Dart ガイド · 5/6

Errors and Testing

この章は現在、英語でのみ提供しています。

Dart signals failures with exceptions and handles them with try/catch. There are no checked exceptions, and an uncaught exception propagates upward. This chapter covers throwing and catching exceptions, assert, and writing and running tests with package:test and dart test.

Exception versus Error

Dart's core libraries divide problems into two families.

Kind	Meaning	Examples
`Exception`	An expected failure the caller should handle	`FormatException`, `IOException`, `TimeoutException`
`Error`	A programming bug, usually fixed rather than caught	`RangeError`, `TypeError`, `StateError`, `ArgumentError`

Malformed input or a dropped connection is an Exception for the caller to handle. Indexing past the end of a list is an Error: fix the cause instead of hiding it. throw accepts any object, but by convention you throw only Exception or Error types.

class InsufficientFundsException implements Exception {
  InsufficientFundsException(this.balance, this.amount);
  final int balance;
  final int amount;

  @override
  String toString() => 'Insufficient funds: balance $balance, requested $amount';
}

class Account {
  int _balance = 0;
  int get balance => _balance;

  void deposit(int amount) {
    if (amount <= 0) {
      throw ArgumentError.value(amount, 'amount', 'must be positive');
    }
    _balance += amount;
  }

  void withdraw(int amount) {
    if (amount > _balance) {
      throw InsufficientFundsException(_balance, amount);
    }
    _balance -= amount;
  }
}

try, on, catch, finally, rethrow

Use on Type to catch a specific exception, and catch (e, s) to receive the exception object and the stack trace. A finally block always runs, whether or not an exception occurred, so it is the place for cleanup. To log an exception and pass it on, use rethrow, which preserves the original stack trace.

int parseAge(String input) {
  try {
    final age = int.parse(input);
    if (age < 0) throw FormatException('negative age', input);
    return age;
  } on FormatException catch (e) {
    print('Format error: ${e.message}');
    return 0;
  } catch (e, stackTrace) {
    print('Unexpected error: $e');
    print(stackTrace);
    rethrow;
  } finally {
    print('parseAge done');
  }
}

void main() {
  print(parseAge('42'));   // 42
  print(parseAge('abc'));  // prints a format error, then 0

  final account = Account()..deposit(100);
  try {
    account.withdraw(500);
  } on InsufficientFundsException catch (e) {
    print(e); // Insufficient funds: balance 100, requested 500
  }
}

この章について質問がありますか？

Errors and Testing

Exception versus Error

try, on, catch, finally, rethrow

assert

Writing tests with package:test

Running dart test

Summary

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