骏域网站建设专家电话,宁波开发网络推广平台,建立网站的条件,西安 网站建设 培训学校1 数据类型 数字(number) int double 字符串转换成 num int.parse(“1”) double.parse(“1”);double 四舍五入保留两位小数 toStringAsFixed(2) 返回值为stringdouble 直接舍弃小数点后几位的数据 可使用字符串截取的方式 字符串(string) 单引号 双引号 三引号三引号 可以输…1 数据类型 数字(number) int double 字符串转换成 num int.parse(“1”) double.parse(“1”);double 四舍五入保留两位小数 toStringAsFixed(2) 返回值为stringdouble 直接舍弃小数点后几位的数据 可使用字符串截取的方式 字符串(string) 单引号 双引号 三引号三引号 可以输出文本格式 比如换行 布尔值(boolean) 列表(list) 一旦定义了 内容的类型 就不可变list内容是常量 添加const 集合(set) 映射(map)符文(rune)符号(Symbol)
- 数字(number) int double- 字符串转换成 num int.parse(1) double.parse(1);- double 四舍五入保留两位小数 toStringAsFixed(2) 返回值为string- double 直接舍弃小数点后几位的数据 可使用字符串截取的方式
//字符串转换成 num var a int.parse(1);var b double.parse(1);print(a);print(b);
//结果
//1
//1.0- 字符串(string)- 单引号 双引号 三引号- 三引号 可以输出文本格式 比如换行
var a aaabbb;print(a);a aaabbb;print(a);a aaa bbb;print(a);
结果
aaabbb
aaabbb
aaa bbb- 列表(list)- 一旦定义了 内容的类型 就不可变- - list内容是常量 添加const
void main(){var list [1,2,3];// list [1,2,3]; // Error: A value of type String cant be assigned to a variable of type int.var cars [ Car(TSLT),Car(XRB)];print(cars);print(cars[0].name);print(cars[0].drive());var names const [TSLT,XRBS]; //内容不可改变print(names);names[1] TSLC; // Cannot modify an unmodifiable listprint(names);
}
class Car{var name;Car(this.name);bool drive(){print(${name}我着了...);return true;}
}结果[Instance of Car, Instance of Car]
TSLT
TSLT我着了...
true
[TSLT, XRBS]void main(){var a {1,2,3.4,Car(5),true};print(a);a.add(aaa);print(a);var b Set();b.add(1);b.add(2);b.add(3.4);b.add(Car(5));b.add(true);print(b);// var c const {1,2,3.4,Car(5),true}; // Error: The element 3.4 does not have a primitive equality.var c const {1,你好,true}; // Error: The element 3.4 does not have a primitive equality.print(c);
}
结果
{1, 2, 3.4, Instance of Car, true}
{1, 2, 3.4, Instance of Car, true, aaa}
{1, 2, 3.4, Instance of Car, true}
{1, 你好, true}
2 var 关键字
var 关键字一旦赋值类型即确定
void main(){var a 1;var b hello;var c Car();c.drive();print(a.runtimeType);print(b.runtimeType);print(c.runtimeType);
} class Car{void drive(){print(我跑了);}
}结果
我跑了
int
String
Car3 dynamic 和 Object
Object是Dart所有对象的父类Object和dynamic声明变量 可改变类型dynamic 可使用对象类型的属性和方法 Object声明的对象只能用Object的属性和方法
void main(){Object a;a hello;print(a 的类型是 ${a.runtimeType});a 1;print(a 的变化后类型是 ${a.runtimeType});dynamic b;b 1;print(b 的类型是 ${b.runtimeType});b 1.1;print(b 的变化后类型是 ${b.runtimeType});a hello;b world;print(b.length);// print(a.length); //报错 The getter length isnt defined for the class Object.
}结果
a 的类型是 String
a 的变化后类型是 int
b 的类型是 int
b 的变化后类型是 double
54 final 和 const
const 编译时赋值final 运行时 第一次使用时 赋值
void main(){final a;a 1;print(a);// const b; // 报错 Error: The const variable b must be initializedconst b 2;print(b);
}结果
1
25 空安全
void main(){Car? car;print(car?.drive());car Car();print(car?.drive());
}
class Car{bool drive(){print(我着了...);return true;}
}运行结果
lib/demo.dart:8:9: Warning: Operand of null-aware operation ?. has type Car which excludes null.- Car is from package:demo/demo.dart (lib/demo.dart).print(car?.drive());^
null
我着了...
true函数 (方法)
不声明返回值的 默认是dynamic
run(){print(我跑了);
}eat(){return 吃撑了;
}void main(){print(run());print(eat());
}结果
我跑了
null
吃撑了函数可以作为变量
var run (){print(我跑了);
};var eat (){return 吃撑了;
};void main(){print(run());print(eat());
}结果
我跑了
null
吃撑了函数可以作为参数
run(String name){print(${name}跑了);
}eat(String name){return ${name}吃撑了;
}see(var seeWho){seeWho(老六);
}void main(){var car Car(安妮);car.who(run);see((var name)print(name));
}
class Car{var name;Car(this.name);who(var run){run(name);}
}运行结果
安妮跑了
老六可变函数
eat(String name,{String? food,int? num}){return ${name}吃${food}撑了${num}份;
}speak(String name,[String? and,String? chat]){return ${name}吃${and}${chat};
}void main(){print(eat(张三));print(eat(张三,food: 麻辣烫,num: 10));print(speak(张三));print(speak(张三,李四));print(speak(张三,李四,聊天));
}运行结果
张三吃null撑了null份
张三吃麻辣烫撑了10份
张三吃nullnull
张三吃李四null
张三吃李四聊天扩展函数 常用类型转换 / 时间处理 / 或者Widget的扩展例如padding多个扩展函数若已定义 不可重复定义 即函数名不可重复dynamic 不可使用扩展函数
/*** 扩展函数* 格式* extension 函数名 on 类**/
extension toNum on String{int toInt(){return int.parse(this);}double toDouble(){return double.parse(this);}
}void main(){var a 12.toInt();print(a);print(a.runtimeType);dynamic b 12;
}结果
12
int 类 默认构造函数命名构造函数 Student2.fromJson() import dart:convert;void main(){var student Student(张三, 三年二班, 18, 2005-12-12 12:12:12);print(student${student});var student1 Student1(张三, 三年二班, 18, 2005-12-12 12:12:12);print(student1${student1});var student11 Student1.name(李四); //命名构造函数print(student11${student11});String jsonString {name: John,classGrade: 三年二班,birthday: 2005-12-12 12:12:12, age: 30};var student2 Student2.fromJson(jsonString);print(student2.toString());
}class Student{//不添加构造函数 这些属性 会报错 或者可以加上String name,classGrade;int age;String birthday;Student(this.name, this.classGrade, this.age, this.birthday);MapString,dynamic toJson(){return {name:name,classGrade:classGrade,age:age,birthday:birthday};}overrideString toString() {return Student{name: $name, classGrade: $classGrade, age: $age, birthday: $birthday};}
}class Student1{//不添加构造函数 这些属性 会报错 或者可以加上String? name,classGrade;int? age;String? birthday;Student1(this.name, this.classGrade, this.age, this.birthday);Student1.name(this.name);overrideString toString() {return Student1{name: $name, classGrade: $classGrade, age: $age, birthday: $birthday};}
}class Student2{String name ,classGrade ;int age 0;String birthday ;//命名构造函数Student2.fromJson(String jsonStr){var json jsonDecode(jsonStr);print(jsonStr);this.name json[name];this.classGrade json[classGrade];this.age json[age];this.birthday json[birthday];}overrideString toString() {return Student2{name: $name, classGrade: $classGrade, age: $age, birthday: $birthday};}
}运行结果
studentStudent{name: 张三, classGrade: 三年二班, age: 18, birthday: 2005-12-12 12:12:12}
student1Student1{name: 张三, classGrade: 三年二班, age: 18, birthday: 2005-12-12 12:12:12}
student11Student1{name: 李四, classGrade: null, age: null, birthday: null}
{name: John,classGrade: 三年二班,birthday: 2005-12-12 12:12:12, age: 30}
Student2{name: John, classGrade: 三年二班, age: 30, birthday: 2005-12-12 12:12:12}常量构造函数 常量构造函数 const修饰构造函数常量构造函数用const 修饰时 属性值一样 则对象相等
import dart:convert;class Student{String name;int age;Student(this.name, this.age);
}class Student1{final String name;final int age;const Student1(this.name, this.age);
}void main(){var student Student(张三,18);var student0 Student(张三,18);print(student.hashCode);print(student0.hashCode);print(student student0);print(\n);var student1 const Student1(张三,18);var student11 const Student1(张三,18);var student12 const Student1(张三,19);print(student1.hashCode);print(student11.hashCode);print(student12.hashCode);print(student1 student11); //trueprint(student12 student11); //false}结果
333142976
1033297191
false87118132
87118132
590927718
true
false工厂构造函数 import dart:convert;class Student{String? name;int? age;Student.init({String name ,int age 0}){this.name name;this.age age;}static Student instance Student.init();factory Student(){return instance;}}void main(){var student Student.instance;var student1 Student.instance;print(student.hashCode);print(student1.hashCode);
}结果
11925193
11925193类 默认属性方法修饰符是public_下划线表示私有单继承 多实现
class A{void printA(){print(A);}
}class B{void printA(){print(B);}
}abstract class M{void printM(){print(M);}
}abstract class N{void printN(){print(N);}
}interface class X{void printX(){print(X);}
}interface class Y{void printY(){print(Y);}
}
//单继承 多实现
class C extends A implements M,N,X,Y{overridevoid printM() {// TODO: implement printM}overridevoid printN() {// TODO: implement printN}overridevoid printX() {// TODO: implement printX}overridevoid printY() {// TODO: implement printY}}类的混入
mixin class P{void printP(){print(P);}
}mixin class Q{void printQ(){print(Q);}
}class D with P,Q{void printD(){print(D);}
}void main(){var d D();d.printD();d.printP();d.printQ();
}结果
D
P
Q
