02-03 Dart Grammar Intensive - Basic Grammar

Recently, I relearned Dart with my mobile friends. I mainly watched the CoderWhy video tutorial, which was very good, and made some notes

Variable declarations

/* * * ** * * ** * * *支那* Declare variables* * * ** * * ** * * ** * * ** * /
void defineVariable() {
  // Variable declaration (var/final/const)
  // var Declares variables
  var age = 20;
  // age = "abc";
  age = 30;
  // final declares constants
  final height = 1.88;

  // const declares a constant
  const address = Shenzhen City;

  // Final and const
  // const date1 = DateTime.now(); Write error
  final date2 = DateTime.now();

  // 📢 After Dart 2.0, const can be omitted
  const p1 = const Person("hello");
  const p2 = const Person("world");
  const p3 = const Person("zhengzeqin");

  print(identical(p1, p2)); // true
  print(identical(p2, p3)); // false

  // 📢 After Dart 2.12, the default optional type requires? Previously, the default was not added
  String? optionName = null;
  optionName = "zhengzeqin";
}
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Final vs. const

Const must assign a constant value (a certain value is required at compile time)

Final can obtain a value by evaluating/functions (to determine a value at run time)

runtimeType

RuntimeType: Used to get the current type of a variable

var name = 'zhengzeqin';
print(name.runtimeType); // String
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The use of dynamic

Similar to swift Any, declare dynamic variables

question

Difference between Dynamic and Object?

// The difference between Object and dynamic
// The superclass application points to the subclass object
/ / Object and dynamic
// Object calls a method, an error is reported at compile time
// dynamic calls the method without error at compile time, but there are security risks at run time

// Error example
Object obj = "why";
print(obj.substring(1));

// Explicitly declare
dynamic obj2 = 123;
print(obj2.substring(1));
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identical

Const const const const const const const const const const const const const const const const const const

// after Dart2.0, const can be omitted
const p1 = const Person("hello");
const p2 = const Person("world");
const p3 = const Person("zhengzeqin");

print(identical(p1, p2)); // true
print(identical(p2, p3)); // false

class Person {
  final String name;
  const Person(this.name);
}
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The data type

Numeric types

int double

string

/* * * ** * * ** * * *支那* string* * * ** * * ** * * ** * * ** * /
void stringFunction() {
  // 1. How to define a string
  var s1 = 'Hello World';
  var s2 = "Hello Dart";
  var s3 = 'Hello\'Fullter';
  var s4 = "Hello'Fullter";

  // 2. The way to represent multi-line strings
  var message1 = "Ha ha ha ha ha ha ha ha ha ha ha ha ha;

  print('my name is ${message1}');

  // 3. Splice other variables
  var name = 'zhengzeqin';
  var age = 18;
  var height = 1.88;
  print('my name is ${name}, age is $age, height is $height');
}
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Boolean type

There is no non-zero or true

Dart declares that there is no non-zero or true case of a variable

var flag = "abc";
if (flag) { // Error example
  print("Execute code");
}
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string

Support “”, “””” “””

“”” “”” supports line breaks

${variable}, then {} can be omitted, if the function or expression calculation need not be omitted

/ / string
void stringFunction() {
  // 1. How to define a string
  var s1 = 'Hello World';
  var s2 = "Hello Dart";
  var s3 = 'Hello\'Fullter';
  var s4 = "Hello'Fullter";

  // 2. The way to represent multi-line strings
  var message1 = "Ha ha ha ha ha ha ha ha ha ha ha ha ha;

  print('my name is ${message1}');

  // 3. Splice other variables
  var name = 'zhengzeqin';
  var age = 18;
  var height = 1.88;
  print('my name is ${name}, age is $age, height is $height');
}
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A collection of

Dart has three of the most common ones built in: List/Set/Map

The two biggest differences between a Set and a List are that a Set is unordered and its elements are not repeated.

void collectFunction() {
  / / the List definition
  // 1. Use type derivation for definitions
  var letters = ['a'.'b'.'c'.'d'];
  print('$letters ${letters.runtimeType}');

  // 2. Specify the type explicitly
  List<int> numbers = [1.2.3.4];
  print('$numbers ${numbers.runtimeType}');

  The two biggest differences between a Set and a List are that a Set is unordered and its elements are not repeated.
  // 1. Use type derivation for definitions
  var lettersSet = {'a'.'b'.'c'.'d'};
  print('$lettersSet ${lettersSet.runtimeType}');

  // 2. Specify the type explicitly
  Set<int> numbersSet = {1.2.3.4};
  print('$numbersSet ${numbersSet.runtimeType}');

  numbers = Set<int>.from(numbers).toList();
  print(numbers);

  // Add/remove/include elements
  numbers.add(5);
  numbersSet.add(5);
  print('$numbers $numbersSet');

  numbers.remove(1);
  numbersSet.remove(1);
  print('$numbers $numbersSet');

  print(numbers.contains(2));
  print(numbersSet.contains(2));

  // List deletes elements according to index
  numbers.removeAt(3);
  print('$numbers');

  // Map definition
  // 1. Use type derivation for definitions
  var infoMap1 = {'name': 'harden'.'age': 18};
  print('$infoMap1 ${infoMap1.runtimeType}');

  // 2. Specify the type explicitly
  Map<String.Object> infoMap2 = {'height': 1.88.'address': 'Shenzhen'};
  print('$infoMap2 ${infoMap2.runtimeType}');

  // Map operation
  // 1. Obtain value based on key
  print(infoMap1['name']); // harden

  // 2. Get all entries
  print('${infoMap1.entries} ${infoMap1.entries.runtimeType}'); // (MapEntry(name: why), MapEntry(age: 18)) MappedIterable<String, MapEntry<String, Object>>

  // 3. Get all keys
  print('${infoMap1.keys} ${infoMap1.keys.runtimeType}'); // (name, age) _CompactIterable<String>

  // 4. Get all values
  print('${infoMap1.values} ${infoMap1.values.runtimeType}'); // (harden, 18) _CompactIterable<Object>

  // 5. Check whether a key or value is included
  print('${infoMap1.containsKey('age')} ${infoMap1.containsValue(18)}'); // true true

  // 6. Delete elements based on key
  infoMap1.remove('age');
  print('${infoMap1}'); // {name: harden}
}
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question

Is there anything wrong with this?

  var info<String.Object> = {
    "name": "why"."age": 18
  };
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var names = [“abc”, “cba”, “nba”, “cba”]; How do I write this if I need to specify the type?

function

Return value The name of the function (argument list) {function bodyreturnThe return value}Copy the code

Optional parameters

Optional parameters can be classified into named optional parameters and location optional parameters

📢 Location Optional: The corresponding sequence must be correct

Dart has no function overloading

Name Optional parameters: {param1, param2... } Position Optional parameters: [param1, param2,...]// Optional: Location optional - Name optional
// Note: Only optional parameters can have default values
// Position optional: [int age, double height]
// Arguments and parameters are matched by position
void sayHello1(String name, [int age = 10.double height = 2.int money = 0]) {
  print("sayHello1 name: $name  age: $age age: $height money: $money");
}

// Naming is optional
void sayHello2(String name, {int age = 10.double height = 3.14{})print("sayHello2 name: $name  age: $age age: $height");
}

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question

Position optional Parameters Optional parameters, must be in order to pass parameters? What if you only want to pass the money parameter and do not want to pass other parameters?

The function is a first-class citizen

A function can be assigned to a variable or used as an argument or return value of another function

void firstFunction() {
  // 1. Find another defined function and pass it in
  // test(bar);

  // 2. Anonymous function (argument list) {function body};
  translationFunc(() {
    print("Anonymous function called");
    return 10;
  });

  var movies = ['Infernal Affairs 1'.Infernal Affairs 2.'Infernal Affairs 3'];

  // Use forEach to traverse: named functions
  printElement(item) {
    print(item);
  }

  movies.forEach(printElement);

  // Use forEach to traverse: anonymous function
  movies.forEach((item) {
    print(item);
  });
  movies.forEach((item) => print(item));

  // 3. Arrow function: conditional, function body has only one line of code
  translationFunc(() => print("Arrow function executed."));

  // 4. Using aliases to define parameters requires: pass in a function & anonymous function
  calculateFunc((num1, num2) {
    return num1 * num2;
  });

  // 5. Declare the function and call it
  var add = addFunc();
  print(add(20.30));
}

// A function can be an argument to another function
void translationFunc(Function foo) {
  foo();
}

void bar() {
  print("Bar function called");
}

// Use aliases to define function types
typedef Calculate = int Function(int num1, int num2);

void calculateFunc(Calculate calc) {
  calc(20.30);
}

Calculate addFunc() {
  return (num1, num2) {
    return num1 * num2;
  };
}
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question

Does the arrow function support multiple lines?

The operator

conditions

?? =: When the variable isnull, use the following content for the assignment, otherwise no assignment

??: When the variable isnull, the following value is used, otherwise the value of the variable is used

void conditionalOperatorFunc() {
  / / 1.?? = :
  // When the original variable has a value, then?? = does not perform
  // If the original variable is null, then assign the value to the variable
  var name = null; name ?? ="harden";
  print(name);

  / /?? :
  / /?? If the previous data has a value, use?? Previous data
  / /?? If the previous data is NULL, the later value is used
  var name2 = null;
  var temp = name2 ?? "harden";
  print(temp);
}

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cascade

.: similar to chain call

class Animal {
  String? name;

  Animal(this.name);

  void run() {
    print("running");
  }

  void eat() {
    print("eating"); }}// cascade operator
void cascadeOperatorFunc() {
  // cascade operator
  var p = Animal("zhengzeqin")
    ..name = "harden". eat() .. run(); }Copy the code

Process control

 if else
 for
 while
 do-while
 break
 continue
 switch-case
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void processControlFunc() {
  for (var i = 0; i < 5; i++) {
    print(i);
  }

  var names = ['harden'.'kobe'.'curry'];
  for (var name in names) {
    print(name);
  }

  var direction = 'east';
  switch (direction) {
    case 'east':
      print('the east');
      break;
    case 'south':
      print('the south');
      break;
    case 'west':
      print('the west');
      break;
    case 'north':
      print('the north');
      break;
    default:
      print('Other direction'); }}Copy the code

Classes and objects

A constructor

When a constructor is not explicitly specified in a class, it defaults to having a constructor with no arguments.

📢 After Dart 2.12, the default optional type requires? Otherwise, the default no-argument constructor will not compile

📢 When you have your own constructor, the default constructor is invalidated and cannot be used

Of course, you might want to explicitly write a default constructor, but that would conflict with our custom constructor;

This is because Dart itself does not support function overloading (the same name, different arguments)

Named constructors: custom named constructors

Initializer list: the constructor body initializes the instance variable before it runs. The initializer list can be used

Redirected constructor: You can use a redirected constructor when you want to call another constructor from within a constructor

Constant constructor: We refer to the final section when we pass in the same value and want to return the same object

Factory constructor: Dart provides the Factory keyword to retrieve objects from the factory

void classFunc() {
  var hunman = Human(12);
  print("hunman: $hunman");
}

class Runner {
  // 📢 After Dart 2.12, the default optional type requires? Otherwise, the default no-argument constructor will not compile
  String? name;
  int? age;
}

class Human {
  // 📢 After Dart 2.12, the default optional type requires? Otherwise, the default no-argument constructor will not compile
  String? name;
  int? age;

  // Human(String name, int age) {
  // this.name = name;
  // this.age = age;
  // }

  // 📢 defaults to a no-fail constructor. If you declare a parameter constructor, the system default no-argument constructor does not exist
  // Human(this.name, this.age); // Syntax sugar equals comment section

  // Initialize the list page
  Human(this.age) : name = "zhengzeqin age is $age";

  // Name the constructor
  Human.withArgments(String name, int age) {
    this.name = name;
    this.age = age;
  }

  // Name a redirection constructor
  Human.form(int age) : this(age);

  // constant constructor

  // Project constructor, 📢 If there is a constructor of the same name, the factory constructor can no longer be implemented
  // factory Human(Int age) {
  // return Human.withArgments("zhengzeqin", age);
  // }

  // setter && getter
  String? get getName {
    return name;
  }

  set setName(String name) {
    this.name = name;
  }

  @override
  String toString() {
    return 'name=$name age=$age'; }}Copy the code

An abstract class

Abstract methods must exist in abstract classes

Abstract classes are classes that use abstract declarations

📢 If there is an implementation body, the implementation class may not implement the abstract method

/ / abstract classes
abstract class Shape {
  double getArea();
  // 📢 If there is an implementation body, the implemented class may not implement the abstract method
  void getName() {}
}

class Circle extends Shape {
  double? r;

  Circle(this.r);

  @override
  getArea() {
    final rr = r ?? 0;
    return rr * rr * 3.14; }}class Reactangle extends Shape {
  double w = 0;
  double h = 0;

  Reactangle(this.w, this.h);

  @override
  getArea() {
    returnw * h; }}Copy the code

Implicit interface

Dart interfaces are special and do not have a specific keyword to declare them.

By default, each class defined also declares an interface by default that can be implemented by other classes (because Dart does not support multiple inheritance).

📢 When a class is used as an interface, the class that implements the interface must implement all methods in the interface

Extends extends a class, while implements an interface

abstract class Player {
  play();
}

abstract class Flyer {
  fly();
}

class SuperManer implements Player.Flyer {
  @override
  play() {
    print('Superman is playing');
  }

  @override
  fly() {
    print('Superman is flying'); }}Copy the code

Mixin

In addition to defining a class through class, you can also define a class through the mixin keyword.

Mixin-defined classes are used to be mixed with other classes, but mixed with with the with keyword is equivalent to multiple inheritance

// implements requires that methods be re-implemented, whereas with does not
// Mix mixin classes with the with method, equivalent to multiple inheritance
// The on keyword specifies that subclasses of the class can use the current mixin (without the default limiting to Object).
mixin Runing {
  var name = "";
  run() {
    print('On the run'); }}mixin Flying {
  fly() {
    print('Flying'); }}class SuperWoman with Flying.Runing {
  void sayHello() {
    print("hello my name is $name"); }}Copy the code

The generic

List and Map generics

Does the user want to use an int, a double, or even a string?

One solution is to use Object and dynamic, but it’s very inconvenient to use them later

The alternative is to use generics.

void genericFunc() {
    // Restrict the type
  var names1 = <String> ['harden'.'kobe'.'james']; 
  List<String> names2 = ['harden'.'kobe'.'james']; 

 // Display the type
  Map<String.dynamic> infos1 = {'name': 'harden'.'age': 18};
  var infos2 = <String.dynamic> {'name': 'harden'.'age': 18}; 
}

// specify that T is derived from the num class
class Location<T extends num> {
  T x;
  T y;

  Location(this.x, this.y);
}
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The use of libraries

Import ‘library URL ‘;

Dart: prefix indicates the standard library for dart, such as DART: IO, DART: HTML, dart: mathImport ‘DART: IO’.

Of course, you can also reference import ‘lib/student/student.dart’ in relative or absolute path dart files;

Import ‘lib/student/student.dart’ as Stu;

You can use the show and hide keywords if you want to import only certain content from the library, or if you want to hide certain content from the library

import ‘lib/student/student.dart’ show Student, Person;

import ‘lib/student/student.dart’ hide Person;

export

Import multiple libraries through export

Code sample

portal

reference

Dart (1)
Dart (2)

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02-03 Dart Grammar Intensive – Basic Grammar

Variable declarations

Final vs. const

runtimeType

The use of dynamic

question

identical

The data type

Numeric types

string

Boolean type

There is no non-zero or true

string

A collection of

question

function

Optional parameters

question

The function is a first-class citizen

question

The operator

conditions

cascade

Process control

Classes and objects

A constructor

An abstract class

Implicit interface

Mixin

The generic

List and Map generics

The use of libraries

export

Code sample

reference

02-03 Dart Grammar Intensive – Basic Grammar

Variable declarations

Final vs. const

runtimeType

The use of dynamic

question

identical

The data type

Numeric types

string

Boolean type

There is no non-zero or true

string

A collection of

question

function

Optional parameters

question

The function is a first-class citizen

question

The operator

conditions

cascade

Process control

Classes and objects

A constructor

An abstract class

Implicit interface

Mixin

The generic

List and Map generics

The use of libraries

export

Code sample

reference

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