Building a Gantt Chart That Beats Notion's Timeline View — Flutter Web + Supabase

Why Compete With Notion's Timeline

Notion's Timeline view is powerful but has two problems:

1. It's paid-only (Team plan+)
2. It has no critical path analysis

I'm building 自分株式会社 — an app that replaces 21 SaaS tools including Notion — so I need a Gantt chart that's free and has features Notion doesn't.

The 3-Tab Architecture

GanttTimelinePage is organized into three tabs:

1. Projects — create and select projects
2. Timeline — visual Gantt bars for tasks and milestones
3. Critical Path — ranking of tasks by latest completion date

Backend: Reusing app_analytics as a Generic Storage Table

Rather than creating a gantt_tasks table, I store Gantt data in the existing app_analytics table using source as a discriminator:

// POST /functions/v1/gantt-timeline-manager (action: "add_task")
const { error } = await adminClient.from("app_analytics").insert({
  user_id: user.id,
  source: "gantt_task",        // discriminator
  metadata: {
    task_id: crypto.randomUUID(),
    project_id,
    name,
    duration_days,
    start_date,
    status: "pending",
    progress: 0,
  },
});

Querying tasks:

const { data } = await adminClient
  .from("app_analytics")
  .select("*")
  .eq("user_id", user.id)
  .eq("source", "gantt_task")
  .contains("metadata", { project_id });

Why this works: app_analytics already has user_id, source, and metadata (JSONB). For feature-specific data that doesn't need complex relational queries, this avoids schema migration and new table overhead.

Frontend: Gantt Bars with LinearProgressIndicator

Flutter's LinearProgressIndicator doubles as a Gantt bar with minimal code:

ClipRRect(
  borderRadius: BorderRadius.circular(4),
  child: LinearProgressIndicator(
    value: progress.clamp(0.0, 1.0),
    minHeight: 12,
    backgroundColor: Colors.grey.shade200,
    valueColor: AlwaysStoppedAnimation<Color>(_statusColor(status)),
  ),
),

Status colors:

Color _statusColor(String status) => switch (status) {
  'done'        => Colors.green,
  'in_progress' => Colors.blue,
  'blocked'     => Colors.red,
  _             => Colors.grey,
};

The ClipRRect rounds the corners; minHeight: 12 gives it enough visual weight to read at a glance.

Critical Path Analysis in the Edge Function

The critical path identifies which tasks, if delayed, would delay the entire project. My implementation uses the simplest valid definition: tasks sorted by latest end date.

const endDates = [];
for (const [taskId, task] of taskMap) {
  const start = new Date(task.start_date ?? Date.now());
  const duration = task.duration_days ?? 1;
  const end = new Date(start.getTime() + duration * 86400000);
  endDates.push({
    taskId,
    name: task.name,
    endDate: end.toISOString(),
    duration,
  });
}
// Sort: latest end date first = most critical
endDates.sort(
  (a, b) => new Date(b.endDate).getTime() - new Date(a.endDate).getTime()
);

This runs server-side in the Edge Function, not in the Flutter client. Keeps the client code thin and makes the logic testable.

The avoid_dynamic_calls Trap

supabase.functions.invoke() returns dynamic. Direct property access triggers the avoid_dynamic_calls lint rule:

// FAILS flutter analyze
_tasks = (taskRes.data?['tasks'] as List?) ?? [];

// PASSES: cast to Map first
final taskData = taskRes.data as Map<String, dynamic>?;
_tasks = (taskData?['tasks'] as List?) ?? [];

Always cast FunctionResponse.data to Map<String, dynamic>? before accessing properties. One extra line prevents a class of runtime errors.

Feature Comparison vs. Notion

The critical path tab is the differentiator. Notion shows you when tasks are scheduled; we show you which tasks you can't afford to slip.

Key Takeaways

1. app_analytics + source discriminator avoids schema migrations for feature-specific data
2. LinearProgressIndicator as Gantt bar — Flutter's built-in widgets go further than you'd expect
3. Critical path server-side — move sorting/analysis to the Edge Function, keep the client rendering only
4. Always cast dynamic from EF responses — avoid_dynamic_calls is enforced by flutter analyze

Try it: 自分株式会社

buildinpublic #Flutter #Supabase #projectmanagement #webdev