8. Master the functions in Dart and you master the Force!

您所在的位置：网站首页 › function assert param declared › 8. Master the functions in Dart and you master the Force!

8. Master the functions in Dart and you master the Force!

2023-03-23 14:36| 来源: 网络整理| 查看: 265

Directory Portals: A Guide to The Quick Start To Flutter

As mentioned in the previous section, in Dart, a Function is also an object of type Function.

It can be assigned to a variable and passed as an argument.

Declare a function

This is a normal function declaration:

bool isNoble(int atomicNumber) { returnatomicNumber ! = null; }Copy the code

If a function does not display a declared return value, Dart automatically deduces its return value type.

void main() { bool b = isNoble(2); print(b); } isNoble(int atomicNumber) { returnatomicNumber ! = null; } true Copy the code

If a function does not display a return, it will return NULL by default.

fun(){} assert(fun() == null) Copy the code

If a function has only one expression, use the shorthand:

bool isNoble(int atomicNumber) = atomicNumber ! = null; isNoble(int atomicNumber) = atomicNumber ! = null;Copy the code Optional named parameter

The {} symbol can be used to specify the name of a function parameter.

void enableFlags({bool bold = false, bool hidden = false, @required String content}) { // ... } Copy the code

As you can see, Dart allows us to set default values for parameters.

Call:

enableFlags(bold: true, content: 'required'); enableFlags(bold: true, hidden: false, content: 'required'); Copy the code

Use the @required annotation argument to indicate the required argument, which you cannot omit when calling.

This form of writing is also allowed and not used:

void doStuff( {Listint list = const [1, 2, 3], MapString, String gifts = const { 'first': 'paper'.'second': 'cotton'.'third': 'leather'{}})print('list: $list'); print('gifts: $gifts'); } Copy the code Optional position parameter

With the [] symbol, you can specify that the arguments to the position are optional:

String say(String from, String msg, [String device = 'carrier pigeon', String mood]) { var result = '$from says $msg'; if(device ! = null) { result ='$result with a $device'; } if(mood ! = null) { result ='$result (in a $mood mood)'; } return result; } assert(say('Bob'.'Howdy') = ='Bob says Howdy with a carrier pigeon'); Copy the code

The parameters of device and mood are optional, and device has a default value.

Function as variable

As mentioned earlier, a Function in Dart is an object of type Function, so it can be assigned to a variable.

var say= (str){ print(str); }; say("hi world"); Copy the code Function as an input parameter

Function as an object, it can of course also be passed as a parameter:

void execute(var callback){ callback(); } execute(()=print("xxx")) Copy the code A function closure

The Dart function has features that make it easy to implement closures:

Function makeAdder(num addBy) {return (num i) = addBy + i; } void mainVar add2 = makeAdder(2); var add2 = makeAdder(2); Var add4 = makeAdder(4); assert(add2(3) == 5); assert(add4(3) == 7); }Copy the code

From this example, you can get a sense of the power of closures, which define a dynamically configurable function object.

The typedef keyword

Dart provides the typedef keyword to define a function format (with strict format checking), such as:

typedef int Compare(Object a, Object b); Copy the code

A function of this form is then defined as a Compare. It's actually a similar concept to the definition of a class.

Consider this example:

Compare test = (Object a, Object b){ return 0; } Copy the code

A function of this type is a function of type Compare.

When you assign a function to a variable of Compare, it checks to see if the argument types and return types correspond exactly to Compare. If they do not, the compiler will report an error.

Directory Portals: A Guide to The Quick Start To Flutter

How can I be found?

Portal:CoorChice homepage

Portal:Lot of CoorChice

【本文地址】

今日新闻

function assert param declared

8. Master the functions in Dart and you master the Force!

8. Master the functions in Dart and you master the Force!

今日新闻

推荐新闻