在 Dart 中重写 hashcode 的好方法是什么？

Since Dart is so similar to Java, you can surely find good references on hashCodes for Java that are applicable for Dart too.

A little googling took me to the Wikipedia page on Java's Object.hashCode(). Has a very basic example for the hashcode of a simple object. A popular methodology is to perform a multiplication with a prime number (different ones) and adding some value for each property of the object.

This question f.e. explains why the number 31 is chosen for multiplication for the String.hashCode() method.

More detailed examples of hashcode implementations can be easily found using Google.

The quiver package provides helper functions hash2, hash3, etc., which simplify the task of implementing hashCode, with some assurance that it works properly under the Dart VM and when compiled to JavaScript.

import 'package:quiver/core.dart';


class Person {
String name;
int age;


Person(this.name, this.age);


bool operator ==(o) => o is Person && name == o.name && age == o.age;
int get hashCode => hash2(name.hashCode, age.hashCode);
}

Also see this post for a slightly lengthier discussion.

The equatable package can help

import 'package:equatable/equatable.dart';


class Person extends Equatable {
final String name;
final int age;


Person(this.name, this.age);


@override
List<Object> get props => [name, age];
}

Now Person will use == and hashCode from Equatable, which takes props list that you give

I recomend "equatable" plugin

https://pub.dev/packages/equatable

Example:

Raw mode:

class Person {
final String name;


const Person(this.name);


@override
bool operator ==(Object other) =>
identical(this, other) ||
other is Person &&
runtimeType == other.runtimeType &&
name == other.name;


@override
int get hashCode => name.hashCode;
}

With equatable :

import 'package:equatable/equatable.dart';


class Person extends Equatable {
final String name;


Person(this.name);


@override
List<Object> get props => [name];
}

In the interests of minimizing dependencies, if you are already depending on flutter, but not depending on something like quiver, the dart:ui library contains utilites, hashValues and hashList for creating and combining hash values. If combining list values, one must be careful to ensure the equality operator and hashcode match behaviour. If the hash code computes it's hash deeply, then use deep equality, else, use shallow equality.

class Example {
final String value1;
final Object value2;
final List<Object> deep;
final List<Object> shallow;


Example({this.value1, this.value2, this.deep, this.shallow});


@override
operator ==(o) =>
o is Example &&
o.value1 == value1 &&
o.value2 == value2 &&
listEquals(o.deep, deep) &&
o.shallow == shallow;


@override
int get hashCode => hashValues(value1, value2, hashList(deep), shallow);
}

Flutter API docs for hashValues

Flutter API docs for hashList

Since version 2.14, Dart language added support for Object.hash(), along with Object.hashAll() and Object.hashAllUnordered()

hash() doc:

Creates a combined hash code for a number of objects.

Example:

class SomeObject {
final Object a, b, c;
SomeObject(this.a, this.b, this.c);


bool operator==(Object other) =>
other is SomeObject && a == other.a && b == other.b && c == other.c;


int get hashCode => Object.hash(a, b, c); // <----- here
}

Note from doc on implementation:

The hash value generated by this function is not guranteed to be stable over different runs of the same program, or between code run in different isolates of the same program. The exact algorithm used may differ between different platforms, or between different versions of the platform libraries, and it may depend on values that change on each program execution.