简单英语学Rust

默认值和建造者模式

你可以实现 Default trait，给你认为最常见的 struct 或 enum 赋值。建造者模式可以很好地与之配合，让用户在需要时轻松地进行修改。首先我们来看看Default。实际上，Rust中的大多数通用类型已经有Default。它们并不奇怪。0, ""(空字符串), false, 等等。

fn main() {
    let default_i8: i8 = Default::default();
    let default_str: String = Default::default();
    let default_bool: bool = Default::default();

    println!("'{}', '{}', '{}'", default_i8, default_str, default_bool);
}

这将打印'0', '', 'false'。

所以Default就像new函数一样，除了你不需要输入任何东西。首先我们要创建一个struct，它还没有实现Default。它有一个new函数，我们用它来创建一个名为Billy的角色，并提供一些统计信息。

struct Character {
    name: String,
    age: u8,
    height: u32,
    weight: u32,
    lifestate: LifeState,
}

enum LifeState {
    Alive,
    Dead,
    NeverAlive,
    Uncertain
}

impl Character {
    fn new(name: String, age: u8, height: u32, weight: u32, alive: bool) -> Self {
        Self {
            name,
            age,
            height,
            weight,
            lifestate: if alive { LifeState::Alive } else { LifeState::Dead },
        }
    }
}

fn main() {
    let character_1 = Character::new("Billy".to_string(), 15, 170, 70, true);
}

但也许在我们的世界里，我们希望大部分角色都叫比利，年龄15岁，身高170，体重70，还活着。我们可以实现Default，这样我们就可以直接写Character::default()。它看起来是这样的:

#[derive(Debug)]
struct Character {
    name: String,
    age: u8,
    height: u32,
    weight: u32,
    lifestate: LifeState,
}

#[derive(Debug)]
enum LifeState {
    Alive,
    Dead,
    NeverAlive,
    Uncertain,
}

impl Character {
    fn new(name: String, age: u8, height: u32, weight: u32, alive: bool) -> Self {
        Self {
            name,
            age,
            height,
            weight,
            lifestate: if alive {
                LifeState::Alive
            } else {
                LifeState::Dead
            },
        }
    }
}

impl Default for Character {
    fn default() -> Self {
        Self {
            name: "Billy".to_string(),
            age: 15,
            height: 170,
            weight: 70,
            lifestate: LifeState::Alive,
        }
    }
}

fn main() {
    let character_1 = Character::default();

    println!(
        "The character {:?} is {:?} years old.",
        character_1.name, character_1.age
    );
}

打印出The character "Billy" is 15 years old.，简单多了!

现在我们来看建造者模式。我们会有很多Billy，所以我们会保留默认的。但是很多其他角色只会有一点不同。建造者模式让我们可以把小方法链接起来，每次改变一个值。这里是一个Character的方法:

#![allow(unused)]
fn main() {
fn height(mut self, height: u32) -> Self {    // 🚧
    self.height = height;
    self
}
}

一定要注意，它取的是mut self。我们之前看到过一次，它不是一个可变引用(&mut self)。它占用了Self的所有权，有了mut，它将是可变的，即使它之前不是可变的。这是因为.height()拥有完全的所有权，别人不能碰它，所以它是安全的，可变。它只是改变self.height，然后返回Self(就是Character)。

所以我们有三个这样的构建方法。它们几乎是一样的:

#![allow(unused)]
fn main() {
fn height(mut self, height: u32) -> Self {     // 🚧
    self.height = height;
    self
}

fn weight(mut self, weight: u32) -> Self {
    self.weight = weight;
    self
}

fn name(mut self, name: &str) -> Self {
    self.name = name.to_string();
    self
}
}

每一个都会改变一个变量，并回馈给Self:这就是你在建造者模式中看到的。所以现在我们类似这样写来创建一个角色:let character_1 = Character::default().height(180).weight(60).name("Bobby");。如果你正在构建一个库给别人使用，这可以让他们很容易用起来。对最终用户来说很容易，因为它几乎看起来像自然的英语。"给我一个默认角色，身高为180，体重为60，名字为Bobby." 到目前为止，我们的代码看起来是这样的:

#[derive(Debug)]
struct Character {
    name: String,
    age: u8,
    height: u32,
    weight: u32,
    lifestate: LifeState,
}

#[derive(Debug)]
enum LifeState {
    Alive,
    Dead,
    NeverAlive,
    Uncertain,
}

impl Character {
    fn new(name: String, age: u8, height: u32, weight: u32, alive: bool) -> Self {
        Self {
            name,
            age,
            height,
            weight,
            lifestate: if alive {
                LifeState::Alive
            } else {
                LifeState::Dead
            },
        }
    }

    fn height(mut self, height: u32) -> Self {
        self.height = height;
        self
    }

    fn weight(mut self, weight: u32) -> Self {
        self.weight = weight;
        self
    }

    fn name(mut self, name: &str) -> Self {
        self.name = name.to_string();
        self
    }
}

impl Default for Character {
    fn default() -> Self {
        Self {
            name: "Billy".to_string(),
            age: 15,
            height: 170,
            weight: 70,
            lifestate: LifeState::Alive,
        }
    }
}

fn main() {
    let character_1 = Character::default().height(180).weight(60).name("Bobby");

    println!("{:?}", character_1);
}

最后一个要添加的方法通常叫.build()。这个方法是一种最终检查。当你给用户提供一个像.height()这样的方法时，你可以确保他们只输入一个u32()，但是如果他们输入5000的身高怎么办？这在你正在做的游戏中可能就不对了。我们最后将使用一个名为.build()的方法，返回一个Result。在它里面我们将检查用户输入是否正常，如果正常，我们将返回一个 Ok(Self)。

不过首先我们要改变.new()方法。我们不希望用户再自由创建任何一种角色。所以我们将把impl Default的值移到.new()。而现在.new()不接受任何输入。

#![allow(unused)]
fn main() {
    fn new() -> Self {    // 🚧
        Self {
            name: "Billy".to_string(),
            age: 15,
            height: 170,
            weight: 70,
            lifestate: LifeState::Alive,
        }
    }
}

这意味着我们不再需要impl Default了，因为.new()有所有的默认值。所以我们可以删除impl Default。

现在我们的代码是这样的。

#[derive(Debug)]
struct Character {
    name: String,
    age: u8,
    height: u32,
    weight: u32,
    lifestate: LifeState,
}

#[derive(Debug)]
enum LifeState {
    Alive,
    Dead,
    NeverAlive,
    Uncertain,
}

impl Character {
    fn new() -> Self {
        Self {
            name: "Billy".to_string(),
            age: 15,
            height: 170,
            weight: 70,
            lifestate: LifeState::Alive,
        }
    }

    fn height(mut self, height: u32) -> Self {
        self.height = height;
        self
    }

    fn weight(mut self, weight: u32) -> Self {
        self.weight = weight;
        self
    }

    fn name(mut self, name: &str) -> Self {
        self.name = name.to_string();
        self
    }
}

fn main() {
    let character_1 = Character::new().height(180).weight(60).name("Bobby");

    println!("{:?}", character_1);
}

这样打印出来的结果是一样的:Character { name: "Bobby", age: 15, height: 180, weight: 60, lifestate: Alive }。

我们几乎已经准备好写.build()方法了，但是有一个问题:如何让用户使用它？现在用户可以写let x = Character::new().height(76767);，然后得到一个Character。有很多方法可以做到这一点，也许你能想出自己的方法。但是我们会在Character中增加一个can_use: bool的值。

#![allow(unused)]
fn main() {
#[derive(Debug)]       // 🚧
struct Character {
    name: String,
    age: u8,
    height: u32,
    weight: u32,
    lifestate: LifeState,
    can_use: bool, // Set whether the user can use the character
}

\\ Cut other code

    fn new() -> Self {
        Self {
            name: "Billy".to_string(),
            age: 15,
            height: 170,
            weight: 70,
            lifestate: LifeState::Alive,
            can_use: true, // .new() always gives a good character, so it's true
        }
    }
}

而对于其他的方法，比如.height()，我们会将can_use设置为false。只有.build()会再次设置为true，所以现在用户要用.build()做最后的检查。我们要确保height不高于200，weight不高于300。另外，在我们的游戏中，有一个不好的字叫smurf，我们不希望任何角色使用它。

我们的.build()方法是这样的:

#![allow(unused)]
fn main() {
fn build(mut self) -> Result<Character, String> {      // 🚧
    if self.height < 200 && self.weight < 300 && !self.name.to_lowercase().contains("smurf") {
        self.can_use = true;
        Ok(self)
    } else {
        Err("Could not create character. Characters must have:
1) Height below 200
2) Weight below 300
3) A name that is not Smurf (that is a bad word)"
            .to_string())
    }
}
}

!self.name.to_lowercase().contains("smurf") 确保用户不会写出类似 "SMURF"或 "IamSmurf"的字样。它让整个 String 都变成小写(小字母)，并检查 .contains() 而不是 ==。而前面的!表示 "不是"。

如果一切正常，我们就把can_use设置为true，然后把Ok里面的字符给用户。

现在我们的代码已经完成了，我们将创建三个不工作的角色，和一个工作的角色。最后的代码是这样的。

#[derive(Debug)]
struct Character {
    name: String,
    age: u8,
    height: u32,
    weight: u32,
    lifestate: LifeState,
    can_use: bool, // Here is the new value
}

#[derive(Debug)]
enum LifeState {
    Alive,
    Dead,
    NeverAlive,
    Uncertain,
}

impl Character {
    fn new() -> Self {
        Self {
            name: "Billy".to_string(),
            age: 15,
            height: 170,
            weight: 70,
            lifestate: LifeState::Alive,
            can_use: true,  // .new() makes a fine character, so it is true
        }
    }

    fn height(mut self, height: u32) -> Self {
        self.height = height;
        self.can_use = false; // Now the user can't use the character
        self
    }

    fn weight(mut self, weight: u32) -> Self {
        self.weight = weight;
        self.can_use = false;
        self
    }

    fn name(mut self, name: &str) -> Self {
        self.name = name.to_string();
        self.can_use = false;
        self
    }

    fn build(mut self) -> Result<Character, String> {
        if self.height < 200 && self.weight < 300 && !self.name.to_lowercase().contains("smurf") {
            self.can_use = true;   // Everything is okay, so set to true
            Ok(self)               // and return the character
        } else {
            Err("Could not create character. Characters must have:
1) Height below 200
2) Weight below 300
3) A name that is not Smurf (that is a bad word)"
                .to_string())
        }
    }
}

fn main() {
    let character_with_smurf = Character::new().name("Lol I am Smurf!!").build(); // This one contains "smurf" - not okay
    let character_too_tall = Character::new().height(400).build(); // Too tall - not okay
    let character_too_heavy = Character::new().weight(500).build(); // Too heavy - not okay
    let okay_character = Character::new()
        .name("Billybrobby")
        .height(180)
        .weight(100)
        .build();   // This character is okay. Name is fine, height and weight are fine

    // Now they are not Character, they are Result<Character, String>. So let's put them in a Vec so we can see them:
    let character_vec = vec![character_with_smurf, character_too_tall, character_too_heavy, okay_character];

    for character in character_vec { // Now we will print the character if it's Ok, and print the error if it's Err
        match character {
            Ok(character_info) => println!("{:?}", character_info),
            Err(err_info) => println!("{}", err_info),
        }
        println!(); // Then add one more line
    }
}

这将打印:

Could not create character. Characters must have:
1) Height below 200
2) Weight below 300
3) A name that is not Smurf (that is a bad word)

Could not create character. Characters must have:
1) Height below 200
2) Weight below 300
3) A name that is not Smurf (that is a bad word)

Could not create character. Characters must have:
1) Height below 200
2) Weight below 300
3) A name that is not Smurf (that is a bad word)

Character { name: "Billybrobby", age: 15, height: 180, weight: 100, lifestate: Alive, can_use: true }