Flutter UI框架 pure的使用

#Flutter UI框架 pure的使用

描述

在Flutter中，pure 包并不是直接与界面纯净或原生风格相关的插件。实际上，pure 是一个Dart包，它通过扩展方法引入了函数式编程的核心要素。该包提供了函数组合/管道、函数记忆化、部分应用和递归蹦床等功能。

虽然 pure 包本身不是专门用于UI设计的，但它的功能可以在构建纯净和高效的Flutter应用程序时发挥重要作用。以下是对 pure 包功能的详细介绍，并附有完整的示例代码。

功能介绍

扩展方法 (Extensions)

pure 包提供了多种扩展方法，支持从0到9个参数的函数操作。以下是主要的功能：

1. 组合 (Composition)

组合允许将多个函数串联起来，简化代码结构并减少嵌套。

double intToDouble(int x) => x.toDouble();
String doubleToString(double x) => x.toString();

/// Traditional application
String intToStringApplication(int x) => doubleToString(intToDouble(x));

/// Composition
final intToStringComposition = doubleToString.dot(intToDouble);

/// Pipes
String intToStringPipe1(int x) => x.pipe(intToDouble).pipe(doubleToString); 
// or 
String intToStringPipe2(int x) => intToDouble(x).pipe(doubleToString); 

print(intToStringApplication(1)); // "1.0"
print(intToStringComposition(2)); // "2.0"
print(intToStringPipe1(3)); // "3.0"

2. 常量 (Constant)

创建一个总是返回特定值的函数，忽略输入参数。

const numbers = [1, 2, 3];
final zeros = numbers.map(0.constant);
print(zeros.toList()); // [0, 0, 0]

3. Curry 和 Uncurry

Curry 将多参数函数转换为一系列单参数函数；Uncurry 则相反。

String createPerson(String country, int age, String name) => "$country, $name, $age";

final createTwin = createPerson.curry("Canada")(12);

final tim = createTwin("Tim");
final jim = createTwin("Jim");

print(tim); // "Canada, Tim, 12"
print(jim); // "Canada, Jim, 12"

4. 参数翻转 (Flipping)

翻转函数参数的顺序。

String createPerson(String firstName, String lastName) => "$firstName, $lastName";
final createFormalPerson = createPerson.flip;

print(createFormalPerson('John', 'Doe')); // "Doe, John"

5. 可空调用 (Nullable Invocation)

允许对可空参数进行安全调用。

int sum(int first, int second) => first + second;
final sumNullable = sum.nullable;

print(sumNullable(2, 2)); // 4
print(sumNullable(2, null)); // null

6. 递归蹦床 (Recursion Trampolines)

提供栈安全的递归功能。

Tram<int> tramSum(int number, int sum) => number == 0
    ? Tram.done(sum)
    : Tram.call(() => tramSum(number - 1, sum + number));

final n = 99999999;

final trampolined1 = tramSum.bounce(n, 0); // 4999999950000000
// or
final trampolined12 = tramSum(n, 0).bounce(); // 4999999950000000

7. 记忆化 (Memoization)

缓存函数结果以优化性能。

int Function(int base) newCounter() {
  var counter = 0;

  return (base) => base + counter++;
}

final counter = newCounter().memoize();

print(counter(0)); // 0
print(counter(10)); // 11
print(counter(0)); // 0
print(counter(11)); // 13

8. 部分应用 (Partial Application)

对非Curry化的函数进行部分应用。

int sumThree(int a, int b, int c) => a + b + c;

int buffer(int b, int c) => sumThree(1, b, c);
final partial = sumThree.apply(1);

print(partial(2, 3)); // 6

9. Thunk

创建一个不带参数的函数，返回一个值。

int meaningOfLife(int value) => value;

final meaning = meaningOfLife.thunk(42);

print(meaning()); // 42

示例代码

以下是一个综合使用的示例代码，展示了如何在Flutter项目中使用 pure 包：

import 'package:flutter/material.dart';
import 'package:pure/pure.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('Pure Package Example')),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              Text('Result of composition: ${composition()}'),
              Text('Result of constant: ${constant()}'),
              Text('Result of curry and uncurry: ${curryUncurry()}'),
              Text('Result of trampoline recursion: ${trampolineRecursion()}'),
              Text('Result of memoization: ${memoization().join(', ')}'),
              Text('Result of partial application: ${partialApplication()}'),
              Text('Result of nullable: ${nullable().toString()}'),
              Text('Result of flip: ${flip()}'),
            ],
          ),
        ),
      ),
    );
  }
}

String composition() {
  double intToDouble(int x) => x.toDouble();
  String doubleToString(double x) => x.toString();

  final intToStringComposition = doubleToString.dot(intToDouble);
  String intToStringPipe(int x) => intToDouble(x).pipe(doubleToString);

  return intToStringComposition(2) + intToStringPipe(3);
}

int constant() {
  int runWithCurrentDate(int Function(DateTime now) callback) =>
      callback(DateTime.now());

  return runWithCurrentDate(10.constant);
}

int curryUncurry() {
  int sum4(int a, int b, int c, int d) => a + b + c + d;

  return sum4.curry(1)(2).uncurry(3, 4);
}

int trampolineRecursion() {
  Tram<int> tramSum(int number, int sum) => number == 0
      ? Tram.done(sum)
      : Tram.call(() => tramSum(number - 1, sum + number));

  return tramSum.bounce(9999999, 0);
}

List<int> memoization() {
  int Function(int base) newCounter() {
    var counter = 0;
    return (base) => base + counter++;
  }

  final counter = newCounter().memoize();

  return [
    counter(0),
    counter(10),
    counter(0),
    counter(11),
  ];
}

int partialApplication() {
  int addQuaternary(int first, int second, int third, int fourth) =>
      first + second + third + fourth;
  int multiply(int first, int second) => first * second;

  final addToTen = addQuaternary.curry(8)(2).uncurry;
  final multiplyByTwo = multiply.apply(2);

  return addToTen(multiplyByTwo(5), multiplyByTwo(15));
}

Future<int?> nullable() async {
  int add(int first, int second) => first + second;

  return await Future<int?>.value(10).then(add.apply(90).nullable);
}

String flip() {
  String concatenate(String first, String second) => first + ' ' + second;

  return concatenate.flip('First', 'Second');
}

这个示例展示了如何在Flutter应用中集成 pure 包的功能，从而编写更简洁、高效且易于维护的代码。希望这些信息能帮助你在Flutter开发中更好地利用 pure 包！