Dart 速查表

Dart 語言的設計目標是讓來自其他語言的程式設計人員容易學習，但它有一些獨特的功能。本教學會引導您了解這些最重要的語言功能。

本教學中嵌入的編輯器具有部分完成的程式碼片段。您可以完成程式碼並按一下 [執行] 按鈕，來使用這些編輯器測試您的知識。編輯器也包含詳盡的測試程式碼；請勿編輯測試程式碼，但您可以研究它來學習測試。

如果您需要協助，請展開每個 DartPad 下方的 [解決方案...] 下拉式選單，以取得說明和答案。

若要將運算式的值放入字串中，請使用 ${expression}。如果運算式是識別碼，您可以省略 {}。

以下是一些使用字串插值的範例

下列函式會將兩個整數當作參數。請讓它傳回一個字串，其中包含以空格分隔的兩個整數。例如，stringify(2, 3) 應該傳回 '2 3'。

String stringify(int x, int y) {
  TODO('Return a formatted string here');
}


// Tests your solution (Don't edit!): 
void main() {
  assert(stringify(2, 3) == '2 3',
      "Your stringify method returned '${stringify(2, 3)}' instead of '2 3'");
  print('Success!');
}

String stringify(int x, int y) {
  return '$x $y';
}

Dart 會強制執行健全的空值安全。這表示除非您說它們可以，否則值不能為 null。換句話說，類型預設為不可為 null。

例如，請考慮下列程式碼。在空值安全的情況下，此程式碼會傳回錯誤。類型為 int 的變數不能有值 null

int a = null; // INVALID.

建立變數時，請在類型中加入 ?，以指出變數可以為 null

int? a = null; // Valid.

您可以簡化該程式碼一些，因為在所有 Dart 版本中，null 都是未初始化變數的預設值

int? a; // The initial value of a is null.

若要深入瞭解 Dart 中的空值安全，請閱讀健全的空值安全指南。

在此 DartPad 中宣告兩個變數

• 一個名為 name 且值為 'Jane' 的可為 null 的 String。
• 一個名為 address 且值為 null 的可為 null 的 String。

忽略 DartPad 中的所有初始錯誤。

// TODO: Declare the two variables here


// Tests your solution (Don't edit!): 
void main() {
  try {
    if (name == 'Jane' && address == null) {
      // verify that "name" is nullable
      name = null;
      print('Success!');
    } else {
      print('Not quite right, try again!');
    }
  } catch (e) {
    print('Exception: ${e.runtimeType}');
  }
}

String? name = 'Jane';
String? address;

Dart 提供一些方便的運算子，用於處理可能為 null 的值。其中一個是 ??= 指派運算子，只有在變數目前為 null 時，才會將值指派給變數

int? a; // = null
a ??= 3;
print(a); // <-- Prints 3.

a ??= 5;
print(a); // <-- Still prints 3.

另一個可感知空值的運算子是 ??，除非左邊運算式的值為 null，否則它會傳回左邊的運算式，如果左邊運算式的值為 null，則會評估並傳回右邊的運算式

print(1 ?? 3); // <-- Prints 1.
print(null ?? 12); // <-- Prints 12.

嘗試代入 ??= 和 ?? 運算子，以在下列程式碼片段中實作所述的行為。

忽略 DartPad 中的所有初始錯誤。

String? foo = 'a string';
String? bar; // = null

// Substitute an operator that makes 'a string' be assigned to baz.
String? baz = foo /* TODO */ bar;

void updateSomeVars() {
  // Substitute an operator that makes 'a string' be assigned to bar.
  bar /* TODO */ 'a string';
}


// Tests your solution (Don't edit!):
void main() {
  try {
    updateSomeVars();
    
    if (foo != 'a string') {
      print('Looks like foo somehow ended up with the wrong value.');
    } else if (bar != 'a string') {
      print('Looks like bar ended up with the wrong value.');
    } else if (baz != 'a string') {
      print('Looks like baz ended up with the wrong value.');
    } else {
      print('Success!');
    }
  } catch (e) {
    print('Exception: ${e.runtimeType}.');
  }
  
}

// Substitute an operator that makes 'a string' be assigned to baz.
String? baz = foo ?? bar;

void updateSomeVars() {
  // Substitute an operator that makes 'a string' be assigned to bar.
  bar ??= 'a string';
}

若要保護對可能為 null 的物件的屬性或方法的存取，請在點號 (.) 前面加上問號 (?)

myObject?.someProperty

上述程式碼等同於下列程式碼

(myObject != null) ? myObject.someProperty : null

您可以在單一運算式中串連多次使用 ?.

myObject?.someProperty?.someMethod()

如果 myObject 或 myObject.someProperty 為 null，上述程式碼會傳回 null (且永遠不會呼叫 someMethod())。

下列函式會將可為 null 的字串當作參數。嘗試使用條件屬性存取，使其傳回 str 的大寫版本，如果 str 為 null，則傳回 null。

String? upperCaseIt(String? str) {
  // TODO: Try conditionally accessing the `toUpperCase` method here.
}


// Tests your solution (Don't edit!):
void main() {
  try {
    String? one = upperCaseIt(null);
    if (one != null) {
      print('Looks like you\'re not returning null for null inputs.');
    } else {
      print('Success when str is null!');
    }
  } catch (e) {
    print('Tried calling upperCaseIt(null) and got an exception: \n ${e.runtimeType}.');
  }
  
  try {
    String? two = upperCaseIt('a string');
    if (two == null) {
      print('Looks like you\'re returning null even when str has a value.');
    } else if (two != 'A STRING') {
      print('Tried upperCaseIt(\'a string\'), but didn\'t get \'A STRING\' in response.');
    } else {
      print('Success when str is not null!');
    }
  } catch (e) {
    print('Tried calling upperCaseIt(\'a string\') and got an exception: \n ${e.runtimeType}.');
  }
}

String? upperCaseIt(String? str) {
  return str?.toUpperCase();
}

Dart 內建支援清單、對應和集合。您可以使用字面值建立它們

final aListOfStrings = ['one', 'two', 'three'];
final aSetOfStrings = {'one', 'two', 'three'};
final aMapOfStringsToInts = {
  'one': 1,
  'two': 2,
  'three': 3,
};

Dart 的類型推斷可以為您指派這些變數的類型。在此情況下，推斷的類型為 List<String>、Set<String> 和 Map<String, int>。

或者您可以自行指定類型

final aListOfInts = <int>[];
final aSetOfInts = <int>{};
final aMapOfIntToDouble = <int, double>{};

當您使用子類型的內容初始化清單，但仍然希望清單為 List<BaseType> 時，指定類型很方便

final aListOfBaseType = <BaseType>[SubType(), SubType()];

嘗試將下列變數設定為指示的值。取代現有的 null 值。

// Assign this a list containing 'a', 'b', and 'c' in that order:
final aListOfStrings = null;

// Assign this a set containing 3, 4, and 5:
final aSetOfInts = null;

// Assign this a map of String to int so that aMapOfStringsToInts['myKey'] returns 12:
final aMapOfStringsToInts = null;

// Assign this an empty List<double>:
final anEmptyListOfDouble = null;

// Assign this an empty Set<String>:
final anEmptySetOfString = null;

// Assign this an empty Map of double to int:
final anEmptyMapOfDoublesToInts = null;


// Tests your solution (Don't edit!):
void main() {
  final errs = <String>[];
  
  if (aListOfStrings is! List<String>) {
    errs.add('aListOfStrings should have the type List<String>.');
  } else if (aListOfStrings.length != 3) {
    errs.add('aListOfStrings has ${aListOfStrings.length} items in it, \n rather than the expected 3.');
  } else if (aListOfStrings[0] != 'a' || aListOfStrings[1] != 'b' || aListOfStrings[2] != 'c') {
    errs.add('aListOfStrings doesn\'t contain the correct values (\'a\', \'b\', \'c\').');
  }

  if (aSetOfInts is! Set<int>) {
    errs.add('aSetOfInts should have the type Set<int>.');
  } else if (aSetOfInts.length != 3) {
    errs.add('aSetOfInts has ${aSetOfInts.length} items in it, \n rather than the expected 3.');
  } else if (!aSetOfInts.contains(3) || !aSetOfInts.contains(4) || !aSetOfInts.contains(5)) {
    errs.add('aSetOfInts doesn\'t contain the correct values (3, 4, 5).');
  }

  if (aMapOfStringsToInts is! Map<String, int>) {
    errs.add('aMapOfStringsToInts should have the type Map<String, int>.');
  } else if (aMapOfStringsToInts['myKey'] != 12) {
    errs.add('aMapOfStringsToInts doesn\'t contain the correct values (\'myKey\': 12).');
  }

  if (anEmptyListOfDouble is! List<double>) {
    errs.add('anEmptyListOfDouble should have the type List<double>.');
  } else if (anEmptyListOfDouble.isNotEmpty) {
    errs.add('anEmptyListOfDouble should be empty.');
  }

  if (anEmptySetOfString is! Set<String>) {
    errs.add('anEmptySetOfString should have the type Set<String>.');
  } else if (anEmptySetOfString.isNotEmpty) {
    errs.add('anEmptySetOfString should be empty.');
  }

  if (anEmptyMapOfDoublesToInts is! Map<double, int>) {
    errs.add('anEmptyMapOfDoublesToInts should have the type Map<double, int>.');
  } else if (anEmptyMapOfDoublesToInts.isNotEmpty) {
    errs.add('anEmptyMapOfDoublesToInts should be empty.');
  }

  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }

  // ignore_for_file: unnecessary_type_check
}

// Assign this a list containing 'a', 'b', and 'c' in that order:
final aListOfStrings = ['a', 'b', 'c'];

// Assign this a set containing 3, 4, and 5:
final aSetOfInts = {3, 4, 5};

// Assign this a map of String to int so that aMapOfStringsToInts['myKey'] returns 12:
final aMapOfStringsToInts = {'myKey': 12};

// Assign this an empty List<double>:
final anEmptyListOfDouble = <double>[];

// Assign this an empty Set<String>:
final anEmptySetOfString = <String>{};

// Assign this an empty Map of double to int:
final anEmptyMapOfDoublesToInts = <double, int>{};

您可能在 Dart 程式碼中看過 => 符號。這個箭頭語法是一種定義函式的方式，該函式會執行其右邊的運算式，並傳回其值。

例如，請考慮對 List 類別的 any() 方法的此呼叫

bool hasEmpty = aListOfStrings.any((s) {
  return s.isEmpty;
});

以下是撰寫該程式碼的更簡單方式

bool hasEmpty = aListOfStrings.any((s) => s.isEmpty);

嘗試完成下列使用箭頭語法的陳述式。

class MyClass {
  int value1 = 2;
  int value2 = 3;
  int value3 = 5;
  
  // Returns the product of the above values:
  int get product => TODO();
  
  // Adds 1 to value1:
  void incrementValue1() => TODO();
  
  // Returns a string containing each item in the
  // list, separated by commas (e.g. 'a,b,c'): 
  String joinWithCommas(List<String> strings) => TODO();
}


// Tests your solution (Don't edit!):
void main() {
  final obj = MyClass();
  final errs = <String>[];
  
  try {
    final product = obj.product;
    
    if (product != 30) {
      errs.add('The product property returned $product \n instead of the expected value (30).'); 
    } 
  } catch (e) {
    print('Tried to use MyClass.product, but encountered an exception: \n ${e.runtimeType}.');
    return;
  }

  try {
    obj.incrementValue1();
    
    if (obj.value1 != 3) {
      errs.add('After calling incrementValue, value1 was ${obj.value1} \n instead of the expected value (3).'); 
    } 
  } catch (e) {
    print('Tried to use MyClass.incrementValue1, but encountered an exception: \n ${e.runtimeType}.');
    return;
  }

  try {
    final joined = obj.joinWithCommas(['one', 'two', 'three']);
    
    if (joined != 'one,two,three') {
      errs.add('Tried calling joinWithCommas([\'one\', \'two\', \'three\']) \n and received $joined instead of the expected value (\'one,two,three\').'); 
    } 
  } catch (e) {
    print('Tried to use MyClass.joinWithCommas, but encountered an exception: \n ${e.runtimeType}.');
    return;
  }

  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

class MyClass {
  int value1 = 2;
  int value2 = 3;
  int value3 = 5;

  // Returns the product of the above values:
  int get product => value1 * value2 * value3;
  
  // Adds 1 to value1:
  void incrementValue1() => value1++; 
  
  // Returns a string containing each item in the
  // list, separated by commas (e.g. 'a,b,c'): 
  String joinWithCommas(List<String> strings) => strings.join(',');
}

若要在同一個物件上執行一連串的作業，請使用串聯 (..)。我們都看過類似這樣的運算式

myObject.someMethod()

它會在 myObject 上叫用 someMethod()，且運算式的結果是 someMethod() 的傳回值。

以下是使用串聯的相同運算式

myObject..someMethod()

雖然它仍然會在 myObject 上叫用 someMethod()，但運算式的結果不是傳回值，而是 myObject 的參考！

使用串聯，您可以將原本需要個別陳述式的作業鏈結在一起。例如，請考慮下列程式碼，它會使用條件成員存取運算子 (?.) 來讀取 button 的屬性，如果它不是 null

var button = querySelector('#confirm');
button?.text = 'Confirm';
button?.classes.add('important');
button?.onClick.listen((e) => window.alert('Confirmed!'));
button?.scrollIntoView();

若要改為使用串聯，您可以從空值短路串聯 (?..) 開始，這可確保不會對 null 物件嘗試任何串聯作業。使用串聯可以縮短程式碼並讓 button 變數變得不必要

querySelector('#confirm')
  ?..text = 'Confirm'
  ..classes.add('important')
  ..onClick.listen((e) => window.alert('Confirmed!'))
  ..scrollIntoView();

使用串聯來建立單一陳述式，該陳述式會將 BigObject 的 anInt、aString 和 aList 屬性分別設定為 1、'String!' 和 [3.0]，然後呼叫 allDone()。

class BigObject {
  int anInt = 0;
  String aString = '';
  List<double> aList = [];
  bool _done = false;
  
  void allDone() {
    _done = true;
  }
}

BigObject fillBigObject(BigObject obj) {
  // Create a single statement that will update and return obj:
  return TODO('obj..');
}


// Tests your solution (Don't edit!):
void main() {
  BigObject obj;

  try {
    obj = fillBigObject(BigObject());
  } catch (e) {
    print('Caught an exception of type ${e.runtimeType} \n while running fillBigObject');
    return;
  }

  final errs = <String>[];

  if (obj.anInt != 1) {
    errs.add(
        'The value of anInt was ${obj.anInt} \n rather than the expected (1).');
  }

  if (obj.aString != 'String!') {
    errs.add(
        'The value of aString was \'${obj.aString}\' \n rather than the expected (\'String!\').');
  }

  if (obj.aList.length != 1) {
    errs.add(
        'The length of aList was ${obj.aList.length} \n rather than the expected value (1).');
  } else {
    if (obj.aList[0] != 3.0) {
      errs.add(
          'The value found in aList was ${obj.aList[0]} \n rather than the expected (3.0).');
    }
  }
  
  if (!obj._done) {
    errs.add('It looks like allDone() wasn\'t called.');
  }

  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

BigObject fillBigObject(BigObject obj) {
  return obj
    ..anInt = 1
    ..aString = 'String!'
    ..aList.add(3)
    ..allDone();
}

當您需要比簡單的欄位允許的更多屬性控制時，可以隨時定義 getter 和 setter。

例如，您可以確保屬性的值是有效的

class MyClass {
  int _aProperty = 0;

  int get aProperty => _aProperty;

  set aProperty(int value) {
    if (value >= 0) {
      _aProperty = value;
    }
  }
}

您還可以使用 getter 來定義計算屬性

class MyClass {
  final List<int> _values = [];

  void addValue(int value) {
    _values.add(value);
  }

  // A computed property.
  int get count {
    return _values.length;
  }
}

假設您有一個購物車類別，它保留一個私有的 List<double> 價格列表。添加以下內容

• 一個名為 total 的 getter，它會返回價格的總和
• 一個 setter，它會將列表替換為新的列表，只要新的列表中不包含任何負價格（在這種情況下，setter 應拋出 InvalidPriceException）。

忽略 DartPad 中的所有初始錯誤。

class InvalidPriceException {}

class ShoppingCart {
  List<double> _prices = [];
  
  // TODO: Add a "total" getter here:

  // TODO: Add a "prices" setter here:
}


// Tests your solution (Don't edit!):
void main() {
  var foundException = false;
  
  try {
    final cart = ShoppingCart();
    cart.prices = [12.0, 12.0, -23.0];
  } on InvalidPriceException {
    foundException = true;
  } catch (e) {
    print('Tried setting a negative price and received a ${e.runtimeType} \n instead of an InvalidPriceException.');
    return;
  }
  
  if (!foundException) {
    print('Tried setting a negative price \n and didn\'t get an InvalidPriceException.');
    return;
  }
  
  final secondCart = ShoppingCart();
  
  try {
    secondCart.prices = [1.0, 2.0, 3.0];
  } catch(e) {
    print('Tried setting prices with a valid list, \n but received an exception: ${e.runtimeType}.');
    return;
  }
  
  if (secondCart._prices.length != 3) {
    print('Tried setting prices with a list of three values, \n but _prices ended up having length ${secondCart._prices.length}.');
    return;
  }

  if (secondCart._prices[0] != 1.0 || secondCart._prices[1] != 2.0 || secondCart._prices[2] != 3.0) {
    final vals = secondCart._prices.map((p) => p.toString()).join(', ');
    print('Tried setting prices with a list of three values (1, 2, 3), \n but incorrect ones ended up in the price list ($vals) .');
    return;
  }
  
  var sum = 0.0;
  
  try {
    sum = secondCart.total;
  } catch (e) {
    print('Tried to get total, but received an exception: ${e.runtimeType}.');
    return;
  }
  
  if (sum != 6.0) {
    print('After setting prices to (1, 2, 3), total returned $sum instead of 6.');
    return;
  }
  
  print('Success!');
}

// Add a "total" getter here:
double get total => _prices.fold(0, (e, t) => e + t);

// Add a "prices" setter here:
set prices(List<double> value) {
  if (value.any((p) => p < 0)) {
    throw InvalidPriceException();
  }
  
  _prices = value;
}

Dart 有兩種函式參數：位置參數和具名參數。位置參數是您可能熟悉的那種

int sumUp(int a, int b, int c) {
  return a + b + c;
}
  // ···
  int total = sumUp(1, 2, 3);

使用 Dart，您可以將這些位置參數用括號括起來，使其成為可選的

int sumUpToFive(int a, [int? b, int? c, int? d, int? e]) {
  int sum = a;
  if (b != null) sum += b;
  if (c != null) sum += c;
  if (d != null) sum += d;
  if (e != null) sum += e;
  return sum;
}
  // ···
  int total = sumUpToFive(1, 2);
  int otherTotal = sumUpToFive(1, 2, 3, 4, 5);

可選的位置參數始終在函式參數列表中的最後。它們的預設值為 null，除非您提供另一個預設值

int sumUpToFive(int a, [int b = 2, int c = 3, int d = 4, int e = 5]) {
  // ···
}

void main() {
  int newTotal = sumUpToFive(1);
  print(newTotal); // <-- prints 15
}

實作一個名為 joinWithCommas() 的函式，它接受一到五個整數，然後返回一個以逗號分隔的這些數字的字串。以下是一些函式呼叫和傳回值的範例

String joinWithCommas(int a, [int? b, int? c, int? d, int? e]) {
  return TODO();
}


// Tests your solution (Don't edit!):
void main() {
  final errs = <String>[];
  
  try {
    final value = joinWithCommas(1);
    
    if (value != '1') {
      errs.add('Tried calling joinWithCommas(1) \n and got $value instead of the expected (\'1\').'); 
    } 
  } on UnimplementedError {
    print('Tried to call joinWithCommas but failed. \n Did you implement the method?');
    return;
  } catch (e) {
    print('Tried calling joinWithCommas(1), \n but encountered an exception: ${e.runtimeType}.');
    return;
  }

  try {
    final value = joinWithCommas(1, 2, 3);
    
    if (value != '1,2,3') {
      errs.add('Tried calling joinWithCommas(1, 2, 3) \n and got $value instead of the expected (\'1,2,3\').'); 
    } 
  } on UnimplementedError {
    print('Tried to call joinWithCommas but failed. \n Did you implement the method?');
    return;
  } catch (e) {
    print('Tried calling joinWithCommas(1, 2 ,3), \n but encountered an exception: ${e.runtimeType}.');
    return;
  }

  try {
    final value = joinWithCommas(1, 2, 3, 4, 5);
    
    if (value != '1,2,3,4,5') {
      errs.add('Tried calling joinWithCommas(1, 2, 3, 4, 5) \n and got $value instead of the expected (\'1,2,3,4,5\').'); 
    } 
  } on UnimplementedError {
    print('Tried to call joinWithCommas but failed. \n Did you implement the method?');
    return;
  } catch (e) {
    print('Tried calling stringify(1, 2, 3, 4 ,5), \n but encountered an exception: ${e.runtimeType}.');
    return;
  }

  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

String joinWithCommas(int a, [int? b, int? c, int? d, int? e]) {
  var total = '$a';
  if (b != null) total = '$total,$b';
  if (c != null) total = '$total,$c';
  if (d != null) total = '$total,$d';
  if (e != null) total = '$total,$e';
  return total;
}

在參數列表的末尾使用大括號語法，您可以定義具有名稱的參數。

具名參數是可選的，除非它們被明確標記為 required。

void printName(String firstName, String lastName, {String? middleName}) {
  print('$firstName ${middleName ?? ''} $lastName');
}

void main() {
  printName('Dash', 'Dartisan');
  printName('John', 'Smith', middleName: 'Who');
  // Named arguments can be placed anywhere in the argument list
  printName('John', middleName: 'Who', 'Smith');
}

如您所料，可為 null 的具名參數的預設值為 null，但您可以提供自訂預設值。

如果參數的類型不可為 null，則您必須提供預設值（如下列程式碼所示）或將參數標記為 required（如 建構子區段 中所示）。

void printName(String firstName, String lastName, {String middleName = ''}) {
  print('$firstName $middleName $lastName');
}

函式不能同時具有可選的位置參數和具名參數。

將 copyWith() 實例方法添加到 MyDataObject 類別。它應該採用三個可為 null 的具名參數

• int? newInt
• String? newString
• double? newDouble

您的 copyWith() 方法應根據目前的實例傳回一個新的 MyDataObject，並將前述參數（如果有）中的資料複製到物件的屬性中。例如，如果 newInt 不為 null，則將其值複製到 anInt 中。

忽略 DartPad 中的所有初始錯誤。

class MyDataObject {
  final int anInt;
  final String aString;
  final double aDouble;

  MyDataObject({
     this.anInt = 1,
     this.aString = 'Old!',
     this.aDouble = 2.0,
  });

  // TODO: Add your copyWith method here:
}


// Tests your solution (Don't edit!):
void main() {
  final source = MyDataObject();
  final errs = <String>[];
  
  try {
    final copy = source.copyWith(newInt: 12, newString: 'New!', newDouble: 3.0);
    
    if (copy.anInt != 12) {
      errs.add('Called copyWith(newInt: 12, newString: \'New!\', newDouble: 3.0), \n and the new object\'s anInt was ${copy.anInt} rather than the expected value (12).');
    }
    
    if (copy.aString != 'New!') {
      errs.add('Called copyWith(newInt: 12, newString: \'New!\', newDouble: 3.0), \n and the new object\'s aString was ${copy.aString} rather than the expected value (\'New!\').');
    }
    
    if (copy.aDouble != 3) {
      errs.add('Called copyWith(newInt: 12, newString: \'New!\', newDouble: 3.0), \n and the new object\'s aDouble was ${copy.aDouble} rather than the expected value (3).');
    }
  } catch (e) {
    print('Called copyWith(newInt: 12, newString: \'New!\', newDouble: 3.0) \n and got an exception: ${e.runtimeType}');
  }
  
  try {
    final copy = source.copyWith();
    
    if (copy.anInt != 1) {
      errs.add('Called copyWith(), and the new object\'s anInt was ${copy.anInt} \n rather than the expected value (1).');
    }
    
    if (copy.aString != 'Old!') {
      errs.add('Called copyWith(), and the new object\'s aString was ${copy.aString} \n rather than the expected value (\'Old!\').');
    }
    
    if (copy.aDouble != 2) {
      errs.add('Called copyWith(), and the new object\'s aDouble was ${copy.aDouble} \n rather than the expected value (2).');
    }
  } catch (e) {
    print('Called copyWith() and got an exception: ${e.runtimeType}');
  }
  
  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

  MyDataObject copyWith({int? newInt, String? newString, double? newDouble}) {
    return MyDataObject(
      anInt: newInt ?? this.anInt,
      aString: newString ?? this.aString,
      aDouble: newDouble ?? this.aDouble,
    );
  }

Dart 程式碼可以拋出和捕獲例外。與 Java 相反，Dart 的所有例外都是未檢查的。方法不會宣告它們可能拋出哪些例外，並且您不需要捕獲任何例外。

Dart 提供 Exception 和 Error 類型，但您可以使用任何非 null 的物件

throw Exception('Something bad happened.');
throw 'Waaaaaaah!';

處理例外時，請使用 try、on 和 catch 關鍵字

try {
  breedMoreLlamas();
} on OutOfLlamasException {
  // A specific exception
  buyMoreLlamas();
} on Exception catch (e) {
  // Anything else that is an exception
  print('Unknown exception: $e');
} catch (e) {
  // No specified type, handles all
  print('Something really unknown: $e');
}

try 關鍵字的工作方式與大多數其他語言相同。使用 on 關鍵字按類型篩選特定的例外，並使用 catch 關鍵字取得例外物件的參照。

如果您無法完全處理例外，請使用 rethrow 關鍵字傳播例外

try {
  breedMoreLlamas();
} catch (e) {
  print('I was just trying to breed llamas!');
  rethrow;
}

要執行程式碼而不論是否拋出例外，請使用 finally

try {
  breedMoreLlamas();
} catch (e) {
  // ... handle exception ...
} finally {
  // Always clean up, even if an exception is thrown.
  cleanLlamaStalls();
}

實作下面的 tryFunction()。它應該執行一個不可信任的方法，然後執行以下操作

• 如果 untrustworthy() 拋出 ExceptionWithMessage，則使用例外類型和訊息呼叫 logger.logException（嘗試使用 on 和 catch）。
• 如果 untrustworthy() 拋出 Exception，則使用例外類型呼叫 logger.logException（嘗試使用 on 來處理這個）。
• 如果 untrustworthy() 拋出任何其他物件，則不要捕獲例外。
• 在所有內容都被捕獲和處理後，請呼叫 logger.doneLogging（嘗試使用 finally）。

typedef VoidFunction = void Function();

class ExceptionWithMessage {
  final String message;
  const ExceptionWithMessage(this.message);
}

// Call logException to log an exception, and doneLogging when finished.
abstract class Logger {
  void logException(Type t, [String? msg]);
  void doneLogging();
}

void tryFunction(VoidFunction untrustworthy, Logger logger) {
  try {
    untrustworthy();
  } on ExceptionWithMessage catch (e) {
    logger.logException(e.runtimeType, e.message); 
  } on Exception catch (e) {
    logger.logException(e.runtimeType);
  } finally {
    logger.doneLogging();
  }
}

// Tests your solution (Don't edit!):
class MyLogger extends Logger {
  Type? lastType;
  String lastMessage = '';
  bool done = false;
  
  void logException(Type t, [String? message]) {
    lastType = t;
    lastMessage = message ?? lastMessage;
  }
  
  void doneLogging() => done = true;  
}

void main() {
  final errs = <String>[];
  var logger = MyLogger();
  
  try {
    tryFunction(() => throw Exception(), logger);
  
    if ('${logger.lastType}' != 'Exception' && '${logger.lastType}' != '_Exception') {
      errs.add('Untrustworthy threw an Exception, but a different type was logged: \n ${logger.lastType}.');
    }
    
    if (logger.lastMessage != '') {
      errs.add('Untrustworthy threw an Exception with no message, but a message \n was logged anyway: \'${logger.lastMessage}\'.');
    }
    
    if (!logger.done) {
      errs.add('Untrustworthy threw an Exception, \n and doneLogging() wasn\'t called afterward.');
    }
  } catch (e) {
    print('Untrustworthy threw an exception, and an exception of type \n ${e.runtimeType} was unhandled by tryFunction.');
  }
  
  logger = MyLogger();
  
  try {
    tryFunction(() => throw ExceptionWithMessage('Hey!'), logger);
  
    if (logger.lastType != ExceptionWithMessage) {
      errs.add('Untrustworthy threw an ExceptionWithMessage(\'Hey!\'), but a \n different type was logged: ${logger.lastType}.');
    }
    
    if (logger.lastMessage != 'Hey!') {
      errs.add('Untrustworthy threw an ExceptionWithMessage(\'Hey!\'), but a \n different message was logged: \'${logger.lastMessage}\'.');
    }
    
    if (!logger.done) {
      errs.add('Untrustworthy threw an ExceptionWithMessage(\'Hey!\'), \n and doneLogging() wasn\'t called afterward.');
    }
  } catch (e) {
    print('Untrustworthy threw an ExceptionWithMessage(\'Hey!\'), \n and an exception of type ${e.runtimeType} was unhandled by tryFunction.');
  }
  
  logger = MyLogger();
  bool caughtStringException = false;

  try {
    tryFunction(() => throw 'A String', logger);
  } on String {
    caughtStringException = true;
  }

  if (!caughtStringException) {
    errs.add('Untrustworthy threw a string, and it was incorrectly handled inside tryFunction().');
  }
  
  logger = MyLogger();
  
  try {
    tryFunction(() {}, logger);
  
    if (logger.lastType != null) {
      errs.add('Untrustworthy didn\'t throw an Exception, \n but one was logged anyway: ${logger.lastType}.');
    }
    
    if (logger.lastMessage != '') {
      errs.add('Untrustworthy didn\'t throw an Exception with no message, \n but a message was logged anyway: \'${logger.lastMessage}\'.');
    }
    
    if (!logger.done) {
      errs.add('Untrustworthy didn\'t throw an Exception, \n but doneLogging() wasn\'t called afterward.');
    }
  } catch (e) {
    print('Untrustworthy didn\'t throw an exception, \n but an exception of type ${e.runtimeType} was unhandled by tryFunction anyway.');
  }
  
  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

void tryFunction(VoidFunction untrustworthy, Logger logger) {
  try {
    untrustworthy();
  } on ExceptionWithMessage catch (e) {
    logger.logException(e.runtimeType, e.message);
  } on Exception {
    logger.logException(Exception);
  } finally {
    logger.doneLogging();
  }
}

Dart 提供了一個方便的捷徑，可在建構子中將值賦予屬性：宣告建構子時使用 this.propertyName

class MyColor {
  int red;
  int green;
  int blue;

  MyColor(this.red, this.green, this.blue);
}

final color = MyColor(80, 80, 128);

此技術也適用於具名參數。屬性名稱會成為參數的名稱

class MyColor {
  ...

  MyColor({required this.red, required this.green, required this.blue});
}

final color = MyColor(red: 80, green: 80, blue: 80);

在上述程式碼中，red、green 和 blue 被標記為 required，因為這些 int 值不能為 null。如果您新增預設值，則可以省略 required

MyColor([this.red = 0, this.green = 0, this.blue = 0]);
// or
MyColor({this.red = 0, this.green = 0, this.blue = 0});

將單行建構子新增到 MyClass，該建構子使用 this. 語法來接收並賦值給類別的所有三個屬性。

忽略 DartPad 中的所有初始錯誤。

class MyClass {
  final int anInt;
  final String aString;
  final double aDouble;
  
  // TODO: Create the constructor here.
}


// Tests your solution (Don't edit!):
void main() {
  final errs = <String>[];
  
  try {
    final obj = MyClass(1, 'two', 3);
    
    if (obj.anInt != 1) {
      errs.add('Called MyClass(1, \'two\', 3) and got an object with anInt of ${obj.anInt} \n instead of the expected value (1).');
    }

    if (obj.anInt != 1) {
      errs.add('Called MyClass(1, \'two\', 3) and got an object with aString of \'${obj.aString}\' \n instead of the expected value (\'two\').');
    }

    if (obj.anInt != 1) {
      errs.add('Called MyClass(1, \'two\', 3) and got an object with aDouble of ${obj.aDouble} \n instead of the expected value (3).');
    }
  } catch (e) {
    print('Called MyClass(1, \'two\', 3) and got an exception \n of type ${e.runtimeType}.');
  }
  
  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

MyClass(this.anInt, this.aString, this.aDouble);

有時，當您實作建構子時，您需要在建構子主體執行之前執行一些設定。例如，final 欄位必須在建構子主體執行之前具有值。在初始化程式清單中執行此工作，該清單位於建構子的簽章及其主體之間

Point.fromJson(Map<String, double> json)
    : x = json['x']!,
      y = json['y']! {
  print('In Point.fromJson(): ($x, $y)');
}

初始化程式清單也是放置斷言的好地方，斷言只會在開發期間執行

NonNegativePoint(this.x, this.y)
    : assert(x >= 0),
      assert(y >= 0) {
  print('I just made a NonNegativePoint: ($x, $y)');
}

完成下面的 FirstTwoLetters 建構子。使用初始化程式清單將 word 中的前兩個字元賦予 letterOne 和 LetterTwo 屬性。如需額外加分，請新增一個 assert 來捕獲少於兩個字元的單字。

忽略 DartPad 中的所有初始錯誤。

class FirstTwoLetters {
  final String letterOne;
  final String letterTwo;

  // TODO: Create a constructor with an initializer list here:
  FirstTwoLetters(String word)

}


// Tests your solution (Don't edit!):
void main() {
  final errs = <String>[];

  try {
    final result = FirstTwoLetters('My String');
    
    if (result.letterOne != 'M') {
      errs.add('Called FirstTwoLetters(\'My String\') and got an object with \n letterOne equal to \'${result.letterOne}\' instead of the expected value (\'M\').');
    }

    if (result.letterTwo != 'y') {
      errs.add('Called FirstTwoLetters(\'My String\') and got an object with \n letterTwo equal to \'${result.letterTwo}\' instead of the expected value (\'y\').');
    }
  } catch (e) {
    errs.add('Called FirstTwoLetters(\'My String\') and got an exception \n of type ${e.runtimeType}.');
  }

  bool caughtException = false;
  
  try {
    FirstTwoLetters('');
  } catch (e) {
    caughtException = true;
  }
  
  if (!caughtException) {
    errs.add('Called FirstTwoLetters(\'\') and didn\'t get an exception \n from the failed assertion.');
  }
  
  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

  FirstTwoLetters(String word)
      : assert(word.length >= 2),
        letterOne = word[0],
        letterTwo = word[1];

為了讓類別有多個建構子，Dart 支援具名建構子

class Point {
  double x, y;

  Point(this.x, this.y);

  Point.origin()
      : x = 0,
        y = 0;
}

若要使用具名建構子，請使用其完整名稱來調用它

final myPoint = Point.origin();

為 Color 類別提供一個名為 Color.black 的建構子，該建構子將所有三個屬性設為零。

忽略 DartPad 中的所有初始錯誤。

class Color {
  int red;
  int green;
  int blue;
  
  Color(this.red, this.green, this.blue);

  // TODO: Create a named constructor called "Color.black" here:

}


// Tests your solution (Don't edit!):
void main() {
  final errs = <String>[];

  try {
    final result = Color.black();
    
    if (result.red != 0) {
      errs.add('Called Color.black() and got a Color with red equal to \n ${result.red} instead of the expected value (0).');
    }

    if (result.green != 0) {
      errs.add('Called Color.black() and got a Color with green equal to \n ${result.green} instead of the expected value (0).');
    }

    if (result.blue != 0) {
  errs.add('Called Color.black() and got a Color with blue equal to \n ${result.blue} instead of the expected value (0).');
    }
  } catch (e) {
    print('Called Color.black() and got an exception of type \n ${e.runtimeType}.');
    return;
  }

  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

  Color.black()
      : red = 0,
        green = 0,
        blue = 0;

Dart 支援工廠建構子，它們可以傳回子類型或甚至 null。若要建立工廠建構子，請使用 factory 關鍵字

class Square extends Shape {}

class Circle extends Shape {}

class Shape {
  Shape();

  factory Shape.fromTypeName(String typeName) {
    if (typeName == 'square') return Square();
    if (typeName == 'circle') return Circle();

    throw ArgumentError('Unrecognized $typeName');
  }
}

將名為 IntegerHolder.fromList 的工廠建構子中的 TODO(); 行取代為傳回以下內容

• 如果列表有一個值，則使用該值建立 IntegerSingle 實例。
• 如果列表有兩個值，則依序使用這些值建立 IntegerDouble 實例。
• 如果列表有三個值，則依序使用這些值建立 IntegerTriple 實例。
• 否則，拋出 Error。

如果成功，主控台應顯示 Success!。

class IntegerHolder {
  IntegerHolder();
  
  // Implement this factory constructor.
  factory IntegerHolder.fromList(List<int> list) {
    TODO();
  }
}

class IntegerSingle extends IntegerHolder {
  final int a;

  IntegerSingle(this.a);
}

class IntegerDouble extends IntegerHolder {
  final int a;
  final int b;

  IntegerDouble(this.a, this.b);
}

class IntegerTriple extends IntegerHolder {
  final int a;
  final int b;
  final int c;

  IntegerTriple(this.a, this.b, this.c);
}

// Tests your solution (Don't edit from this point to end of file):
void main() {
  final errs = <String>[];

  // Run 5 tests to see which values have valid integer holders
  for (var tests = 0; tests < 5; tests++) {
    if (!testNumberOfArgs(errs, tests)) return;
  }

  // The goal is no errors with values 1 to 3,
  // but have errors with values 0 and 4.
  // The testNumberOfArgs method adds to the errs array if
  // the values 1 to 3 have an error and
  // the values 0 and 4 don't have an error
  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

bool testNumberOfArgs(List<String> errs, int count) {
  bool _threw = false;
  final ex = List.generate(count, (index) => index + 1);
  final callTxt = "IntegerHolder.fromList(${ex})";
  try {
    final obj = IntegerHolder.fromList(ex);
    final String vals = count == 1 ? "value" : "values";
    // Uncomment the next line if you want to see the results realtime
    // print("Testing with ${count} ${vals} using ${obj.runtimeType}.");
    testValues(errs, ex, obj, callTxt);
  } on Error {
    _threw = true;
  } catch (e) {
    switch (count) {
      case (< 1 && > 3):
        if (!_threw) {
          errs.add('Called ${callTxt} and it didn\'t throw an Error.');
        }
      default:
        errs.add('Called $callTxt and received an Error.');
    }
  }
  return true;
}

void testValues(List<String> errs, List<int> expectedValues, IntegerHolder obj,
    String callText) {
  for (var i = 0; i < expectedValues.length; i++) {
    int found;
    if (obj is IntegerSingle) {
      found = obj.a;
    } else if (obj is IntegerDouble) {
      found = i == 0 ? obj.a : obj.b;
    } else if (obj is IntegerTriple) {
      found = i == 0
          ? obj.a
          : i == 1
              ? obj.b
              : obj.c;
    } else {
      throw ArgumentError(
          "This IntegerHolder type (${obj.runtimeType}) is unsupported.");
    }

    if (found != expectedValues[i]) {
      errs.add(
          "Called $callText and got a ${obj.runtimeType} " + 
          "with a property at index $i value of $found " +
          "instead of the expected (${expectedValues[i]}).");
    }
  }
}

  switch (list.length) {
    case 1:
      return IntegerSingle(list[0]);
    case 2:
      return IntegerDouble(list[0], list[1]);
    case 3:
      return IntegerTriple(list[0], list[1], list[2]);
    default:
      throw ArgumentError("List must between 1 and 3 items. This list was ${list.length} items.");
  }

有時，建構子的唯一目的是重新導向到同一類別中的另一個建構子。重新導向建構子的主體為空，且建構子呼叫會出現在冒號 (:) 之後。

class Automobile {
  String make;
  String model;
  int mpg;

  // The main constructor for this class.
  Automobile(this.make, this.model, this.mpg);

  // Delegates to the main constructor.
  Automobile.hybrid(String make, String model) : this(make, model, 60);

  // Delegates to a named constructor
  Automobile.fancyHybrid() : this.hybrid('Futurecar', 'Mark 2');
}

還記得上面的 Color 類別嗎？建立一個名為 black 的具名建構子，但不要手動賦值屬性，而是將其重新導向到以零作為引數的預設建構子。

忽略 DartPad 中的所有初始錯誤。

class Color {
  int red;
  int green;
  int blue;
  
  Color(this.red, this.green, this.blue);

  // TODO: Create a named constructor called "black" here
  // and redirect it to call the existing constructor
}


// Tests your solution (Don't edit!):
void main() {
  final errs = <String>[];

  try {
    final result = Color.black();
    
    if (result.red != 0) {
      errs.add('Called Color.black() and got a Color with red equal to \n ${result.red} instead of the expected value (0).');
    }

    if (result.green != 0) {
      errs.add('Called Color.black() and got a Color with green equal to \n ${result.green} instead of the expected value (0).');
    }

    if (result.blue != 0) {
  errs.add('Called Color.black() and got a Color with blue equal to \n ${result.blue} instead of the expected value (0).');
    }
  } catch (e) {
    print('Called Color.black() and got an exception of type ${e.runtimeType}.');
    return;
  }

  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

  Color.black() : this(0, 0, 0);

如果您的類別產生永不變更的物件，您可以將這些物件設為編譯時常數。若要執行此操作，請定義 const 建構子並確保所有實例變數都是 final。

class ImmutablePoint {
  static const ImmutablePoint origin = ImmutablePoint(0, 0);

  final int x;
  final int y;

  const ImmutablePoint(this.x, this.y);
}

修改 Recipe 類別，使其能夠常數化其實例，並建立一個常數建構子，執行下列操作

• 有三個參數：ingredients、calories 和 milligramsOfSodium（依序）。
• 使用 this. 語法自動將參數值賦予具有相同名稱的物件屬性。
• 是常數，在建構子宣告中的 Recipe 之前使用 const 關鍵字。

忽略 DartPad 中的所有初始錯誤。

class Recipe {
  List<String> ingredients;
  int calories;
  double milligramsOfSodium;

  // TODO: Create a const constructor here"

}


// Tests your solution (Don't edit!):
void main() {
  final errs = <String>[];

  try {
    const obj = Recipe(['1 egg', 'Pat of butter', 'Pinch salt'], 120, 200);
    
    if (obj.ingredients.length != 3) {
      errs.add('Called Recipe([\'1 egg\', \'Pat of butter\', \'Pinch salt\'], 120, 200) \n and got an object with ingredient list of length ${obj.ingredients.length} rather than the expected length (3).');
    }
    
    if (obj.calories != 120) {
      errs.add('Called Recipe([\'1 egg\', \'Pat of butter\', \'Pinch salt\'], 120, 200) \n and got an object with a calorie value of ${obj.calories} rather than the expected value (120).');
    }
    
    if (obj.milligramsOfSodium != 200) {
      errs.add('Called Recipe([\'1 egg\', \'Pat of butter\', \'Pinch salt\'], 120, 200) \n and got an object with a milligramsOfSodium value of ${obj.milligramsOfSodium} rather than the expected value (200).');
    }
  } catch (e) {
    print('Tried calling Recipe([\'1 egg\', \'Pat of butter\', \'Pinch salt\'], 120, 200) \n and received a null.');
  }

  if (errs.isEmpty) {
    print('Success!');
  } else {
    errs.forEach(print);
  }
}

class Recipe {
  final List<String> ingredients;
  final int calories;
  final double milligramsOfSodium;

  const Recipe(this.ingredients, this.calories, this.milligramsOfSodium);
}

我們希望您喜歡使用本教學課程來學習或測試您對 Dart 語言一些最有趣的功能的知識。

您可以嘗試的下一步包括

• 嘗試其他 Dart 教學課程。
• 閱讀Dart 語言導覽。
• 使用 DartPad.
• 取得 Dart SDK.

除非另有說明，否則本網站上的文件反映的是 Dart 3.6.0。頁面最後更新時間為 2024-08-04。檢視來源或回報問題。