examples: add initial ecs-app

This is a temporary application to begin iterating on high-level ECS applications.

Eventually, this will be removed - for now it's just here so we can see how this API
looks today and improve it.

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

examples/ecs-app/main.zig

@@ -0,0 +1,54 @@
+// Experimental ECS app example. Not yet ready for actual use.
+
+const std = @import("std");
+const mach = @import("mach");
+const gpu = mach.gpu;
+const ecs = mach.ecs;
+
+// TODO: rename *ecs.World to *engine.Engine or something
+
+const renderer = @import("renderer.zig");
+const physics2d = @import("physics2d.zig");
+
+// Define all the modules in our application. Modules can have components, systems, state,
+// and/or global values in them. They can also send and receive messages to coordinate
+// with each-other.
+//
+// Single-word module names (`.mach`, `.renderer`, etc.) are reserved for the application itself.
+//
+// Modules that come from libraries must be prefixed (e.g. `.bullet_physics`, `.ziglibs_box2d`)
+// similar to GitHub repositories, to avoid conflicts with one another. Note that modules themselves
+// will interact with the ECS using e.g. `.getComponent(.bullet_physics, .location)` internally and
+// so cannot be renamed here.
+// TODO: just make this a list so one cannot even think renaming is possible here
+const modules = ecs.Modules(.{
+    .mach = mach.module,
+    .renderer = renderer.module,
+    .physics2d = physics2d.module,
+});
+
+// Our Mach app, which tells Mach where to find our modules and init entry point.
+pub const App = mach.App(modules, init);
+
+pub fn init(engine: *ecs.World(modules)) !void {
+    // The Mach .core is where we set window options, etc.
+    const core = engine.get(.mach, .core);
+    try core.setOptions(.{ .title = "Hello, ECS!" });
+
+    // We can get the GPU device:
+    const device = engine.get(.mach, .device);
+    _ = device; // TODO: actually show off using the GPU device
+
+    // We can create entities, and set components on them. Note that components live in a module
+    // namespace, so we set the `.renderer, .location` component which is different than the
+    // `.physics2d, .location` component.
+
+    // TODO: cut out the `.entities.` in this API to make it more brief
+    const player = try engine.entities.new();
+    try engine.entities.setComponent(player, .renderer, .location, .{ .x = 0, .y = 0, .z = 0 });
+    try engine.entities.setComponent(player, .physics2d, .location, .{ .x = 0, .y = 0 });
+    _ = player;
+
+    // TODO: there could be an entities wrapper to interact with a single namespace so you don't
+    // have to pass it in as a parameter always?
+}