Rikucherry's blog

Flutter Basics: Dart(2)

 技术学习/Flutter  722  2 分钟

2-1 Functions

  • Function scope 
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      void main(List<String> arguments) {
    String nestedFunction() {
      return 'nested level';
    }

    print(topFunction()); //top level scope
    print(nestedFunction()); //only valid inside of the outer function
    otherFunction(); //top level scope
  }

  String topFunction() {
    return 'top level';
  }

  void otherFunction() {
    print(topFunction());
    //nestedFunction(); <--invalid scope
  }
    • output
      top level
  nested level
  top level
  • Function parameters
    • arguments: passed in a function
    • parameter: used within a function
    • positional prameters: can't change the order
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    void function(int x, double y, String z) { //arguments passed in
  function(1, 1.5, 'called');//use parameters
}
    • optional positional parameters
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    //z is optional, should be nullable or defined a default value
void function(int x, double y, [String z = 'default']) { 
  function(1, 1.5);
  function(1, 1.5, 'optional');
}
    • named parameters: the order doesn't matter. All optional by default.
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    void function({
    int? x,
    double? y,
    String? z 
  }) {
    function(y: 1.5); //put arguments by designating
  }
    • required named parameters
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    void function({
    required int x,
    required double y,
    required String z 
  }) {
    function(x: 1, y: 1.5, z: 'required'); 
  }
    • mixed types parameters: positional parameters always come first
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    void function(int positional, {
   int？x,
   required double y,
   required String z 
 }) {
   function(1, y: 1.5, z: 'required'); 
 }
  • Functional programming
    • High order functions: take functions as parameters or return functions
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    void main(List<String> arguments) {
  //(x) {return x + 5;} is an anonymous function
  //twicePlusFive is also a function
  final twicePlusFive = twice((x) {return x + 5;});
  print(twicePlusFive(3)); //output is 13.
}
//(return type) Function(param type)
int Function(int) twice(int Function(int) f) {
  return (int x) {return f(f(x));};
}
    • convert blocks to expressions: cmd+.
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    void main(List<String> arguments) {
  final twicePlusFive = twice((x) => x + 5);
  print(twicePlusFive(3)); //output is 13.
}

int Function(int) twice(int Function(int) f) {
  return (int x) => f(f(x));
}

2-2 Collections

  • List
    • define
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    List<int> myList = [1, 2, 3]; //dart has no arrays but only lists
final inferList = [1, 2, 3]; //will be inferred as List<int>
final inferList2 = [1, 2, 'text']; //will be inferred as List<Object>
<int>[1, 2, 3]; //no variable assignment
    • access
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    int length = myList.length;
final firstElement = myList[0];
  • Map
    • define
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    Map<String, dynamic> myMap = {
  'name': 'riku',
  'age': 0,
  'is Chinese': true
};
    • access
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    final name = myMap['name'];
  • Set
    • define
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    Set<int> mySet = {1, 2, 3}; //a set can't contains duplicated values
Set<int> dupSet = {1, 2, 3, 3};
print(dupSet.length); //output is 3

2-3 Transformations

  • map 
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    final names = ['Riku', 'CC', 'Cherry'];
final nameLengths = names.map((e) => e.length).toList();
//will return a 'Literable<int>' if it's without 'toList'
print(nameLengths[1]);//2
  • where, forEach 
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    final names = ['Riku', 'CC', 'Ch'];
final nameFiltered = names.where((e) => e.length == 2).toList();
//direct print
print(nameFiltered);//[CC, Ch] 
//Loops
nameFiltered.forEach((element) => print(element)); 
nameFiltered.forEach(print);
  • If, For, Spread
    • If-Else inside of a list
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      bool isSignedIn = true;
  final dynamicList = <String>[
    'This is a fake content.',
    if (isSignedIn) 'Sign In' else 'Sign out'
  ];
    • For inside of a list
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      final dynamicList = <String>[
    for (int i = 0; i < 5; i++) i.toString(),
    for (final number in [6, 7, 8]) number.toString()
  ];
    • Spread inside of a list
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    final list1 = <String>['e1', 'e2'];
final list2 = <String>['e3', 'e4'];
final concatList = <String>[
  ...list1,
  'em',
  ...list2,
];
print(concatList); //[e1,e2,em,e3,e4]

2-4 Enums

  • enums 
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    enum AccountType { free, premium, vip }

void main(List<String> arguments) {
  final userAccountType = AccountType.free;
  print(userAccountType.index);//0
  print(AccountType.values);//[AccountType.free, AccountType.premium, AccountType.vip]
  print(AccountType);//AccountType
}

2-5 The Pub Package System

  • dependencies/dev_dependencies 
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    dependencies: #Is packaged into your app when it's built for publishing
path: ^1.7.0

dev_dependencies: #Is not a part of your code
pedantic: ^1.9.0
  • add/update dependencies(VS code)
    • hit F1 -> type Pubspec Assist -> Add/update...
    • search from website: https://pub.dev/
    • manually sync dependencies: flutter pub get for native flutter projects / dart pub get for dart projects

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