Dart 中的函数 typedef/函数类型别名是什么？

A common usage pattern of typedef in Dart is defining a callback interface. For example:

typedef void LoggerOutputFunction(String msg);


class Logger {
LoggerOutputFunction out;
Logger() {
out = print;
}
void log(String msg) {
out(msg);
}
}


void timestampLoggerOutputFunction(String msg) {
String timeStamp = new Date.now().toString();
print('${timeStamp}: $msg');
}


void main() {
Logger l = new Logger();
l.log('Hello World');
l.out = timestampLoggerOutputFunction;
l.log('Hello World');
}

Running the above sample yields the following output:

Hello World
2012-09-22 10:19:15.139: Hello World

The typedef line says that LoggerOutputFunction takes a String parameter and returns void.

timestampLoggerOutputFunction matches that definition and thus can be assigned to the out field.

Let me know if you need another example.

Dart 1.24 introduces a new typedef syntax to also support generic functions. The previous syntax is still supported.

typedef F = List<T> Function<T>(T);

For more details see https://github.com/dart-lang/sdk/blob/master/docs/language/informal/generic-function-type-alias.md

Function types can also be specified inline

void foo<T, S>(T Function(int, S) aFunction) {...}

See also https://www.dartlang.org/guides/language/language-tour#typedefs

Just slightly modified answer, according to the latest typedef syntax, The example could be updated to:

typedef LoggerOutputFunction = void Function(String msg);


class Logger {
LoggerOutputFunction out;
Logger() {
out = print;
}
void log(String msg) {
out(msg);
}
}


void timestampLoggerOutputFunction(String msg) {
String timeStamp = new Date.now().toString();
print('${timeStamp}: $msg');
}


void main() {
Logger l = new Logger();
l.log('Hello World');
l.out = timestampLoggerOutputFunction;
l.log('Hello World');
}

typedef LoggerOutputFunction = void Function(String msg);

this looks much more clear than previous version

Typedef in Dart is used to create a user-defined function (alias) for other application functions,

Syntax: typedef function_name (parameters);

With the help of a typedef, we can also assign a variable to a function.

Syntax:typedef variable_name = function_name;

After assigning the variable, if we have to invoke it then we go as:

Syntax: variable_name(parameters);

Example:

// Defining alias name
typedef MainFunction(int a, int b);


functionOne(int a, int b) {
print("This is FunctionOne");
print("$a and $b are lucky numbers !!");
}


functionTwo(int a, int b) {
print("This is FunctionTwo");
print("$a + $b is equal to ${a + b}.");
}


// Main Function
void main() {
// use alias
MainFunction number = functionOne;


number(1, 2);


number = functionTwo;
// Calling number
number(3, 4);
}

Output:

This is FunctionOne
1 and 2 are lucky numbers !!
This is FunctionTwo
3 + 4 is equal to 7

Since dart version 2.13 you can use typedef not only with functions but with every object you want.

Eg this code is now perfectly valid:

typedef IntList = List<int>;
IntList il = [1, 2, 3];

For more details see updated info: https://dart.dev/guides/language/language-tour#typedefs

https://www.tutorialspoint.com/dart_programming/dart_programming_typedef.htm

 typedef ManyOperation(int firstNo , int secondNo);   //function signature
Add(int firstNo,int second){
print("Add result is ${firstNo+second}");
}
Subtract(int firstNo,int second){
print("Subtract result is ${firstNo-second}");
}
Divide(int firstNo,int second){
print("Divide result is ${firstNo/second}");
}
Calculator(int a,int b ,ManyOperation oper){
print("Inside calculator");
oper(a,b);
}
main(){
Calculator(5,5,Add);
Calculator(5,5,Subtract);
Calculator(5,5,Divide);
}