Overview of the Dart Language (Variables, built-in types, functions, Operators, flow control statements)

variable

Declaration of variables

var
dynamic
Object

Declare an uninitialized variable whose type can be changed

/ / variable
/// Three slash document comments
/// The return type void can be omitted, and the return value is null

void main(){
  ///----------------------- variable declaration -----------------
  // declare an uninitialized variable whose type can be changed
  var data;
  data = "HelloWorld";

  dynamic data1;
  data1 = "HelloWorld1";
  data1 = 123;

  Object data2;
  data2 = 'HelloWorld2';
  data2 = 123;
  print([data,data1,data2]);
}

/// Print effect
lib/1-variables.dart:1: Warning: Interpreting this as package URI, 'package:flutter_test3/1-variables.dart'.
[HelloWorld, 123.123]

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Declare an initialized variable whose type cannot be changed

 var variablel =
      'HelloWorld'; // A variable is a reference. A variable named variableL refers to a String with the content 'HelloWorld'.
// variablel = 123; // After initialization, the type of the variableL variable is inferred to be String, and its type cannot be changed
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The types of variables declared by dynamic and Object can still be changed after initialization

dynamic variable2 = "HelloWorld";
  variable2 = 123;
// variable2.test(); // Call the test() method that does not exist, compile and pass, run an exception. Type is not checked during compilation

  Object variable3 = 'HelloWorld';
  variable3 = 123;
// variable3.test(); // Call to nonexistent test() method, failed compilation. Types are checked at compile time
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Declare a variable with a deterministic type. The type of the variable cannot be changed

String name3 = "HelloWorld";
// name3 =123; // The type of the variable cannot be changed
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Summary of variable Declarations

Var: can be any type if there is no initial value
Dynamic: Any dynamic type. The type is not checked during compilation
Object dynamic arbitrary type. The type is checked at compile time

The difference between:
1. The only difference is that if var has an initial value, the type is locked

The default value

Variables that are not initialized default to NULL

Everything is an object, and the default value for an object is NULL
```
bool isEmpty;
print((isEmpty == null));
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```

Final and const

For variables that are final or const, the variable type may be omitted

 final FVariablel = "HelloWorld";
// final String FVariablel = "HelloWorld";

  const cVariablel = "HelloWorld";
// const String cVariablel = "HelloWorld";
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Variables that are final or const cannot be modified.

// fVariable1 = '123';
// cVariable1 = '123456';
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If it is a class-level constant, use static, const.

DateTime; DateTime static const int Monday = 1;
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A const may be initialized with the value of another const.

const width = 100;
const height = 100;
const square = width * height;
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Const assignment declarations may be omitted

const List clist = [1.2.3];
// const List clist = const [1, 2, 3]; // Before dart 2, const assignments must be declared const
print("\n\n\n");
print(clist);
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You can change the value of a non-final, nonconst variable even if it once had a const value

var varList = const [1.2.3];
  final finalList = const [1.2.3];
  const constList = [1.2.3];
  print([varList, finalList, constList]);
  varList = [1];
// constList[1];
// finalList[1];
  print("\n\n");
  print([varList, finalList, constList]); Print effect --/* * ** [[1, 2, 3], [1, 2, 3], [1, 2, 3]] * [[1], [1, 2, 3], [1, 2, 3]] * /

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The immutability caused by const is transitive

final List ls = [1.2.3];
  ls[2] = 444;
  print(ls);
  const List cLs = [4.5.6];
// cLs[1] = 4;
  print("\n");
  print(ls); Error:/* * *
   * Unhandled exception:
      Unsupported operation: Cannot modify an unmodifiable list
#0 UnmodifiableListBase.[]= (dart:_internal/list.dart:90:5)
#1 main (package:flutter_test3/1-variables.dart:103:6)* /
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The same const constant is not created repeatedly in memory

final finalList1 = [1.2.3];
final finalList2 = [4.5.6];
print("\n");
print(identical(finalList1, finalList2)); // IDENTICAL checks to see if two references point to the same object

const constList1 = [1.2];
const constList2 = [1.2];
print("\n");
print(identical(constList1, constList2)); // IDENTICAL checks to see if two references point to the same object
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Const needs to be a compile-time constant

final DateTime finalDateTime = DateTime.now();
// const DateTime constDateTime = DateTime.now(); // Datetime.now () is the calculated value at runtime
  const sum = 1 + 2; // The value obtained by a basic operation using a literal of the built-in data type
  const aConstNum = 0;
  const aConstBool = true;
  const aConstString = 'a constant string';
  const aConstNull = null;
  const validConstString =
      '$aConstNum, $aConstBool, $aConstString, $aConstNull';
  print(validConstString); // Interpolation using compile-time constants that evaluate to null or a number, string, or Boolean
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Final and const summaries

In common

The declared type can be omitted
No value can be assigned after initialization
Cannot be used with var

Differences (Points to note)

Class level constants, using static, const.
Const may be initialized with the value of another const
Const may be initialized with the value of another const
You can change the value of a nonfinal, nonconst variable, even if it once had a const value
The immutability caused by const is transitive
The same const constant is not created repeatedly in memory
Const needs to be a compile-time constant

Built-in types

Number value (num, int, double)

int i = 1; / / integer
double d = 1.0 ;// Double b4-bit floating-point number
int bitLength = i.bitLength;
print('bitLength:${bitLength}'); //bitLength specifies the number of bits needed for int.
double maxFinite = double.maxFinite;
print('maxFinite: ${maxFinite}'); //maxFinitedouble Specifies the maximum value
Int and double are subclasses of num
num n1 = 1;
num n2 = 1.0;
// The decimal and hexadecimal values are supported
int il = oxfff;
// Scientific enumeration
double dl = 1.2 e2; / / 120.0
/ / conversion
//String > int
int i2 = int.pasrse('1');
double d2 = 1;// When double is int, int is automatically converted to double
print(D2: '${d2}');
int i2 = int.try.parse('1.0');/ / returns null

//int > String
int is = 123;
String s = 123.toString;

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String String

Dart strings are utF-16 encoded sequences of characters that can be created using single or double quotes
var name = 'HelloWorld';
// You can use an expression in a string: ${expression}. If the expression is an identifier, you can omit {}. If the result of the expression is an object, Dart calls the object's toString() function to get a string
var names = 'HelloWorld ${name}';
// The r prefix creates a "raw" string
var rawNames = r"HelloWorld ${name}";
print('name:${names}');
print('rawNames :${rawNames}');
// If the result of the expression is an object, Dart calls the object's toString() function to get a string.
pint(Map);
// You can create a multi-line string object with three single or double quotes
var multiLinesString = '''
Java Android
Flutter''';
pint('mutiLinesString:${mutiLinesString}');

/// StringBuffer
var sb = StringBuffer(a);// Dart 2 can omit newsb.. write('aaa').. write('bbb').. write('ccc');/ /.. Cascading implements chained calls
sb.writeAll([aaa,bbb,ccc],', ');// The second argument represents the delimiter, which is used to concatenate the data in the first argument list
pint('sb:${sb}');

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Booleans Boolean value (bool)

/ / bool: true and false
bool isNull;
print('isNull: ${isNull}');
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List (array List)

// Declare an automatic length array
List growableList = new List(a);//List growbleList = new List().. length = 3;growbleList.. add(1).. add(2).. add('HelloWorld');
pint('growbleList: ${growbleList}');

// Declare an array of fixed length
var list = List(6);// This can be declared with var or List
list[0] = "Java";
list[1] = "Android";
list[2] = "Dart";
list[3] = "Flutter";
list[4] = "C++";
list[5] = "C"
pint('list:${list}');

// Element fixed type
var typeList = List<String>; typeList.. add("1").. add("2").. add("3");
pint('typeList:${typeList}');

// Common attributes - gets the first element
String first = typeList.fisrt;
pint('typeList.fisrt:$typeFirst');
// The last element
String last = typeList.last;
pint('typeList.last:${last}');
// Number of elements
int typeListLength = typeList.length;
pint('typeListLength:${typeListLength}');
// Whether the element is empty
bool isEmpty = typeList.isEmpty;
pint('typeList.isEmpty:${isEmpty}');
// Whether the element is not empty
bool isNotEmpty = typeList.isNotEmpty;
pint('typeList.isNotEmpty:${isNotEmpty}');
// The array is reversed
可迭代 reversed = typeList.reversed;
print('typeList.reversed:${reversed}');

// The common methods to add, delete, modify, sort, shuffle, copy the child list
var list4 = [];
/ / to add
list4.add(1);
pint('add 1:${list4}');
list4.addAll([2.3.4]);
print('the addAll [4] 2:${list4}');
list4.insert(0.0);
print('the insert (0, 0) :${list4}');
list4.insertAll(1[5.6.7.8]);
print('insertAll (1,,6,7,8 [5]) :${list4}');
/ / delete
list4.remove(5);
print('remove 5 :${list4}');
list4.removeAt(2);
print('remove at 0:${list4}');
/ / change
list4[4] = 5;
print('updata list4[4] to 5 :${list4}');
//range
list4.fillRange(0.3.9);
print('fillRange updata list4[0] - list[2] to 9 :$list4');
可迭代 getRange = list4.getRange(0.3);
print('getRange list4[0]-list[2]:${getRange}');
/ / check
var contains = list.contains(5);
print('contains 5 :${contains}');
var indexOf = list4.indexOf(1);
print('list4 indexOf 1 :${indexOf}');
int indexWhere = list4.indexWhere((test) => test == 5);
print('list4 indexWhere 5 :${indexWhere}');
/ / sorting
list4.sort();
print('list4 sort:${list4}');
/ / shuffle
list4.shuffle();
print('list4 shuffle:${list4}');
// Copy the sublist
var list5 = list4.shulist(1);
print('sublist(1) list5: ${list5}');
/ / operators
var list6 =[8.9];
print('list6:${list6}');
var list7 = list5 + list7;
print('list5 + list6:${list7}');

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Set of key value pairs (Map)

// Declare a dynamically typed Map
var dynamicMap = Map(a); dynamicMap['name'] = 'HelloWorld';
dynamicMap[1] = 'android';
print('dynamicMap :${dymaicMap}');
/ / type
var map = Map<int.String> (); map[1] = 'java';
map[2] = 'Android';
print('map :${map}');
// This can also be declared
var map1 = {'name':'Java'.1:'android'};
map1.addAll({'name'.'Flutter'});
print('map1:${map1}');
  // Common attributes
// print(map.isEmpty); // Whether to be empty
// print(map.isNotEmpty); // Whether the value is not null
// print(map.length); // Number of key-value pairs
// print(map.keys); / / key collection
// print(map.values); / / the value set
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The Set Set (Set)

// Set No duplicate list
var dynamicSet = Set(a); dynamicSet.add('java');
dynamicSet.add('Android');
dynamicSet.add('Flutter');
dynamicSet.add('C/C++');
dynamicSet.add('1');
dynamicSet.add('1');
print('dynamicSet :${dynamicSet}');

// Common attributes are similar to List
// The common method of adding, deleting, modifying and checking is similar to the List
var set1 = {'java'.'Android'};
print('set1: ${set1}');
var differencel2 = set1.difference(set2);
var difference21 = set1.difference(set1);
print('set1 difference set2 :${differencel2}');// Return the set of elements in set1 but not in set2
print('set2 difference set1 :${difference2l}');// Return the set of elements in set2 but not in set1
var intersection = set1.intersection(set2);
print('set1 set2 intersection:${intersection}');Return the intersection of set1 and set2
var union = set1.union(set2);
print('set2 set1 union:${union}');
set2.retainAll(['java'.'Android']);// Retain only (the element to be retained must exist in the original set)
print('set2 only keeps Java Android:${set2}');
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Runes Symbol character

/ / with Runes for Unicode characters in the string / / https://copychar.cc/emoji/
String runesStr = '👄';
print(runesStr);
print(runesStr.length); // Indicates two 16-bit characters
print(runesStr.runes.length); // Indicates one 32-bit character

Runes runes = new Runes('\u{1f605} \u6211');
var str1 = String.fromCharCodes(runes); // Use String. FromCharCodes to display character graphs
print(str1);
String str2 = '\u{1f605} \u6211'; // If there are not four values, put the encoding values in curly braces
print(str2)

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Symbols identifier

// Mirrors have been removed
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function

define

Can be defined within a function

void main(){
    int add(int x,int y){
        return x + y;
    }
    
    print(add(1.2));
}
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You can omit types when defining functions

void main(){
    add( x, y){ // Omit is not recommended
        return x + y;
    }
    print(add(3.4));
}
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Support abbreviated syntax =>

void main(){
    int add2(int x,int y) => x + y;
    / / / equivalent
    int add3(intX,int y){
        return x + y;
    }
     print(add2(3.4));
     print(add3(3.4));
}
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Optional parameters

Optionally named parameters, use {param1,param2,… } to specify named parameters

void main(){
    int add3({int x,int y,int z}){
        x ?? = 1;
        x ?? = 2;
        x ?? = 3;
        return x + y + z;
    }
    print(add3());
}
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For optional positional parameters, put optional parameters in [], and put required parameters before optional parameters

void main(){
    int add(int x,[int y ,int z]){
        y ?? = 2;
        z ?? = 3;
        return x + y + z;
    }   
    print(add(1));
}
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Optional named parameter defaults (the default must be a compile-time constant). Currently, you can use the equal sign ‘=’ or ‘:’. Before the Dart SDK 1.21, you can only use colons, colon support will be removed later, so you are advised to use the equal sign

void main(){
    int add5(int x,[int y =2.int z = 3]) {return x + y +z;
    }
    // The preceding required parameter has no name
    print(add(5,y:10,z:20));
}

// The default value of the optional positional parameter (the default value must be a compile-time constant) can only be used with the equal sign "="
void mian (){
    int add6(int x,[int y = 2.int z = 3]) {return x + y + z;
    }
    
    print(add6(1));
}

// Use list or map as the default, but must be const
void func({List list = const [1.2.3].Map map = const {1:1.'fun' :'the whole stack{}})//TODO ----
}
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Anonymous functions

Can be assigned to a variable, called through the variable

// Anonymous function with no parameters
var anonFunc1 = () => print('Anonymous function with no arguments');
anonFunction();

// Anonymous functions with parameters
var anonFunc = (name) => 'I am ${name}';
print(anonFunc('DevYK'));

// Call through (), not recommended(() = >print('Not recommended'()));Copy the code

Can be called directly within or passed to other functions

void main(){
    List test(List list,String func(str)){
        for(var i =0; i < list.length; i++){ list[i] = func(list[i]); }return list;
    }
    
    var list = ['a'.'b'.'c'.'d'.'e'];
    print(test(list,(str) => str * 2)); // String * int,Dart and Py


  // The anonymous function used by list.foreach ()
  List list1 = [11.12.13];
  list1.forEach((item) => print('$item'));
}

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closure

Return Function object (closure)

void main(){
    Function makeAddFunc(int x){
        x++;
        return (int y) => x + y;
    }
    var addFunc = makeAddFunc(2);
    print(addFunc(3));
}
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Function is an alias

It can point to any function with the same signature

void main(){
    MyFunc myFunc;
    myFunc = subtsract;
    myFunc(4.2);
    myFunc = divide;
    myFunc(4.2);
    // The typeDef is passed as an argument
    calculator(4.2,subtsract);
}

// Function alias
typedef MyFunc(int a,int b);
// Define two functions based on the same function signature of MyFunc
subtsract(int a,int b){
    print('subtsract: ${a-b}');
}
divide(int a,int b){
    print('divide: ${a / b}');
}
// Typedefs can also be passed as arguments to functions
calculator(int a,int b,MyFunc func){
    func(a,b);
}
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The operator

The suffix operation

Conditional member access is similar to., but the left operation object cannot be null, such as foo? Bar Returns null if foo is null, otherwise returns the bar member.
```
String a;
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```

Take the quotient operator

Dividend ÷ divisor = quotient… The remainder, A ~/ B is equal to C, and this C is the quotient. That’s the Java equivalent of /
```
print(2 / 3);
print(2~ /3);
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```

Type determination operator

As, is, is! Determine the object type at run time

void main(){
    // As type conversion
num iNum = 1;
num dNum = 1.0;
int i = iNum as int;
double d = dNum as double;
print([i,d]);

// is Returns true if the object is of the specified type
print(iNum is int);
Child child;
Child childl = new Child();
print(child is Parent);//child is Null
print(childl is Parent);

// is! Returns False if the object is of the specified type
print(iNum is! int);
}
class Parent {}
class Child extends Parent {}

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Conditional expression

The ternary operator condition? expr1 : expr2

bool isFinish = true;
StringTextVal = isFinish?'yes':'no';
// expr1 ?? Expr2, if expr1 is non-null, return its value; Otherwise execute expr2 and return its result.
bool isPaused;
isPaused = isPaused ?? false;
/ / orisPaused ?? =false;
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The cascading operator

. You can call multiple functions and access member variables on the same object in succession. Strictly speaking, a cascade of two dots is not an operator. Just a Dart special syntax.
```
StringBuffer sb = new StringBuffer(a); sb .. write('Java')
  ..write('Android')
  ..write('\n')
  ..writeln('DevYK');
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```

Flow control statement

if else

for , forEach , for-in

forEach

collection.forEach(item) => print('forEach: ${item}');
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for-in

  for (var item in collection) {
    print('for-in: $item');
  }
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Overview of the Dart Language (Variables, built-in types, functions, Operators, flow control statements)

variable

Declaration of variables

Summary of variable Declarations

The default value

Final and const

Final and const summaries

Built-in types

Number value (num, int, double)

String String

Booleans Boolean value (bool)

List (array List)

Set of key value pairs (Map)

The Set Set (Set)

Runes Symbol character

Symbols identifier

function

define

Optional parameters

Anonymous functions

closure

Function is an alias

The operator

The suffix operation

Take the quotient operator

Type determination operator

Conditional expression

The cascading operator

Flow control statement

if else

for , forEach , for-in

While, do, while

Break the continue

whitch case

Overview of the Dart Language (Variables, built-in types, functions, Operators, flow control statements)

variable

Declaration of variables

Summary of variable Declarations

The default value

Final and const

Final and const summaries

Built-in types

Number value (num, int, double)

String String

Booleans Boolean value (bool)

List (array List)

Set of key value pairs (Map)

The Set Set (Set)

Runes Symbol character

Symbols identifier

function

define

Optional parameters

Anonymous functions

closure

Function is an alias

The operator

The suffix operation

Take the quotient operator

Type determination operator

Conditional expression

The cascading operator

Flow control statement

if else

for , forEach , for-in

While, do, while

Break the continue

whitch case

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