Dart language Basics – Learn it and you’ll almost be able to use it

Thing in common:

1. The declared type can be omitted, as shown in the following example

final str = "str"; //final String str = "str"; The String type can be omitted
const str2 = "str2"; //const String str2 = "str2"; Like final, String can be omitted
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2. No more assignment after initialization (as in Java);
3. It cannot be used with var, as shown in the following example

final ~~var~~ f1 = ""; // Error cannot be used together with var
const~ ~var~~ f2 = ""; // Error cannot be used together with var
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Differences (note) :

1. Class level constants, using static const; The following figure shows DateTime source code as an example

1. Const can initialize its value with the value of another const constant; Look at the following example

const a = 1; 
const b = 2; 
var c = 3; 
final d =4; 
const sum = a+b;// Yes, it is running properly
const sum2 = a+c;// c is not const
const sum3 = a+d;// error: d is not const
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3. Const assignment declaration, const can be omitted; Look at the following example

const list1 = []; 
const list2 = const [];/ / const can be omitted
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4. You can change the value of a nonfinal, nonconst variable, even if it once had a const value; Look at the following example

var list = const [1.2.3]; 
list = [3.2.1]; 
print(list);// the output is [3,2,1]
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5. Immutability resulting from const is transitive; Look at the following example

  final list1 = [1.2.3];
  list1[0] = 0;
  print(list1);// output is [0,2,3]
  const list2 = [1.2.3];
  list2[0] = 0;// An error is reported and cannot be modified
   print(list2);List2 cannot be modified
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6. The same const constant is not created twice in memory; Look at the following example

const a = 1; 
const b =1; 
const c =2; 
print(identical(a,b));//true 
print(identical(a,c));//false
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7. Const needs to be a compile-time constant;

const dt = DateTime.now();Const must be a compile-time constant
final dt1 = DateTime.now();// Yes, it is running properly
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2 Built-in Types

2.1 Built-in types -num, int, and double

• Int: integer value;
• Double: 64-bit double precision floating point number;
• Int and double are subclasses of num;

2.2 Built-in Type -String

1. The Dart string is a SEQUENCE of UTF-16 encoded characters that can be created using either single or double quotation marks;
2. You can create a multi-line string object with three single or double quotation marks;
3. You can create “raw” strings using the r prefix; Look at the following example

String s1 = "123" 'abc' "okok";
String s2 = "123" + 'abc';
String s3 = ' ''ewrwerAeawrwer eee'' ';
String s4 = ' ''ewrwerAeawrwer\neee'' ';
String s5 = r' ''ewrwerAeawrwer\neee'' ';
print(s1);
print(s2);
print(s3);
print(s4);
print(s5);
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Running results:

123abcokok
123abc
ewrwerAeawrwer
  eee
ewrwerAeawrwer
eee
ewrwerAeawrwer\neee
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Now that you’ve tried String, try StringBuffer as shown in the following example

StringBuffer sb1 = StringBuffer();
StringBuffer sb2 = StringBuffer();
sb1.write("333");
sb1.write("aaa");
sb1.write("666"); sb2.. write("333").. write("aaa").. writeAll(['6'.'6'.'6']);/ /.. Cascade character, can be called inside the chain method
print(sb1);//333aaa666
print(sb2);//333aaa666
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A StringBuffer object can be printed without calling the.toString() method. Why? Let’s take a look at how print() is implemented internally. See below, it turns out that it prints the toString() method of the called object.

4. We can use expressions in strings:${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. Look at the following example

var a = 1; 
var b = 2; 
print("${a + b}"); // Output: 3
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2.3 Built-in Type -bool

• Unlike Java, bool objects default to null for uninitialization; Look at the following example

bool bo; 
print(bo);// output: null if(bo){// error, because bool is not initialized default is null, so this call will run error}
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Dart’s List doesn’t look exactly like a List. It’s a bit like an array in Java. Dart can print lists directly, including elements of lists, which are also objects. In Java, printing a list directly results in the address value. The subscript index of a List in Dart starts at 0, as in Java. Generics are supported as in Java. Add, delete, change, check, support reverse order, own sort, shuffle, you can use + to merge two lists.

/ / declare
  // Automatic length
  List growableList = List();
//  List growableList = new List()..length = 3;growableList.. add(1).. add(2).. add('damon');
  print('growableList: ${growableList}');
  // Fixed length
  var list = List(3); //List declaration, can use var or List.
  list[0] = 1; // The subscript index starts at 0
  list[1] = 2;
  list[2] = 'damon';
  print('list: ${list}');
  // The element type is fixed
  var typeList = List<int>();
  typeList.add(1);
  typeList.add(2);
  typeList.add(3);
  print('typeList: ${typeList}');
  // Common attributes
  int first = typeList.first;
  print('typeList.first: ${first}'); // The first element
  int last = typeList.last;
  print('typeList.last: ${last}'); // The last element
  int length = typeList.length;
  print('typeList.length: ${length}'); // Number of elements
  bool isEmpty = typeList.isEmpty;
  print('typeList.isEmpty: ${isEmpty}'); // Whether it is empty
  bool isNotEmpty = typeList.isNotEmpty;
  print('typeList.isNotEmpty: ${isNotEmpty}'); // Whether it is not null
  Iterable reversed = typeList.reversed;
  print('typeList.reversed: ${reversed}'); / / reverse
  // Add, delete, change, sort, shuffle, copy sublist
  var list4 = [];
  / / to add
  list4.add(1);
  print('add 1 :${list4}');
  list4.addAll([2.3.4]);
  print('addAll [2, 3, 4] :${list4}');
  list4.insert(0.0);
  print('insert(0, 0) :${list4}');
  list4.insertAll(1[5.6.7]);
  print('insertAll(1, [5, 6, 7]) :${list4}');
  / / delete
  list4.remove(5);
  print('remove 5 :${list4}');
  list4.removeAt(2);
  print('remove at 0 :${list4}');
  / / change
  list4[4] = 5;
  print('update list4[4] to 5 :$list4}');
  //range
  list4.fillRange(0.3.9);
  print('fillRange update list4[0]-list4[2] to 9 :$list4}');
  Iterable getRange = list4.getRange(0.3);
  print('getRange list4[0]-list4[2] :$getRange}');
  / / check
  var contains = list4.contains(5);
  print('list4 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 :${list4}');
  // Copy the sublist
  var list5 = list4.sublist(1);
  print('sublist(1) list5 :${list5}');
  / / operators
  var list6 = [8.9];
  print('list6 :${list6}');
  var list7 = list5 + list6;
  print('list5 + list6 :${list7}');
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If you look at the figure below, this is a List that has some common attributes that we know when we use them, so I’m not going to illustrate them here.

2.5 Built-in Types -Map

/ / declare
  // Dynamic type
  var dynamicMap = Map(a); dynamicMap['name'] = 'dongnao';
  dynamicMap[1] = 'android';
  print('dynamicMap :${dynamicMap}');
  / / type
  var map = Map<int, String> (); map[1] = 'android';
  map[2] = 'flutter';
  print('map :${map}');
  // It can also be declared like this
  var map1 = {'name': 'dongnao'.1: 'android'};
  map1.addAll({'name':'damon'});
  print('map1 :${map1}');
  // Common attributes
// print(map.isEmpty); // Whether it is empty
// print(map.isNotEmpty); // Whether it 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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Map is similar to Java in that it has key-value pairs, which we know when we use them, but we won’t go through all the examples here.

2.6 Built-in Type -Set No duplicate list

var dynamicSet = Set(a); dynamicSet.add('dongnao');
  dynamicSet.add('flutter');
  dynamicSet.add(1);
  dynamicSet.add(1);
  print('dynamicSet :${dynamicSet}');
  // Common attributes are similar to list
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The basic usage of Set is similar to Java, with a few differences. Set1.difference (set2): returns the set of elements present in set1 but not in set2; Set1. intersection(set2) : Returns the intersection of set1 and set2; Set1. union(set2) : returns the union of set1 and set2; Set1.retainall () : set1 only preserves certain elements (the elements to be retained must exist in the original set);

• Look at the following example:

  Set set1 = Set(a); set1.addAll(['a'.'b'.'a'.'c'.'d']);
  Set set2 = Set(a); set2.addAll(['a'.'b'.'a'.'e'.'f']);
// print(set1.difference(set2)); {c, d}
// print(set1.intersection(set2)); {a, b}
// print(set1.union(set2)); {a, b, c, d, e, f}
  set1.retainAll(['a'.'b']);//set1 preserves only certain elements (the elements to be preserved must exist in the original set)
  print(set1);{a, b}
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2.7 Built-in types -Runes

• Dart Runes is a String object in the UTF-32 character set.

1. Runes is used to represent Unicode characters in strings;
2. Display character graphs using string.fromCharcodes;
3. If there are more than four values, place the encoded value in braces.

• Look at the following example

Runes runes = new Runes('\u6787 \u2665 \u{1f605} \u{1f60e} \u{1f44d}'); 
print(String.fromCharCodes(runes));// Py? ? ?
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Output result:

Py ♥ 😅 😎 👍Copy the code

CopyChar | free special symbol, can refer to it.

2.8 Built-in type -Symbol

1. The symbol literal is a compile-time constant preceded by the #; Look at the following example

#foo_lib 
#runtimeType
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2. If it is determined dynamically, use the Symbol constructor to instantiate it via new. Look at the following example

Symbol obj = new Symbol('name');
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The mirrors module has been removed. The mirrors module is no longer used for reflection.

3 function