make it clear how to use module system without mach.Core (remove mach.App)

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

examples/custom-renderer/App.zig

@@ -22,6 +22,7 @@ pub const components = .{
 };
 
 pub const systems = .{
+    .start = .{ .handler = start },
     .init = .{ .handler = init },
     .deinit = .{ .handler = deinit },
     .tick = .{ .handler = tick },
@@ -42,6 +43,16 @@ pub fn deinit(core: *mach.Core.Mod, renderer: *Renderer.Mod) void {
     core.schedule(.deinit);
 }
 
+fn start(
+    core: *mach.Core.Mod,
+    renderer: *Renderer.Mod,
+    app: *Mod,
+) !void {
+    core.schedule(.init);
+    renderer.schedule(.init);
+    app.schedule(.init);
+}
+
 fn init(
     // These are injected dependencies - as long as these modules were registered in the top-level
     // of the program we can have these types injected here, letting us work with other modules in
@@ -49,10 +60,10 @@ fn init(
     entities: *mach.Entities.Mod,
     core: *mach.Core.Mod,
     renderer: *Renderer.Mod,
-    game: *Mod,
+    app: *Mod,
 ) !void {
-    core.schedule(.init);
-    renderer.schedule(.init);
+    core.state().on_tick = app.system(.tick);
+    core.state().on_exit = app.system(.deinit);
 
     // Create our player entity.
     const player = try entities.new();
@@ -68,9 +79,9 @@ fn init(
     try renderer.set(player, .scale, 1.0);
 
     // Initialize our game module's state - these are the struct fields defined at the top of this
-    // file. If this is not done, then game.state() will panic indicating the state was never
+    // file. If this is not done, then app.state() will panic indicating the state was never
     // initialized.
-    game.init(.{
+    app.init(.{
         .timer = try mach.Timer.start(),
         .spawn_timer = try mach.Timer.start(),
         .player = player,
@@ -83,11 +94,11 @@ fn tick(
     entities: *mach.Entities.Mod,
     core: *mach.Core.Mod,
     renderer: *Renderer.Mod,
-    game: *Mod,
+    app: *Mod,
 ) !void {
     var iter = core.state().pollEvents();
-    var direction = game.state().direction;
-    var spawning = game.state().spawning;
+    var direction = app.state().direction;
+    var spawning = app.state().spawning;
     while (iter.next()) |event| {
         switch (event) {
             .key_press => |ev| {
@@ -118,18 +129,18 @@ fn tick(
     // Keep track of which direction we want the player to move based on input, and whether we want
     // to be spawning entities.
     //
-    // Note that game.state() simply returns a pointer to a global singleton of the struct defined
+    // Note that app.state() simply returns a pointer to a global singleton of the struct defined
     // by this file, so we can access fields defined at the top of this file.
-    game.state().direction = direction;
-    game.state().spawning = spawning;
+    app.state().direction = direction;
+    app.state().spawning = spawning;
 
     // Get the current player position
-    var player_pos = renderer.get(game.state().player, .position).?;
+    var player_pos = renderer.get(app.state().player, .position).?;
 
     // If we want to spawn new entities, then spawn them now. The timer just makes spawning rate
     // independent of frame rate.
-    if (spawning and game.state().spawn_timer.read() > 1.0 / 60.0) {
-        _ = game.state().spawn_timer.lap(); // Reset the timer
+    if (spawning and app.state().spawn_timer.read() > 1.0 / 60.0) {
+        _ = app.state().spawn_timer.lap(); // Reset the timer
         for (0..5) |_| {
             // Spawn a new entity at the same position as the player, but smaller in scale.
             const new_entity = try entities.new();
@@ -137,19 +148,19 @@ fn tick(
             try renderer.set(new_entity, .scale, 1.0 / 6.0);
 
             // Tag the entity as one that follows the player
-            try game.set(new_entity, .follower, {});
+            try app.set(new_entity, .follower, {});
         }
     }
 
     // Multiply by delta_time to ensure that movement is the same speed regardless of the frame rate.
-    const delta_time = game.state().timer.lap();
+    const delta_time = app.state().timer.lap();
 
     // Calculate the player position, by moving in the direction the player wants to go
     // by the speed amount.
     const speed = 1.0;
     player_pos.v[0] += direction.x() * speed * delta_time;
     player_pos.v[1] += direction.y() * speed * delta_time;
-    try renderer.set(game.state().player, .position, player_pos);
+    try renderer.set(app.state().player, .position, player_pos);
 
     // Query all the entities that have the .follower tag indicating they should follow the player.
     // TODO(important): better querying API