HarmonyOS Application Development

Core Principles

• ArkTS First — Use ArkTS with strict type safety, no any or dynamic types
• Declarative UI — Build UI with ArkUI's declarative components and state management
• Stage Model — Use modern Stage model (UIAbility), not legacy FA model
• Distributed by Design — Leverage cross-device capabilities from the start
• Atomic Services — Consider atomic services and cards for lightweight experiences
• One-time Development — Design for multi-device adaptation (phone, tablet, watch, TV)

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Hard Rules (Must Follow)

These rules are mandatory. Violating them means the skill is not working correctly.

No Dynamic Types

ArkTS prohibits dynamic typing. Never use any, type assertions, or dynamic property access.

// ❌ FORBIDDEN: Dynamic types
let data: any = fetchData();
let obj: object = {};
obj['dynamicKey'] = value;  // Dynamic property access
(someVar as SomeType).method();  // Type assertion

// ✅ REQUIRED: Strict typing
interface UserData {
  id: string;
  name: string;
}
let data: UserData = fetchData();

// Use Record for dynamic keys
let obj: Record<string, string> = {};
obj['key'] = value;  // OK with Record type

No Direct State Mutation

Never mutate @State/@Prop variables directly in nested objects. Use immutable updates.

// ❌ FORBIDDEN: Direct mutation
@State user: User = { name: 'John', age: 25 };

updateAge() {
  this.user.age = 26;  // UI won't update!
}

// ✅ REQUIRED: Immutable update
updateAge() {
  this.user = { ...this.user, age: 26 };  // Creates new object, triggers UI update
}

// For arrays
@State items: string[] = ['a', 'b'];

// ❌ FORBIDDEN
this.items.push('c');  // UI won't update

// ✅ REQUIRED
this.items = [...this.items, 'c'];

Stage Model Only

Always use Stage model (UIAbility). Never use deprecated FA model (PageAbility).

// ❌ FORBIDDEN: FA Model (deprecated)
// config.json with "pages" array
export default {
  onCreate() { ... }  // PageAbility lifecycle
}

// ✅ REQUIRED: Stage Model
// module.json5 with abilities configuration
import { UIAbility } from '@kit.AbilityKit';

export default class EntryAbility extends UIAbility {
  onCreate(want: Want, launchParam: AbilityConstant.LaunchParam): void {
    // Modern Stage model lifecycle
  }

  onWindowStageCreate(windowStage: window.WindowStage): void {
    windowStage.loadContent('pages/Index');
  }
}

Component Reusability

Extract reusable UI into @Component. No inline complex UI in build() methods.

// ❌ FORBIDDEN: Monolithic build method
@Entry
@Component
struct MainPage {
  build() {
    Column() {
      // 200+ lines of inline UI...
      Row() {
        Image($r('app.media.avatar'))
        Column() {
          Text(this.user.name)
          Text(this.user.email)
        }
      }
      // More inline UI...
    }
  }
}

// ✅ REQUIRED: Extract components
@Component
struct UserCard {
  @Prop user: User;

  build() {
    Row() {
      Image($r('app.media.avatar'))
      Column() {
        Text(this.user.name)
        Text(this.user.email)
      }
    }
  }
}

@Entry
@Component
struct MainPage {
  @State user: User = { name: 'John', email: 'john@example.com' };

  build() {
    Column() {
      UserCard({ user: this.user })
    }
  }
}

---

Quick Reference

When to Use What

Scenario	Pattern	Example
Component-local state	@State	Counter, form inputs
Parent-to-child data	@Prop	Read-only child data
Two-way binding	@Link	Shared mutable state
Cross-component state	@Provide/@Consume	Theme, user context
Persistent state	PersistentStorage	User preferences
App-wide state	AppStorage	Global state
Complex state logic	@Observed/@ObjectLink	Nested object updates

State Decorator Selection

@State        → Component owns the state, triggers re-render on change
@Prop         → Parent passes value, child gets copy (one-way)
@Link         → Parent passes reference, child can modify (two-way)
@Provide      → Ancestor provides value to all descendants
@Consume      → Descendant consumes value from ancestor
@StorageLink  → Syncs with AppStorage, two-way binding
@StorageProp  → Syncs with AppStorage, one-way binding
@Observed     → Class decorator for observable objects
@ObjectLink   → Links to @Observed object in parent

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Project Structure

Recommended Architecture

MyApp/
├── entry/                          # Main entry module
│   ├── src/main/
│   │   ├── ets/
│   │   │   ├── entryability/       # UIAbility definitions
│   │   │   │   └── EntryAbility.ets
│   │   │   ├── pages/              # Page components
│   │   │   │   ├── Index.ets
│   │   │   │   └── Detail.ets
│   │   │   ├── components/         # Reusable UI components
│   │   │   │   ├── common/         # Common components
│   │   │   │   └── business/       # Business-specific components
│   │   │   ├── viewmodel/          # ViewModels (MVVM)
│   │   │   ├── model/              # Data models
│   │   │   ├── service/            # Business logic services
│   │   │   ├── repository/         # Data access layer
│   │   │   ├── utils/              # Utility functions
│   │   │   └── constants/          # Constants and configs
│   │   ├── resources/              # Resources (strings, images)
│   │   └── module.json5            # Module configuration
│   └── build-profile.json5
├── common/                         # Shared library module
│   └── src/main/ets/
├── features/                       # Feature modules
│   ├── feature_home/
│   └── feature_profile/
└── build-profile.json5             # Project configuration

Layer Separation

┌─────────────────────────────────────┐
│           UI Layer (Pages)          │  ArkUI Components
├─────────────────────────────────────┤
│         ViewModel Layer             │  State management, UI logic
├─────────────────────────────────────┤
│         Service Layer               │  Business logic
├─────────────────────────────────────┤
│        Repository Layer             │  Data access abstraction
├─────────────────────────────────────┤
│    Data Sources (Local/Remote)      │  Preferences, RDB, Network
└─────────────────────────────────────┘

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ArkUI Component Patterns

Basic Component Structure

import { router } from '@kit.ArkUI';

@Component
export struct ProductCard {
  // Props from parent
  @Prop product: Product;
  @Prop onAddToCart: (product: Product) => void;

  // Local state
  @State isExpanded: boolean = false;

  // Computed values (use getters)
  get formattedPrice(): string {
    return `¥${this.product.price.toFixed(2)}`;
  }

  // Lifecycle
  aboutToAppear(): void {
    console.info('ProductCard appearing');
  }

  aboutToDisappear(): void {
    console.info('ProductCard disappearing');
  }

  // Event handlers
  private handleTap(): void {
    router.pushUrl({ url: 'pages/ProductDetail', params: { id: this.product.id } });
  }

  private handleAddToCart(): void {
    this.onAddToCart(this.product);
  }

  // UI builder
  build() {
    Column() {
      Image(this.product.imageUrl)
        .width('100%')
        .aspectRatio(1)
        .objectFit(ImageFit.Cover)

      Text(this.product.name)
        .fontSize(16)
        .fontWeight(FontWeight.Medium)

      Text(this.formattedPrice)
        .fontSize(14)
        .fontColor('#FF6B00')

      Button('Add to Cart')
        .onClick(() => this.handleAddToCart())
    }
    .padding(12)
    .backgroundColor(Color.White)
    .borderRadius(8)
    .onClick(() => this.handleTap())
  }
}

List with LazyForEach

import { BasicDataSource } from '../utils/BasicDataSource';

class ProductDataSource extends BasicDataSource<Product> {
  private products: Product[] = [];

  totalCount(): number {
    return this.products.length;
  }

  getData(index: number): Product {
    return this.products[index];
  }

  addData(product: Product): void {
    this.products.push(product);
    this.notifyDataAdd(this.products.length - 1);
  }

  updateData(index: number, product: Product): void {
    this.products[index] = product;
    this.notifyDataChange(index);
  }
}

@Component
struct ProductList {
  private dataSource: ProductDataSource = new ProductDataSource();

  build() {
    List() {
      LazyForEach(this.dataSource, (product: Product, index: number) => {
        ListItem() {
          ProductCard({ product: product })
        }
      }, (product: Product) => product.id)  // Key generator
    }
    .lanes(2)  // Grid with 2 columns
    .cachedCount(4)  // Cache 4 items for smooth scrolling
  }
}

Custom Dialog

@CustomDialog
struct ConfirmDialog {
  controller: CustomDialogController;
  title: string = 'Confirm';
  message: string = '';
  onConfirm: () => void = () => {};

  build() {
    Column() {
      Text(this.title)
        .fontSize(20)
        .fontWeight(FontWeight.Bold)
        .margin({ bottom: 16 })

      Text(this.message)
        .fontSize(16)
        .margin({ bottom: 24 })

      Row() {
        Button('Cancel')
          .onClick(() => this.controller.close())
          .backgroundColor(Color.Gray)
          .margin({ right: 16 })

        Button('Confirm')
          .onClick(() => {
            this.onConfirm();
            this.controller.close();
          })
      }
    }
    .padding(24)
  }
}

// Usage
@Entry
@Component
struct MainPage {
  dialogController: CustomDialogController = new CustomDialogController({
    builder: ConfirmDialog({
      title: 'Delete Item',
      message: 'Are you sure you want to delete this item?',
      onConfirm: () => this.deleteItem()
    }),
    autoCancel: true
  });

  private deleteItem(): void {
    // Delete logic
  }

  build() {
    Button('Delete')
      .onClick(() => this.dialogController.open())
  }
}

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Extended Reference

Detailed material starting at ## State Management Patterns has been moved to reference/extended.md to keep this skill concise. Load that reference when the task requires the moved examples, command catalogs, checklists, platform details, or implementation templates.