core: add Null backend to show what implementing a backend looks like

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

build.zig

@@ -277,9 +277,11 @@ pub const Platform = enum {
     wayland,
     web,
     win32,
+    null,
 
     pub fn fromTarget(target: std.Target) Platform {
         if (target.cpu.arch == .wasm32) return .web;
+        if (target.os.tag == .windows) return .win32;
         return .x11;
     }
 };

src/Core.zig

@@ -17,11 +17,9 @@ const Platform = switch (build_options.core_platform) {
     .wayland => @import("core/Wayland.zig"),
     .web => @panic("TODO: revive wasm backend"),
     .win32 => @import("core/win32.zig"),
+    .null => @import("core/Null.zig"),
 };
 
-// TODO(important): mach.core has a lot of standard Zig APIs, and some global variables, which are
-// part of its old API design. We should elevate them into this module instead.
-
 pub const name = .mach_core;
 
 pub const Mod = mach.Mod(@This());

src/core/Null.zig

@@ -0,0 +1,165 @@
+// The Null backend serves no purpose other than to show what the barebones structure of a Mach
+// platform backend looks like.
+
+const std = @import("std");
+const mach = @import("../main.zig");
+const Core = @import("../Core.zig");
+const InputState = @import("InputState.zig");
+const Frequency = @import("Frequency.zig");
+const unicode = @import("unicode.zig");
+const detectBackendType = @import("common.zig").detectBackendType;
+const gpu = mach.gpu;
+const InitOptions = Core.InitOptions;
+const Event = Core.Event;
+const KeyEvent = Core.KeyEvent;
+const MouseButtonEvent = Core.MouseButtonEvent;
+const MouseButton = Core.MouseButton;
+const Size = Core.Size;
+const DisplayMode = Core.DisplayMode;
+const CursorShape = Core.CursorShape;
+const VSyncMode = Core.VSyncMode;
+const CursorMode = Core.CursorMode;
+const Position = Core.Position;
+const Key = Core.Key;
+const KeyMods = Core.KeyMods;
+const Joystick = Core.Joystick;
+
+const log = std.log.scoped(.mach);
+
+const EventQueue = std.fifo.LinearFifo(Event, .Dynamic);
+pub const EventIterator = struct {
+    queue: *EventQueue,
+
+    pub inline fn next(self: *EventIterator) ?Event {
+        return self.queue.readItem();
+    }
+};
+
+pub const Null = @This();
+
+allocator: std.mem.Allocator,
+core: *Core,
+
+events: EventQueue,
+input_state: InputState,
+modifiers: KeyMods,
+
+title: [:0]u8,
+display_mode: DisplayMode,
+vsync_mode: VSyncMode,
+cursor_mode: CursorMode,
+cursor_shape: CursorShape,
+border: bool,
+headless: bool,
+refresh_rate: u32,
+size: Size,
+surface_descriptor: gpu.Surface.Descriptor,
+
+// Called on the main thread
+pub fn init(_: *Null, _: InitOptions) !void {
+    return;
+}
+
+pub fn deinit(_: *Null) void {
+    return;
+}
+
+// Called on the main thread
+pub fn update(_: *Null) !void {
+    return;
+}
+
+// May be called from any thread.
+pub inline fn pollEvents(n: *Null) EventIterator {
+    return EventIterator{ .queue = &n.events };
+}
+
+// May be called from any thread.
+pub fn setTitle(_: *Null, _: [:0]const u8) void {
+    return;
+}
+
+// May be called from any thread.
+pub fn setDisplayMode(_: *Null, _: DisplayMode) void {
+    return;
+}
+
+// May be called from any thread.
+pub fn setBorder(_: *Null, _: bool) void {
+    return;
+}
+
+// May be called from any thread.
+pub fn setHeadless(_: *Null, _: bool) void {
+    return;
+}
+
+// May be called from any thread.
+pub fn setVSync(_: *Null, _: VSyncMode) void {
+    return;
+}
+
+// May be called from any thread.
+pub fn setSize(_: *Null, _: Size) void {
+    return;
+}
+
+// May be called from any thread.
+pub fn size(_: *Null) Size {
+    return Size{ .width = 100, .height = 100 };
+}
+
+// May be called from any thread.
+pub fn setCursorMode(_: *Null, _: CursorMode) void {
+    return;
+}
+
+// May be called from any thread.
+pub fn setCursorShape(_: *Null, _: CursorShape) void {
+    return;
+}
+
+// May be called from any thread.
+pub fn joystickPresent(_: *Null, _: Joystick) bool {
+    return false;
+}
+
+// May be called from any thread.
+pub fn joystickName(_: *Null, _: Joystick) ?[:0]const u8 {
+    return null;
+}
+
+// May be called from any thread.
+pub fn joystickButtons(_: *Null, _: Joystick) ?[]const bool {
+    return null;
+}
+
+// May be called from any thread.
+pub fn joystickAxes(_: *Null, _: Joystick) ?[]const f32 {
+    return null;
+}
+
+// May be called from any thread.
+pub fn keyPressed(_: *Null, _: Key) bool {
+    return false;
+}
+
+// May be called from any thread.
+pub fn keyReleased(_: *Null, _: Key) bool {
+    return true;
+}
+
+// May be called from any thread.
+pub fn mousePressed(_: *Null, _: MouseButton) bool {
+    return false;
+}
+
+// May be called from any thread.
+pub fn mouseReleased(_: *Null, _: MouseButton) bool {
+    return true;
+}
+
+// May be called from any thread.
+pub fn mousePosition(_: *Null) Position {
+    return Position{ .x = 0, .y = 0 };
+}