dart: function part

There are three functions in Dart:

• Position parameter function
• named function
• Anonymous function

• Definition method of optional and required parameters
  • Optional parameters are wrapped in the form of [], which can be omitted and not added during the call
  • If package description is not in the form of [], this parameter is required
  • If you want to specify default values for optional parameters, you can directly add the default values of parameters by * * = + [default] *

var favoriateFruit = {"Apple", 'Banana', 'A mandarin orange'}; var fruitMap = {"Apple": 5.5, 'Banana': 3.99, 'A mandarin orange': 6.8}; // Functions without parameters dynamic getFruitPrice() { print(favoriateFruit); print(fruitMap); } // Multiple logics need to be defined with function expressions // Unnamed function definition method // The function named by position parameter needs to wrap the corresponding function name through [] if it needs to add anonymous function dynamic getPrice(String fruit, [String notes = 'default notes']) { if (notes != null) { print('notes = $notes'); } if (favoriateFruit.contains(fruit)) { return fruitMap[fruit]; } return null; }

• How to call position parameter function

void main(){ var price = getPrice('Banana', 'Look at the price of bananas'); // Optional parameters can not be assigned first, and default assignment can be enabled. var nullPrice = getPrice('Pitaya'); print(price); print(nullPrice); } /* notes = Look at the price of bananas notes = Default notes 3.99 null */

How to define optional and non optional functions

• When defining a function, an object similar to set is passed in
• If it is a required parameter, it needs to be declared with @ required
• All parameters not declared through required are optional
• @required is defined in meta.dart, so you need to add a meta package when using it, otherwise an error will be reported
• The method of setting the default parameter is the same as that of the named function

import 'package:meta/meta.dart'; void addFruit( ) { favoriateFruit.add(fruit); fruitMap[fruit] = price; print(notes); }

The method of adding meta package can be solved by calling flitter package. Specific method

1. Create a pubspec.yaml file

   2. Just fill in the following code

How to call a named function

addFruit(fruit: 'Strawberry', price: 23.5, notes: 'Increase strawberry'); // Use default when price is not passed in addFruit(notes: 'Winter melon, price unknown', fruit: 'Wax gourd'); getFruitPrice(); /* Increase strawberry Winter melon, price unknown */

List<Map<String, String>> fruitList = [ {"fruit": "Durian", "price": "32"}, {"fruit": "pineapple", "price": "3.5"} ]; // The following two results are identical // How to use anonymous functions fruitList.forEach((elem) { String str = "$The price is $"; print(str); }); // If the expression has only one sentence, you can use the arrow function fruitList.forEach((elem) => print("$The price is $")); /* The price of durian is 32 A catty of pineapple costs 3.5 The price of durian is 32 A catty of pineapple costs 3.5 */

// Function is the first citizen can pass function to function // Define a handler for each void addFruitByList(Map<String, String> elem) { addFruit( notes: 'Bulk fruit', fruit: elem['fruit'], price: double.parse(elem['price'])); } void main() { fruitList.forEach(addFruitByList); getFruitPrice(); } /* Bulk fruit Bulk fruit */

Function scope is different from JS. Variables defined by var or const are deadband from within function scope to before variables are defined

void forScope() { // After the variable is determined, there will be a dead zone between the var and the const in js. Therefore, the value of the external variable can no longer be obtained if the same variable is defined as the external variable // There will be a mistake here // print(fruitMap); var fruitMap = {'test'}; print('I'm on the first floor=$fruitMap'); void Second() { var fruitMap = {'The second floor'}; print('I'm on the second floor=$fruitMap'); } Second(); } /* I'm the first layer = I am the second layer = */

Using closures can achieve corrilization, which is completely consistent with the method used in JS. No more details are required

// Use of closure syntax dynamic definePriceChanger(String fruit) { if (fruitMap.keys.contains(fruit)) { print('$fruit Price can change'); return (num price) { fruitMap[fruit] = price; }; } else { return ([num _noParam]) { print('Fruit price cannot be modified'); }; } } void main() { var changeBanana = definePriceChanger('Banana'); getFruitPrice(); changeBanana(2.99); getFruitPrice(); } /* */

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4 February 2020, 00:27 | Views: 4181