnail rounded"></video>

docs/tutorial/cpp/src/memory_tile.md

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+# 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 is filled with a soothing tone of blue. Note how lengths in the `.60` language have a unit, here
+the `px` suffix. That makes the code easier to read and the compiler can detect when your're accidentally
+mixing values with different units attached to them.
+
+We copy the following code into the `memory.60` file:
+
+```60
+MemoryTile := Rectangle {
+    width: 64px;
+    height: 64px;
+    background: #3960D5;
+
+    Image {
+        source: @image-url("icons/bus.png");
+        width: parent.width;
+        height: parent.height;
+    }
+}
+
+MainWindow := Window {
+    MemoryTile {}
+}
+```
+
+Inside the <span class="hljs-built_in">Rectangle</span> we place an <span class="hljs-built_in">Image</span> element that
+loads an icon with the <span class="hljs-built_in">@image-url()</span> macro. The path is relative to the folder in which
+the `memory.60` is located. This icon and others we're going to use later need to be installed first. You can download a
+[Zip archive](https://sixtyfps.io/blog/memory-game-tutorial/icons.zip) that we have prepared and extract it with the
+following two commands:
+
+```sh
+curl -O https://sixtyfps.io/blog/memory-game-tutorial/icons.zip
+unzip icons.zip
+```
+
+This should unpack an `icons` directory containing a bunch of icons.
+
+We compile the program with `cmake --build .` and running with the `./memory_game` gives us a
+window on the screen that shows the icon of a bus on a blue background.
+    
+![Screenshot of the first tile](https://sixtyfps.io/blog/memory-game-tutorial/memory-tile.png "Memory Tile Screenshot")

docs/tutorial/cpp/src/polishing_the_tile.md

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+# 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 <span class="hljs-built_in">Image</span>, so that they are drawn afterwards and thus on top of the image.
+The <span class="hljs-built_in">TouchArea</span> 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 <em>MainWindow</em>
+that the tile was clicked on. In the <em>MainWindow</em> we react by flipping a custom <em>open_curtain</em> 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.60` file with the following:
+
+```60
+{{#include ../../rust/src/main_polishing_the_tile.rs:tile}}
+```
+
+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.
+
+<video autoplay loop muted playsinline src="https://sixtyfps.io/blog/memory-game-tutorial/polishing-the-tile.mp4"></video>