plan-ui-change

# Plan a Blazor UI Change

When asked to build a complex UI feature, **plan the component decomposition first, then immediately implement it**. A single monolithic page component is almost never the right answer — break the UI into focused, composable components.

## Planning Workflow

### Step 1 — Map the Visual Regions

Read the request and identify every distinct visual region. Each region that has its own data, behavior, or layout responsibility is a candidate component.

Draw the component tree:

```
InventoryDashboard          (page — owns data, orchestrates layout)
├── StockSummaryBar         (read-only stats: total items, low-stock count, value)
├── InventoryFilters        (search box, category dropdown, stock-level toggle)
├── InventoryTable          (sortable table of products)
│   └── InventoryRow        (single product row with inline edit/delete)
└── AddProductForm          (slide-out form for new products)
```

Rules for identifying components:
- **Distinct responsibility** — a region owns its own state or behavior → separate component
- **Repeated structure** — items in a list, cards in a grid → extract the item template
- **Independent interactivity** — a section that handles user input separately from its siblings → separate component
- **Size** — any section that would exceed ~150 lines of markup on its own → split it

### Step 2 — Classify Each Component

For every component in the tree, determine:

| Component | Action | Render Mode | State Owned | Lines (est.) |
|-----------|--------|-------------|-------------|-------------|
| InventoryDashboard | Create | InteractiveServer | product list, filter state | ~80 |
| StockSummaryBar | Create | (inherits) | none — receives data | ~30 |
| InventoryFilters | Create | (inherits) | search text, selected category | ~60 |
| InventoryTable | Create | (inherits) | sort column, sort direction | ~50 |
| InventoryRow | Create | (inherits) | inline-edit mode flag | ~60 |
| AddProductForm | Create | (inherits) | form model | ~80 |

**A page component that exceeds ~200 lines of combined markup + code is too large.** If your estimate puts a single component above that, split further.

### Step 3 — Design Data Flow

Identify the **state owner** for each piece of data, then map how it flows:

```
InventoryDashboard (owns: products[], filters)
  │
  ├─ [Parameter] products ──→ StockSummaryBar (reads aggregate stats)
  │
  ├─ [Parameter] filters ──→ InventoryFilters
  │   └─ EventCallback<Filters> OnFiltersChanged ──→ InventoryDashboard
  │
  ├─ [Parameter] filteredProducts ──→ InventoryTable
  │   └─ [Parameter] product ──→ InventoryRow
  │       ├─ EventCallback<Product> OnSave ──→ InventoryTable ──→ InventoryDashboard
  │       └─ EventCallback<Product> OnDelete ──→ InventoryTable ──→ InventoryDashboard
  │
  └─ EventCallback<Product> OnProductAdded ←── AddProductForm
```

Rules:
- Data always flows **down** through `[Parameter]`
- Events always flow **up** through `EventCallback<T>`
- The page/parent **owns the data** and passes filtered/transformed views to children
- Children **never mutate parameters** — they notify the parent via callbacks
- If data must cross more than 2 levels without intermediate components needing it, use a cascading value or a scoped service

### Step 4 — Identify Reuse Opportunities

Before creating a new component, check if an existing component in the project can serve the purpose. Look for:
- Existing list-item components that match the structure
- Shared filter/search components already in the project
- Generic components (e.g., `DataTable<T>`, `Pagination`) that accept templates

If a component will be used in more than one page, place it in a `Shared/` or `Components/` folder.

### Step 5 — Order the Implementation

Build bottom-up — leaf components first, then parents that compose them:

1. **Models/DTOs** — define the data shapes
2. **Services** — data access, business logic (interface + implementation)
3. **Leaf components** — components with no children (InventoryRow, StockSummaryBar)
4. **Container components** — components that compose leaves (InventoryTable, InventoryFilters)
5. **Page component** — wires everything together, registers routes
6. **Configuration** — DI registration, render mode setup

Each component should be independently compilable. Never reference a component that doesn't exist yet.

## Output Format

Present the plan briefly, then **immediately proceed to implement** — never stop at just the plan or ask for confirmation before writing code. The plan is a thinking tool, not a deliverable.

```markdown
## Component Plan: [Feature Name]

### Component Tree
[ASCII tree showing parent-child relationships]

### Component Table
| Component | Action | Render Mode | Purpose | Est. Lines |
|-----------|--------|-------------|---------|------------|
| ... | ... | ... | ... | ... |

### Data Flow
[State owner] → [Parameters down] → [EventCallbacks up]

### Implementation Order
1. [First file to create — why]
2. [Second file — why]
...
```

After outputting the plan, **immediately begin implementing** the components in the order listed. Do not wait for approval or ask "shall I proceed?" — the plan is a guide for you to follow, not a proposal for the user to approve.

## Anti-Patterns to Avoid

| Anti-Pattern | Why It's Wrong | Correct Approach |
|-------------|----------------|-----------------|
| One page component with 500+ lines | Impossible to test, reuse, or maintain | Decompose into focused components |
| Passing 10+ parameters through intermediate components | Parameter drilling obscures intent | Use cascading values or a scoped state service |
| Child component fetching its own data from an API | Multiple components making redundant calls | Parent owns data, passes via parameters |
| Inline rendering of list items with complex markup | Duplicated logic, no reuse, hard to test | Extract item template into its own component |
| Building everything in one file then "refactoring later"