ch17-02翻译90%

src/ch17-02-trait-objects.md

@@ -18,7 +18,7 @@ get Chapter 8 for editing. /Carol -->
 
 在有继承的语言里，我们可能会定义一个名为`Component`的类，该类上有一个`draw`方法。其他的类比如`Button`、`Image`和`SelectBox`会从`Component`继承并继承`draw`方法。它们会各自覆写`draw`方法来自定义行为，但是框架会把所有的类型当作是`Component`的实例，并在它们上调用`draw`。
 
-## 定义一个带有自定义行为的Trait
+### 定义一个带有自定义行为的Trait
 
 不过，在Rust语言中，我们可以定义一个名为`Draw`的trait，其上有一个名为`draw`的方法。我们定义一个带有*trait对象*的vector，绑定了一种指针的trait，比如`&`引用或者一个`Box<T>`智能指针。
 
@@ -111,7 +111,7 @@ impl<T> Screen<T>
 
 而如果使用内部有`Vec<Box<Draw>>` trait对象的列表的`Screen`结构体，`Screen`实例可以同时包含`Box<Button>`和`Box<TextField>`的`Vec`。我们看它是怎么工作的，然后讨论运行时性能的实现。
 
-###　来自我们或者库使用者的实现
+### 来自我们或者库使用者的实现
 
 现在，我们增加一些实现了`Draw`trait的类型。我们会再次提供`Button`，实际上实现一个GUI库超出了本书的范围，所以`draw`方法的内部不会有任何有用的实现。为了想象一下实现可能的样子，`Button`结构体可能有 width`、`height`和`label`字段，如Listing 17-7所示：
 
@@ -135,8 +135,7 @@ impl Draw for Button {
 }
 ```
 
-<span class="caption">Listing 17-7: A `Button` struct that implements the
-`Draw` trait</span>
+<span class="caption">Listing 17-7: 实现了`Draw` trait的`Button` 结构体</span>
 
 在`Button`上的 `width`、`height`和`label`会和其他组件不同，比如`TextField`可能有`width`、`height`,
 `label`和 `placeholder`字段。每个我们可以在屏幕上绘制的类型会实现`Draw`trait，在`draw`方法中使用不同的代码，定义了如何绘制`Button`（GUI代码的具体实现超出了本章节的范围）。除了`Draw` trait，`Button`可能也有另一个`impl`块，包含了当按钮被点击的时候的响应方法。这类方法不适用于`TextField`这样的类型。
@@ -165,11 +164,7 @@ impl Draw for SelectBox {
 <span class="caption">Listing 17-8: 另外一个crate中，在`SelectBox`结构体上使用`rust_gui`和实现了`Draw` trait
 </span>
 
-The user of our library can now write their `main` function to create a
-`Screen` instance and add a `SelectBox` and a `Button` to the screen by putting
-each in a `Box<T>` to become a trait object. They can then call the `run`
-method on the `Screen` instance, which will call `draw` on each of the
-components. Listing 17-9 shows this implementation:
+我们的库的使用者现在可以写他们的`main`函数来创建一个`Screen`实例，然后通过把自身放入`Box<T>`变成trait对象，向screen增加`SelectBox` 和`Button`。它们可以在每个`Screen`实例上调用`run`方法，这会调用每个组件的`draw`方法。 Listing 17-9展示了实现：
 
 <span class="filename">Filename: src/main.rs</span>
 
@@ -200,32 +195,16 @@ fn main() {
 }
 ```
 
-<span class="caption">Listing 17-9: Using trait objects to store values of
-different types that implement the same trait</span>
-
-Even though we didn't know that someone would add the `SelectBox` type someday,
-our `Screen` implementation was able to operate on the `SelectBox` and draw it
-because `SelectBox` implements the `Draw` type, which means it implements the
-`draw` method.
-
-Only being concerned with the messages a value responds to, rather than the
-value's concrete type, is similar to a concept called *duck typing* in
-dynamically typed languages: if it walks like a duck, and quacks like a duck,
-then it must be a duck! In the implementation of `run` on `Screen` in Listing
-17-5, `run` doesn't need to know what the concrete type of each component is.
-It doesn't check to see if a component is an instance of a `Button` or a
-`SelectBox`, it just calls the `draw` method on the component. By specifying
-`Box<Draw>` as the type of the values in the `components` vector, we've defined
-that `Screen` needs values that we can call the `draw` method on.
-
-The advantage with using trait objects and Rust's type system to do duck typing
-is that we never have to check that a value implements a particular method at
-runtime or worry about getting errors if a value doesn't implement a method but
-we call it. Rust won't compile our code if the values don't implement the
-traits that the trait objects need.
-
-For example, Listing 17-10 shows what happens if we try to create a `Screen`
-with a `String` as a component:
+<span class="caption">Listing 17-9: 使用trait对象来存储实现了相同trait的不同类型
+</span>
+
+虽然我们不知道有些人可能有一天会增加`SelectBox`类型，但是我们的`Screen` 有能力操作`SelectBox`和绘制，因为`SelectBox`实现了`Draw`类型，这意味着它实现了`draw`方法。
+
+只关心值响应的消息，而不关心值的具体类型，这类似于动态类型语言中的*duck typing*：如果它像鸭子一样走路，像鸭子一样叫，那么它肯定是只鸭子！在Listing 17-5的`Screen`的`run`方法的实现中，`run`不需要知道每个组件的具体类型。它也不检查是否一个组件是`Button`或者`SelectBox`的实例，只是调用组件的`draw`方法即可。通过指定`Box<Draw>`作为`components`vector中的值类型，我们定义了：`Screen`需要可以被调用其`draw`方法的值。
+
+使用trait对象和支持duck typing的Rust类型系统的好处是，我们永远不需要在运行时检查一个值是否实现了一个特殊方法，或者担心因为调用了一个值没有实现方法而遇到错误。如果值没有实现trait对象需要的trait，Rust不会编译我们的代码。
+
+比如，Listing 17-10展示了当我们创建一个把`String`当做其成员的`Screen`时发生的情况：
 
 <span class="filename">Filename: src/main.rs</span>
 
@@ -244,10 +223,11 @@ fn main() {
 }
 ```
 
-<span class="caption">Listing 17-10: Attempting to use a type that doesn't
-implement the trait object's trait</span>
+<span class="caption">Listing 17-10: 尝试使用一种没有实现trait对象的trait的类型
 
-We'll get this error because `String` doesn't implement the `Draw` trait:
+</span>
+
+我们会遇到这个错误，因为`String`没有实现 `Draw`trait：
 
 ```text
 error[E0277]: the trait bound `std::string::String: Draw` is not satisfied
@@ -260,31 +240,15 @@ error[E0277]: the trait bound `std::string::String: Draw` is not satisfied
    = note: required for the cast to the object type `Draw`
 ```
 
-This lets us know that either we're passing something we didn't mean to pass to
-`Screen` and we should pass a different type, or we should implement `Draw` on
-`String` so that `Screen` is able to call `draw` on it.
-
-### Trait Objects Perform Dynamic Dispatch
-
-Recall in Chapter 10 when we discussed the process of monomorphization that the
-compiler performs when we use trait bounds on generics: the compiler generates
-non-generic implementations of functions and methods for each concrete type
-that we use in place of a generic type parameter. The code that results from
-monomorphization is doing *static dispatch*: when the method is called, the
-code that goes with that method call has been determined at compile time, and
-looking up that code is very fast.
-
-When we use trait objects, the compiler can't perform monomorphization because
-we don't know all the types that might be used with the code. Instead, Rust
-keeps track of the code that might be used when a method is called and figures
-out at runtime which code needs to be used for a particular method call. This
-is known as *dynamic dispatch*, and there's a runtime cost when this lookup
-happens. Dynamic dispatch also prevents the compiler from choosing to inline a
-method's code, which prevents some optimizations. We did get extra flexibility
-in the code that we wrote and were able to support, though, so it's a tradeoff
-to consider.
-
-### Object Safety is Required for Trait Objects
+这个报错让我们知道，或者我们传入了本来不想传给`Screen`的东西，我们应该传入一个不同的类型，或者是我们应该在`String`上实现`Draw`，这样，`Screen`才能调用它的`draw`方法。
+
+### Trait对象执行动态分发
+
+回忆一下第10章，我们讨论过当我们使用通用类型的trait绑定时，编译器执行单类型的处理过程：在我们需要使用通用类型参数的地方，编译器为每个实体类型产生了非通用的函数实现和方法。由于非单类型而产生的代码是 *static dispatch*：当方法被调用，代码会执行在编译阶段就决定的方法，这样寻找那段代码是非常快速的。
+
+当我们使用trait对象，编译器不能执行单类型的，因为我们不知道可能被代码调用的类型。而，当方法被调用的时候，Rust跟踪可能被使用的代码，然后在运行时找出为了方法被调用时该使用哪些代码。这也是我们熟知的*dynamic dispatch*，当运行时的查找发生时是比较耗费资源的。动态分发也防止编译器选择内联函数的代码，这样防止了一些优化。虽然我们写代码时得到了额外的代码灵活性，不过，这是一个权衡考虑。
+
+### Trait 对象需要对象安全
 
 <!-- Liz: we're conflicted on including this section. Not being able to use a
 trait as a trait object because of object safety is something that