add window.onload getter and setter

src/browser/html/window.zig

@@ -35,6 +35,7 @@ const CSSStyleDeclaration = @import("../cssom/CSSStyleDeclaration.zig");
 const Screen = @import("screen.zig").Screen;
 const domcss = @import("../dom/css.zig");
 const Css = @import("../css/css.zig").Css;
+const EventHandler = @import("../events/event.zig").EventHandler;
 
 const Function = Env.Function;
 
@@ -67,6 +68,7 @@ pub const Window = struct {
     performance: Performance,
     screen: Screen = .{},
     css: Css = .{},
+    onload_callback: ?Function = null,
 
     pub fn create(target: ?[]const u8, navigator: ?Navigator) !Window {
         var fbs = std.io.fixedBufferStream("");
@@ -101,6 +103,37 @@ pub const Window = struct {
         return fetchFn(input, options, page);
     }
 
+    /// Returns `onload_callback`.
+    pub fn get_onload(self: *const Window) ?Function {
+        return self.onload_callback;
+    }
+
+    /// Sets `onload_callback`.
+    pub fn set_onload(self: *Window, maybe_listener: ?EventHandler.Listener, page: *Page) !void {
+        const event_target = parser.toEventTarget(Window, self);
+
+        if (self.onload_callback) |callback| {
+            // If we got here, it means `onload_callback` has been set before
+            // so listener cannot be null.
+            const listener = try parser.eventTargetHasListener(event_target, "load", false, callback.id);
+            std.debug.assert(listener != null);
+
+            try parser.eventTargetRemoveEventListener(event_target, "load", listener.?, false);
+        }
+
+        if (maybe_listener) |listener| {
+            // The only time this can return null if the listener is already
+            // registered. But before calling `register`, all of our functions
+            // remove any existing listener, so it should be impossible to get null
+            // from this function call.
+            _ = try EventHandler.register(page.arena, event_target, "load", listener, null) orelse unreachable;
+            self.onload_callback = listener.function;
+        } else {
+            // Just unset the listener.
+            self.onload_callback = null;
+        }
+    }
+
     pub fn get_window(self: *Window) *Window {
         return self;
     }