A, Hello Dart

1.1 Hello World

Create a new helloworld.dart file in VSCode and add the following

main(List<String> args){
 print('Hello World');
}
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Then run the code and see the result of Hello World

1.2 Analysis of procedures

The Dart language entry is also the main function and must be explicitly defined;
Dart’s entry function main does not return a value
Methods can omit void if they return no value
The command-line arguments passed to main are done through List

List is literally a collection type in Dart where each String is a parameter passed to Main
When defining strings, you can use either single or double quotation marks
Each line must end with a semicolon, and many languages, such as Swift, do not require semicolons

Define variables

1.1 Explicit Statement (Explicit)

How to declare a variable explicitly, in the following format:

Variable type Variable name = assignment;Copy the code

Sample code:

    String name = "Maybe";
    print(name);
    
    int age = 18;
    print(age); A double height = 24.5;print(height);

    // print("$name.$age.$height");
    print("A man${name}${age}${height}");
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Note: A defined variable can change its value, but it cannot be assigned to another type

name = 11; // Variables defined by error can be reassigned, but other types cannot be assignedCopy the code

1.2 Type Inference

A type derivation declares a variable in the following format:

Var /dynamic/const/final = assignment;Copy the code

In Dart syntax, variables that are not initialized automatically get a default value of NUll, everything is an object, and the default value of the object is NUll

    var thisNull;
    print("$thisNull, thisNull type${thisNull.runtimeType}");
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The code output above

Null,thisNull type nullCopy the code

1.2.1 Use of VAR

An example of var

RuntimeType is used to get the current type of a variable

    var type1 = 1; // Declare variablesprint("Current type1 type${type1.runtimeType}"); /* To get the current type of the variable */Copy the code

Incorrect use of var:

type1 = "2"; // You cannot assign a String to an intCopy the code

1.2.2 Using Dynamic

In development, variables are not normally declared using dynamic

   print("Change the variable type before type2${type2.runtimeType}");
   type2 = 1;
   print("Change the variable type of type2${type2.runtimeType}");
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In this case, a variable declared with dynamic will change its type after reassignment

1.2.3 Use of final&const

Final and const are used to define constants, that is, they cannot be changed after they are defined

   const type3 = "3"; // declare constant finaltype4 = "type4"; // declare constantstype3 = "4"; / / errortype4 = "type3"; / / errorCopy the code

This is where you get an error

Error: Can't assign to the const variable 'type3'. Cannot assign to type3 Error: Can't assign to the final variable 'type4'. Cannot be assigned totype4

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What’s the difference between final and const

Const assignments must be determined at compile time, that is, written dead
Final can be retrieved dynamically when assigning, for example to a function

String getName() {
 return "Maybe"; } const getName1 = getName(); Final getName2 = getName();Copy the code

Final and const are not reassigned once they have a definite result

    final date1 = DateTime.now();
    print(The current time is 0.$date1");
    sleep(Duration(seconds:2));
    print("Time after sleep date1:$date1");
    final date2  = DateTime.now();
    print("No time after sleep date2:$date2"); 2020-05-30 14:03:28.271718 Date1:2020-05-30 14:03:28.271718 Date2:2020-05-30 14:03:30.277818Copy the code

Const is placed to the right of assignment statements to share objects and improve performance

class Person{
  const Person();
}

    final a = const Person();
    final b = const Person();
    print(identical(a,b));  //true
    
    final a =  Person();
    final b =  Person();
    print(identical(a,b)); //flase

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Number types

3.1 Number Types

For values, we don’t have to worry about symbols, width and precision. You just need an int for an integer and a double for a floating-point

The range of ints and doubles represented in Dart is not fixed; it depends on the platform on which Dart is running

Main (List<String> args) {/* int*/ int age = 18; int hexAge = 0x12;print(age);
  print(hexAge); /* double*/ double height = 1.88;print(height); Var a = int. Parse ('1111');
 var b = double.parse('22.22');
 print(${a.runtimeType}, $a'); // The data type of a is int,a is 1111print('data type ${b.untimeType}, value $b'); Var num1 = 123; var num1 = 123; Var num2 = 456.789; Var num3 = 1.234567890123; Var num4 = 2.456788656898; var num1Str = num1.toString(); // The data type of num1Str is String, and the value of num1Str 123 var num2Str = num2.tostring (); //num2Str Data type String,num2Str value 456.789 var num2StrD = num2.toStringasFixed (2); Num2StrD = 456.79; num2StrD = 456.79; This is not a simple capture If the value of 456.78 num2 = 456.781 num2StrD var num3Str = num3. ToStringAsExponential (3); Var num4Str = num4.tostringasprecision (4); // Precision is the length of the string, not including the decimal point, 4, output 2.457print(${num1Str. RuntimeType}, the value of num1Str $num1Str');
 print(${num2Str. RuntimeType},num2Str value $num2Str');
 print(${num2strd. runtimeType},num2StrD value $num2StrD');
 print(${num3Str. RuntimeType},num3Str value $num3Str');
 print(${num4Str. RuntimeType}, the value of num4Str $num4Str');

}

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3.2 Boolean types

Dart provides a bool of the Boolean type true and flase

// Boolean var isFlag =true;
 print('$isFlag ${isFlag.runtimeType}'); //true bool
 
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Note: Dart cannot determine that non-zero is true, or that non-null is true

Dart’s type-safety means you can’t use code like IF (non-BooleanValue) or Assert (non-BooleanValue)

var message = 'Hello Dart';
 if(message){
    print('Judgment went here.');
 } //Error: A value of type 'String' can't be assigned to a variable of type 'bool'.

 
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3.3 String Types

The Dart string is a sequence of UTF-16 encoding units, and you can create a string using either single or double quotes

var s1 = 'Hello Flutter';
  var s2 = "Hello Flutter";
  var s3 = 'Hello \'Flutter'; var s4 = "Hello ' Flutter"; var s5 = ''' 111 222 333 444 '''; // This represents a multi-line stringCopy the code

Here you can use three single quotes to represent a multi-line string

When concatenating strings with other variables or expressions, use ${expression}. If the expression is an identifier, {} can be omitted.

   print("s1 = $s1 s2 = $s2 s3 = $s3 s4 = $s4 s5 = ${s5}");         

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3.4 Set Types

3.4.1 Definition of collection types

Dart has three commonly used Set types built in: List, Set, and Map

3.4.1.1 List definition

Var list1 = [var list1 = ['a'.'b'.'c'.'d'];
  print('$list1, ${list1.runtimeType}'); / / / a, b, c, d, a List < String > / / 2, explicitly specify type List < int > list2 = [1, 2, 3, 4];print("$list2.${list2.runtimeType}"); //[1, 2, 3, 4],List<int>
  
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3.4.1.2 set definition

A Set is defined like a List, except that it is unordered and its elements do not repeat

  var set1 = {1, 2, 3, 4};print('$set1,${set1.runtimeType}');

  Set<String> set2 = {'black'.'white'.'gray'};
  print('$set2,${set2.runtimeType}');
  
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If the elements in a Set are the same, I think it will ignore the following identical elements, with a warning: Two elements in a set literal shouldn’t be equal. Change or remove the duplicate element.

3.4.1.3 Map definition

Var map1 = {var map1 = {var map1 = {'name':'Maybe'.'age':'18'};
  print('$map1,${map1.runtimeType}'); //{name: Maybe, age: 18},_InternalLinkedHashMap<String, String>

  Map<String,Object> map2 = {'height':'188'.'weight':'140'};
   print('$map2,${map2.runtimeType}'); //{height: 188, weight: 140},_InternalLinkedHashMap<String, Object>Copy the code

3.4.1 Common operations of collections

3.4.1.1 Obtaining the Length of a Set

All collections support getting their lengthlength

  print('List1 length,${list1.length}'); // The length of list1,4print(${set1. Length}'); //setThe length of 1 is 4print(${map1. Length}'); // The length of map1,2Copy the code

3.4.1.2 Add, Delete, or Include Operations

forListSince the elements are ordered, it provides a method to delete the elements at the specified index position

Common List operations

  list1.add('f');
  list2.add(5);
  print('List add $list1, $list2'); // Add [a, b, c, d, f],[1, 2, 3, 4, 5] to List; //Error: The argumenttype 'List<int>' can't be assigned to the parameter type 'Iterable<String>'. list1.addAll(list3); 1. Append all objects from [iterable] to the end of the list; 2. Expand the list based on the number of objects in [iterable]. If the list is fixed length, raise [UnsupportedError] print('List after concatenation$list1'); // +++++++[a, b, c, d, f, x, y, z] list1.remove('a'); Print (')List specifies that an element should be deleted$list1'); RemoveAt (1); list1.removeat (1); list1.removeat (1); Print (')List deletes the element with subscript 1$list1'); [b, d, f, x, y, z] list1.removelAst (); Print (')List removes the last element$list1'); [b, d, f, x, y] list1.removeRange(0, 3); // print('List is obtained by removing elements from 0 to 3$list1'); // list1.removeWhere((element) => false); // list1.removeWhere((element) => 1); list1.contains(1); Print (')${list1.contains(1)}'); // Return false because the existing list1 contains only x and y, so return flaseCopy the code

Common Map operations

Because it iskey/valueFormal and unordered, so any operation needs to be explicitly based onkey,valueOr is itkey/value.

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Flutter Basic Dart Syntax (1)

A, Hello Dart

1.1 Hello World

1.2 Analysis of procedures

Define variables

1.1 Explicit Statement (Explicit)

1.2 Type Inference

1.2.1 Use of VAR

1.2.2 Using Dynamic

1.2.3 Use of final&const

Number types

3.1 Number Types

3.2 Boolean types

3.3 String Types

3.4 Set Types

3.4.1 Definition of collection types

3.4.1.1 List definition

3.4.1.2 set definition

3.4.1.3 Map definition

3.4.1 Common operations of collections

3.4.1.1 Obtaining the Length of a Set

3.4.1.2 Add, Delete, or Include Operations

Flutter Basic Dart Syntax (1)

A, Hello Dart

1.1 Hello World

1.2 Analysis of procedures

Define variables

1.1 Explicit Statement (Explicit)

1.2 Type Inference

1.2.1 Use of VAR

1.2.2 Using Dynamic

1.2.3 Use of final&const

Number types

3.1 Number Types

3.2 Boolean types

3.3 String Types

3.4 Set Types

3.4.1 Definition of collection types

3.4.1.1 List definition

3.4.1.2 set definition

3.4.1.3 Map definition

3.4.1 Common operations of collections

3.4.1.1 Obtaining the Length of a Set

3.4.1.2 Add, Delete, or Include Operations

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