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The DiscountCalculatorProvider manages the state and business logic for discount calculations. It extends ChangeNotifier and persists user inputs using SharedPreferences.

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Overview

This provider handles two types of discounts:
  • Percentage discounts: Apply sequential percentage-based discounts
  • Fixed amount discounts: Apply dollar-value discounts
It automatically persists all inputs and calculation results across app sessions.

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Enums

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DiscountType

Defines the type of discount being applied. 
enum DiscountType {
  percentage,
  fixedAmount,
}
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percentage
DiscountType
Apply discounts as percentages (e.g., 10% off)
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fixedAmount
DiscountType
Apply discounts as fixed dollar amounts (e.g., $50 off)

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Constructor

DiscountCalculatorProvider(SharedPreferences prefs)
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prefs
SharedPreferences
required
The SharedPreferences instance used for persisting calculator state

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Getters

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Calculation Results

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subtotal
double?
The price after discounts but before tax
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finalPrice
double?
The final price after all discounts and tax
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savedAmount
double?
The total amount saved from all discounts
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taxAmount
double?
The calculated tax amount based on the subtotal
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errorMessage
String?
Error message if validation fails, null otherwise

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Configuration

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discountType
DiscountType
The current discount type (percentage or fixedAmount)

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Input State

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originalPriceInput
String
The persisted original price input string
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primaryDiscountInput
String
The persisted primary discount input string
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additionalDiscountInput
String
The persisted additional discount input string
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taxInput
String
The persisted tax percentage input string
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originalPrice
double?
The parsed original price value for display

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Methods

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calculateDiscount

Performs the discount calculation with the provided inputs. 
void calculateDiscount({
  required String originalPriceStr,
  required String primaryDiscountStr,
  String additionalDiscountStr = '',
  String taxStr = '',
})
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originalPriceStr
String
required
The original price before any discounts
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primaryDiscountStr
String
required
The primary discount value (percentage or fixed amount depending on discountType)
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additionalDiscountStr
String
default:""
Optional additional discount applied sequentially after the primary discount
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taxStr
String
default:""
Optional tax percentage to apply to the subtotal

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Calculation Logic

For Percentage Discounts:
  1. Apply primary discount: saved1 = originalPrice * (primaryDiscount / 100)
  2. Apply additional discount to new price: saved2 = priceAfterFirst * (additionalDiscount / 100)
  3. Calculate subtotal: subtotal = originalPrice - saved1 - saved2
  4. Apply tax: taxAmount = subtotal * (taxPercent / 100)
  5. Calculate final price: finalPrice = subtotal + taxAmount
For Fixed Amount Discounts:
  1. Sum all discounts: totalDiscount = primaryDiscount + additionalDiscount
  2. Calculate subtotal: subtotal = originalPrice - totalDiscount
  3. Apply tax: taxAmount = subtotal * (taxPercent / 100)
  4. Calculate final price: finalPrice = subtotal + taxAmount

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Validation

The method validates:
  • All numeric inputs are valid numbers
  • No negative values
  • Percentage discounts don’t exceed 100%
  • Fixed discounts don’t exceed original price
If validation fails, errorMessage is set and results are cleared.

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setDiscountType

Changes the discount type and persists the selection. 
void setDiscountType(DiscountType type)
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type
DiscountType
required
The new discount type to apply
This method automatically persists the selection to SharedPreferences and notifies listeners.

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clear

Resets all inputs and calculation results. 
void clear()
This method:
  • Clears all input strings
  • Removes persisted values from SharedPreferences
  • Clears all calculation results
  • Clears any error messages
  • Notifies listeners of the state change

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Usage Example

import 'package:provider/provider.dart';
import 'package:numix/features/discount_calculator/providers/discount_provider.dart';

// In your widget
final provider = context.read<DiscountCalculatorProvider>();

// Set discount type
provider.setDiscountType(DiscountType.percentage);

// Calculate discount
provider.calculateDiscount(
  originalPriceStr: '100.00',
  primaryDiscountStr: '20',      // 20% off
  additionalDiscountStr: '10',   // Additional 10% off
  taxStr: '8.5',                 // 8.5% tax
);

// Access results
if (provider.errorMessage != null) {
  print('Error: ${provider.errorMessage}');
} else {
  print('Original Price: \$${provider.originalPrice}');
  print('Amount Saved: \$${provider.savedAmount}');
  print('Subtotal: \$${provider.subtotal}');
  print('Tax: \$${provider.taxAmount}');
  print('Final Price: \$${provider.finalPrice}');
}

// Clear all
provider.clear();

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UI Integration

This provider is typically used with Consumer widgets to reactively update the UI: 
Consumer<DiscountCalculatorProvider>(
  builder: (context, provider, child) {
    if (provider.finalPrice != null) {
      return Text('Final Price: \$${provider.finalPrice!.toStringAsFixed(2)}');
    }
    return const SizedBox.shrink();
  },
)

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Persistence

All user inputs are automatically persisted using the following SharedPreferences keys:
  • disc_orig - Original price
  • disc_pri - Primary discount
  • disc_add - Additional discount
  • disc_tax - Tax percentage
  • disc_type - Discount type (0 = percentage, 1 = fixed amount)
The provider automatically restores these values when initialized.

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