diff --git a/src/browser/Scheduler.zig b/src/browser/Scheduler.zig
index 63632fc4..41ebfbde 100644
--- a/src/browser/Scheduler.zig
+++ b/src/browser/Scheduler.zig
@@ -43,59 +43,127 @@ pub fn init(allocator: std.mem.Allocator) Scheduler {
 }
 
 pub fn deinit(self: *Scheduler) void {
-    self.task_pool.deinit();
+    // Release memory pool after finalization.
+    defer self.task_pool.deinit();
+
+    // Finalize all low-prio tasks.
+    while (self.low_priority.deleteMin()) |task| {
+        if (task.on_finalize) |on_finalize| {
+            @call(.auto, on_finalize, .{ task, self });
+        }
+    }
+
+    // Finalize all high-prio tasks.
+    while (self.high_priority.deleteMin()) |task| {
+        if (task.on_finalize) |on_finalize| {
+            @call(.auto, on_finalize, .{ task, self });
+        }
+    }
 }
 
+/// Scheduled tasks must satisfy this interface.
+const ScheduleInterface = struct {
+    const T = type;
+
+    /// Must be implemented if scheduled with `Scheduler.once`.
+    fn onRun(ctx: *T, scheduler: *Scheduler) !void {
+        _ = ctx;
+        _ = scheduler;
+    }
+
+    /// Must be implemented if scheduled with `Scheduler.after`.
+    fn onReady(ctx: *T, scheduler: *Scheduler) !AfterAction {
+        _ = ctx;
+        _ = scheduler;
+    }
+
+    /// Optional, should be implemented if task needs to do some cleanup.
+    fn onFinalize(ctx: *T) void {
+        _ = ctx;
+    }
+};
+
 pub const Priority = enum(u1) { low, high };
 
 pub const ScheduleOptions = struct {
     name: []const u8 = "unspecified",
-    priority: Priority = .high,
-    // TODO: Backport finalizer.
+    prio: Priority,
 };
 
-/// Schedules a task; runs it once.
+// scheduler.once(
+//     .{ .name = "my-event", .priority = .high },
+//     MyType,
+//     &my_type,
+//     struct {
+//         fn action(my_type: *MyType, scheduler: *Scheduler) !void {
+//             // action taken logic...
+//         }
+//
+//         fn finalize(my_type: *MyType) void {
+//             // finalize logic...
+//         }
+//     },
+// );
+
+/// Schedules a task that'll be executed in the next run.
 pub fn once(
     self: *Scheduler,
     comptime options: ScheduleOptions,
     /// Type of `ctx`.
     comptime T: type,
     ctx: *T,
-    comptime callback: *const fn (scheduler: *Scheduler, ctx: *T) anyerror!void,
+    /// See `Scheduler.ScheduleInterface` (private type) for reference.
+    comptime Interface: anytype,
 ) !void {
     if (comptime IS_DEBUG) {
-        log.debug(.scheduler, "Scheduler.once", .{ .name = options.name, .priority = @tagName(options.priority) });
+        log.debug(.scheduler, "Scheduler.once", .{ .name = options.name, .prio = @tagName(options.prio) });
     }
 
-    const runner = struct {
+    const action_runner = struct {
         fn runner(task: *Task, scheduler: *Scheduler) void {
+            // Return task back to pool once done.
+            defer scheduler.task_pool.destroy(task);
             // Give type-erased ctx a type.
             const typed_ctx: *T = @ptrCast(@alignCast(task.ctx));
 
-            // Run provided callback; this can fail, we'll ignore it though.
-            @call(.auto, callback, .{ scheduler, typed_ctx }) catch |err| {
-                log.warn(.scheduler, "Task.callback", .{
+            // Run provided callback; this can fail, we won't handle it though.
+            @call(.auto, Interface.action, .{ scheduler, typed_ctx }) catch |err| {
+                log.warn(.scheduler, "Task.action", .{
                     .name = options.name,
-                    .priority = @tagName(options.priority),
+                    .priority = @tagName(options.prio),
                     .err = err,
                 });
             };
-
-            // Return task back to pool.
-            scheduler.task_pool.destroy(task);
         }
     }.runner;
 
+    // Finalizer, if provided.
+    const finalize_runner = blk: {
+        if (!std.meta.hasFn(Interface, "finalize")) {
+            break :blk null;
+        }
+
+        break :blk (struct {
+            fn runner(task: *Task, scheduler: *Scheduler) void {
+                defer scheduler.task_pool.destroy(task);
+                const typed_ctx: *T = @ptrCast(@alignCast(task.ctx));
+
+                @call(.always_inline, Interface.finalize, .{typed_ctx});
+            }
+        }).runner;
+    };
+
     const task = try self.task_pool.create();
     task.* = .{
         // This variant always have 0-timeout.
         .run_at = milliTimestamp(.monotonic),
         .ctx = ctx,
-        .callback = runner,
+        .on_action = action_runner,
+        .on_finalize = finalize_runner,
     };
 
     // Push to right heap.
-    switch (options.priority) {
+    switch (options.prio) {
         .low => self.low_priority.insert(task),
         .high => self.high_priority.insert(task),
     }
@@ -129,34 +197,35 @@ pub fn after(
     comptime T: type,
     ctx: *T,
     run_in_ms: u32,
-    /// If an integer is returned, the task will be repeated after that much ms.
-    /// If null is returned, task won't be repeated.
-    comptime callback: *const fn (scheduler: *Scheduler, ctx: *T) anyerror!AfterAction,
+    /// See `Scheduler.ScheduleInterface` (private type) for reference.
+    comptime Interface: anytype,
+    // If an integer is returned, the task will be repeated after that much ms.
+    // If null is returned, task won't be repeated.
+    //comptime callback: *const fn (scheduler: *Scheduler, ctx: *T) anyerror!AfterAction,
 ) !void {
     if (comptime IS_DEBUG) {
         log.debug(.scheduler, "Scheduler.after", .{
             .name = options.name,
             .run_in_ms = run_in_ms,
-            .priority = @tagName(options.priority),
+            .priority = @tagName(options.prio),
         });
     }
 
-    const runner = struct {
+    const action_runner = struct {
         fn runner(task: *Task, scheduler: *Scheduler) void {
             // Give type-erased ctx a type.
             const typed_ctx: *T = @ptrCast(@alignCast(task.ctx));
 
-            // Run provided callback; this can fail, we'll ignore it though.
-            const result = @call(.auto, callback, .{ scheduler, typed_ctx }) catch |err| {
-                log.warn(.scheduler, "Task.callback", .{
+            // Run provided callback; this can fail, we won't handle it though.
+            const result = @call(.auto, Interface.action, .{ scheduler, typed_ctx }) catch |err| blk: {
+                log.warn(.scheduler, "Task.action", .{
                     .name = options.name,
-                    .priority = @tagName(options.priority),
+                    .priority = @tagName(options.prio),
                     .err = err,
                 });
 
                 // Can't repeat w/o return value.
-                scheduler.task_pool.destroy(task);
-                return;
+                break :blk AfterAction.dont_repeat;
             };
 
             // If task is not repeated, disarm.
@@ -168,22 +237,39 @@ pub fn after(
             // Wants recur.
             const repeat_in_ms = result.ms;
             // Task cannot be repeated immediately, and they should know that.
-            lp.assert(repeat_in_ms != 0, "Task.callback: 0-timer", .{ .name = options.name });
+            lp.assert(repeat_in_ms != 0, "Task.action: 0-timer", .{ .name = options.name });
             task.run_at = milliTimestamp(.monotonic) + repeat_in_ms;
             // Prefer low priority?
             scheduler.low_priority.insert(task);
         }
     }.runner;
 
+    // Finalizer, if provided.
+    const finalize_runner = blk: {
+        if (!std.meta.hasFn(Interface, "finalize")) {
+            break :blk null;
+        }
+
+        break :blk (struct {
+            fn runner(task: *Task, scheduler: *Scheduler) void {
+                defer scheduler.task_pool.destroy(task);
+                const typed_ctx: *T = @ptrCast(@alignCast(task.ctx));
+
+                @call(.always_inline, Interface.finalize, .{typed_ctx});
+            }
+        }).runner;
+    };
+
     const task = try self.task_pool.create();
     task.* = .{
         .run_at = milliTimestamp(.monotonic) + run_in_ms,
         .ctx = ctx,
-        .callback = runner,
+        .on_action = action_runner,
+        .on_finalize = finalize_runner,
     };
 
     // Push to right heap.
-    switch (options.priority) {
+    switch (options.prio) {
         .low => self.low_priority.insert(task),
         .high => self.high_priority.insert(task),
     }
@@ -196,14 +282,8 @@ pub fn run(self: *Scheduler) !?u64 {
 
 /// Runs events of the desired tree.
 fn runTasks(self: *Scheduler, comptime prio: Priority) if (prio == .low) void else ?u64 {
-    const tree = if (comptime prio == .low) &self.low_priority else &self.high_priority;
-
-    // const cached_time = self.cached_time;
-    // const now = milliTimestamp(.monotonic);
-    // Update cache...
-    // self.cached_time = now;
-
     const now = milliTimestamp(.monotonic);
+    const tree = if (comptime prio == .low) &self.low_priority else &self.high_priority;
 
     while (tree.peek()) |task| {
         // No tasks to execute so far.
@@ -223,7 +303,7 @@ fn runTasks(self: *Scheduler, comptime prio: Priority) if (prio == .low) void el
             log.debug(.scheduler, "Scheduler.runTasks", .{ .prio = @tagName(prio) });
         }
 
-        task.callback(task, self);
+        @call(.auto, task.on_action, .{ task, self });
     }
 
     if (comptime prio == .high) {
@@ -237,7 +317,9 @@ const Task = struct {
     run_at: u64,
     /// Userdata.
     ctx: *anyopaque,
-    callback: *const fn (task: *Task, scheduler: *Scheduler) void,
+    on_action: *const fn (task: *Task, scheduler: *Scheduler) void,
+    on_finalize: ?*const fn (task: *Task, scheduler: *Scheduler) void = null,
+    /// Entry in `Heap`.
     heap: heap.IntrusiveField(Task) = .{},
 
     const Heap = heap.Intrusive(Task, void, Task.less);