📱 TDemo - Product List App

A lightweight iOS app built using UIKit and MVVM that lists grocery products and shows their details. It supports offline caching, image storage with ETag-based freshness checks, and full unit test coverage — all without third-party libraries.

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🧠 Architecture & Design

• ✅ MVVM Pattern for clean separation of concerns
• ✅ Protocol-Based Abstraction for testability and flexibility
• ✅ Replaceable Layers: Swap Core Data with Realm or others without changing business logic
• ✅ Multi-environment support ready via APIConstants

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💾 Offline & Persistence Strategy

• ✅ Persistence abstraction makes Core Data replaceable
• ✅ Offline-first: falls back to local DB when network is unavailable
• ✅ insertOrUpdate prevents unnecessary read-update-write cycles
• ✅ All DB operations are done on background context for performance

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📸 Image Caching

• ✅ Saves images to /caches/ImageCache
• ✅ Uses HTTP ETag to prevent redundant downloads
• ✅ Format-agnostic saving (PNG, JPG, etc.)
• ✅ Saves both Thumbnail and Original for performance (Thumbnail uses same aspect ratio)
• ✅ Clean UIImageView extension (setImage(for:)) for use in UI

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⚡️ Performance Optimizations

• ✅ All Core Data writes run off the main thread
• ✅ ETag checks for image freshness
• ✅ Avoids redundant DB interactions by combining logic into insertOrUpdate

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❗️ Error Handling

• ✅ Graceful fallback to DB if API fails
• ✅ ViewModels expose onError closures for clean UI integration
• ✅ Custom domain errors like .noCachedProduct for clarity

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🧪 Test Coverage

✅ DetailViewModel

Case | Repository Result     | ViewModel Callback   | Description                      | Covered?
-----|------------------------|-----------------------|----------------------------------|----------
  1  | Success (Product)      | onProductUpdated      | Updates description              | ✅
  2  | Failure (Error)        | onError               | Triggers error handling          | ✅

✅ HomeViewModel

Case | Repository Result     | ViewModel Callback       | Description                        | Covered?
-----|------------------------|---------------------------|------------------------------------|----------
  1  | Success ([Product])    | onProductsUpdated         | Updates product list               | ✅
  2  | Failure (Error)        | onError                   | Triggers error handling            | ✅

✅ ProductRepositoryImp.fetchProductDetails(productId:)

Case | Internet | Network | JSON Decode | DB Save | DB Read | Result                        | Covered?
-----|----------|---------|-------------|---------|---------|-------------------------------|----------
  1  | OFF      | —       | —           | —       | OK      | Load product from DB          | ✅
  2  | OFF      | —       | —           | —       | FAIL    | Return read failure           | ✅
  3  | ON       | FAIL    | —           | —       | —       | Return network error          | ✅
  4  | ON       | OK      | FAIL        | —       | OK      | Load product from DB          | ✅
  5  | ON       | OK      | OK          | FAIL    | —       | Return product anyway         | ✅
  6  | ON       | OK      | OK          | OK      | —       | Return product                | ✅
  7  | ON       | OK      | FAIL        | —       | FAIL    | Return read failure fallback  | ✅
  8  | ON       | OK      | FAIL        | —       | Empty   | Return .noCachedProduct       | ✅

✅ ProductRepositoryImp.fetchProducts()

Case | Internet | Network | JSON Decode | DB Save | Result                     | Covered?
-----|----------|---------|-------------|---------|----------------------------|----------
  1  | OFF      | —       | —           | —       | Load from DB               | ✅
  2  | ON       | FAIL    | —           | —       | Return network error       | ✅
  3  | ON       | OK      | FAIL        | —       | Load from DB               | ✅
  4  | ON       | OK      | OK          | FAIL    | Return products (no save)  | ✅
  5  | ON       | OK      | OK          | OK      | Return products (saved)    | ✅
  6  | ON       | OK      | OK (empty)  | OK      | Return empty array         | ✅

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🧪 Tech Stack

• UIKit
• MVVM
• Core Data
• Native image caching with ETag support
• XCTest
• No third-party libraries

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▶️ Run the App

• Open TDemo.xcodeproj
• Run on simulator

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