Dart tips

dart Little knowledge -​ Dart Is an object-oriented programming language

• var- According to the character
  var It can be used to represent many different data types ,int,double,list Both can be used. var Express .
  string Types and integers cannot be mixed , Use both var Express , But logic is wrong .

1,final： Assigned once , It can be the object , character string , Numbers etc. , Variable used to modify the expression of a value .
const: Set an explicit value .

2,Boolean Boolean type ： Used to judge , The value is true or false.
var sex = true; var sex = false
if（sex）{ if（sex）{

3,List type （ Array ）

• Basic operation ： increase , Delete , Change , check
  • increase ：add
    addAll—— Batch addition
    insert—— Add... At the specified location insert（n, Number to be added ）
    insertAll—— Batch add
  • reduce ：remove
    remove—— Delete eligible elements
    clear—— Delete all elements
    removeAt—— Specify location search delete removeAt（ Delete , Don't delete ） The principle of moving left and not moving right
    *1.start and end Must be effective , The value range is 0 <= start <= end <= length
    *2. The removed array cannot be a fixed length array
  • Change ：setRange—— Modify an interval element
    setAll—— Modify the italicized elements after searching
    replaceRange—— Replace
    fillRange—— Replace an interval with a fixed value
    getRange—— Get the value of an interval
  • check ：singleWhere

//List<int m = [8, 12, 4, 1, 17, 33, 10];
int a = m.singleWhere((n) => n > 30);                              //33
int b =m.singleWhere((n) => n > 10);                              // Report errors 
int c = m.singleWhere((n) => n > 10, orElse: () => -1);   // Report errors 
int d = m.singleWhere((n) => n > 40, orElse: () => -1);  //-1

** In the use of singleWhere when , it is to be noted that The following points ：
When there is only one element in the array that meets the condition , Return the element
When there are multiple qualified elements in the array , Report errors
When there are multiple qualified elements in the array , Even if it's set orElse , also Will report a mistake
When there is no element in the array that meets the condition , return orElse Specified return value

       - subList（n）—— Return from index n After the value of 
         subList（m,n）—— Return from m To n Value 
         **any—— Determine whether there are elements in the array that meet the requirements 
                 print(list.any((v) => v > 2));
         every—— Determine whether the elements in the array meet the requirements 
         contains—— Determine whether the array contains an element 

4.Map type —— Mapping function

• Define fixed type arrays ：List<int ()–List<string ()
  Define a fixed length array ：List（n）
  Define a mixed type array ：List<dynamic ()
  Define array length ：List.length
  Array in reverse order ：reversed
  Determine whether the array is empty ：isEmoty

// var list = [1, 2, 3, 4;
print(list.isEmpty); //false;

## Determine whether the array is non empty ：isNotEmpty

List[n]=n+1

function and class
function —— By setting the function law , Process the data , To realize a certain function is the function of a programming language .
class —— In object-oriented programming , A function that matches an object .（ It's a special function ）

• Function return value ：
  • All functions have return values return
  • If the return value of the function is not specified , The default return value is null
  • Functions that do not return values , The system will add implicit return sentence

// main(){
    
    bool m(String str1,String str2){
    
      return str1 != str2;
    }
    String str1="YYQX";
    String str2="SXX";
    print(m(str1,str2));
   }

//main(){
    
  bool m(int a, int b) {
    
    return a == b;
  }
  int a = 1;
  int b = 2;
  print(m(a, b));
}

• Optional parameters ——a There are three variables in it ：name1,name2,c; among c It's an optional parameter . b Is a string variable ,$ String names can be used to introduce strings . stay c When it's worth it ,b The second half of the sentence will be displayed , On the contrary, it does not show .

// main() {
    
  String a(String name1, String name2, [String c = "YYQX"]) {
    
    var b = '$name1 like $name2';
    if (c != null) {
    
      b = '$b like $c';
    }
    return b;
  }
  print(a('YYQX', 'SXX'));
}