我需要实现一个返回futures::StreamExt特征的特征。
一般来说，这听起来很简单，对此有几个答案，例如这里的这个。
我试过这个，StreamExt但这确实 - 由于某种原因 - 不起作用。这是我的示例代码：
// [dependencies]
// futures = "0.3.6"
// async-std = { version = "1.6.5", features = ["attributes", "unstable"] } // interval function is unstable atm.
use std::time::Duration;
use futures::StreamExt;
trait StreamProvidingTrait {
fn returnastream(self: &Self) -> Box<dyn StreamExt<Item=i32>>;
}
struct StreamProvider {}
impl StreamProvidingTrait for StreamProvider {
fn returnastream(self: &Self) -> Box<dyn StreamExt<Item=i32>> {
return Box::new(async_std::stream::interval(Duration::from_millis(1000)).map(|_| 1));
}
}
#[async_std::main]
async fn main() {
let mut counter = 0;
let object = StreamProvider {};
// creates a stream
let worx = object.returnastream();
// mappes the stream into something....
let mut mapped_stream = worx.map(|_| {
counter = counter + 1;
counter
});
// subscribing to the items
while let item = mapped_stream.next().await {
match item {
Some(value) => println!("{}", value),
_ => {}
}
}
}
这里的错误：
Compiling traittest v0.1.0 (/Users/andre/repos/traittest)
warning: the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
--> /Users/andre/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.6/src/stream/stream/mod.rs:1326:8
|
1326 | fn poll_next_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>
| ^^^^^^^^^^^^^^^ the trait cannot be made into an object because method `poll_next_unpin` references the `Self` type in its `where` clause
|
= note: `#[warn(where_clauses_object_safety)]` on by default
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #51443 <https://github.com/rust-lang/rust/issues/51443>
error[E0038]: the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
--> src/main.rs:6:36
|
6 | fn returnastream(self: &Self) -> Box<dyn StreamExt<Item=i32>>;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
|
::: /Users/andre/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.6/src/stream/stream/mod.rs:226:8
|
226 | fn next(&mut self) -> Next<'_, Self>
| ---- the trait cannot be made into an object because method `next` references the `Self` type in its return type
...
976 | fn by_ref(&mut self) -> &mut Self {
| ------ the trait cannot be made into an object because method `by_ref` references the `Self` type in its return type
...
1381 | fn select_next_some(&mut self) -> SelectNextSome<'_, Self>
| ---------------- the trait cannot be made into an object because method `select_next_some` references the `Self` type in its return type
error[E0038]: the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
--> src/main.rs:12:36
|
12 | fn returnastream(self: &Self) -> Box<dyn StreamExt<Item=i32>> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
|
::: /Users/andre/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.6/src/stream/stream/mod.rs:226:8
|
226 | fn next(&mut self) -> Next<'_, Self>
| ---- the trait cannot be made into an object because method `next` references the `Self` type in its return type
...
976 | fn by_ref(&mut self) -> &mut Self {
| ------ the trait cannot be made into an object because method `by_ref` references the `Self` type in its return type
...
1381 | fn select_next_some(&mut self) -> SelectNextSome<'_, Self>
| ---------------- the trait cannot be made into an object because method `select_next_some` references the `Self` type in its return type
error: aborting due to 2 previous errors; 1 warning emitted
For more information about this error, try `rustc --explain E0038`.
error: could not compile `traittest`.
To learn more, run the command again with --verbose.
Process finished with exit code 101
当我StreamExt用自己的特性交换特性时，这段代码运行良好。
Compiling traittest v0.1.0 (/Users/andre/repos/traittest)
warning: the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
--> /Users/andre/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.6/src/stream/stream/mod.rs:1326:8
|
1326 | fn poll_next_unpin(&mut self, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>
| ^^^^^^^^^^^^^^^ the trait cannot be made into an object because method `poll_next_unpin` references the `Self` type in its `where` clause
|
= note: `#[warn(where_clauses_object_safety)]` on by default
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, see issue #51443 <https://github.com/rust-lang/rust/issues/51443>
error[E0038]: the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
--> src/main.rs:6:36
|
6 | fn returnastream(self: &Self) -> Box<dyn StreamExt<Item=i32>>;
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
|
::: /Users/andre/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.6/src/stream/stream/mod.rs:226:8
|
226 | fn next(&mut self) -> Next<'_, Self>
| ---- the trait cannot be made into an object because method `next` references the `Self` type in its return type
...
976 | fn by_ref(&mut self) -> &mut Self {
| ------ the trait cannot be made into an object because method `by_ref` references the `Self` type in its return type
...
1381 | fn select_next_some(&mut self) -> SelectNextSome<'_, Self>
| ---------------- the trait cannot be made into an object because method `select_next_some` references the `Self` type in its return type
error[E0038]: the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
--> src/main.rs:12:36
|
12 | fn returnastream(self: &Self) -> Box<dyn StreamExt<Item=i32>> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `futures_util::stream::stream::StreamExt` cannot be made into an object
|
::: /Users/andre/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.6/src/stream/stream/mod.rs:226:8
|
226 | fn next(&mut self) -> Next<'_, Self>
| ---- the trait cannot be made into an object because method `next` references the `Self` type in its return type
...
976 | fn by_ref(&mut self) -> &mut Self {
| ------ the trait cannot be made into an object because method `by_ref` references the `Self` type in its return type
...
1381 | fn select_next_some(&mut self) -> SelectNextSome<'_, Self>
| ---------------- the trait cannot be made into an object because method `select_next_some` references the `Self` type in its return type
error: aborting due to 2 previous errors; 1 warning emitted
For more information about this error, try `rustc --explain E0038`.
error: could not compile `traittest`.
To learn more, run the command again with --verbose.
Process finished with exit code 101
这里有什么问题？显然有什么我不明白的地方。请帮助我理解这一点！
返回特征的函数可以使用impl Trait语法返回实现特征的不透明类型。返回 trait 的 trait 方法目前不支持此功能，并且需要将 trait 作为 trait 对象返回 - 动态分派的引用或智能指针，例如Box或Rc。然而，并非所有 trait 都是对象安全的，坏消息是StreamExt那些不是，因为编译器指出了原因，例如Self在方法返回类型和where子句中引用。
然而，好消息是这不是问题：StreamExt是一个扩展特性，它为所有实现Stream. 所以你不需要费心返回一个dyn StreamExttrait 对象，你可以返回dyn Stream一个，并且你仍然可以StreamExt通过使用use StreamExt. 换句话说，您可以在特征的返回类型中替换Box<dyn StreamExt>with Box<dyn Stream>。
您可能会遇到的另一个问题是，Box<dyn Stream>它不适用于需要移动流的方法，其中包括StreamExt. 这些将需要固定流，这可以通过返回Pin<Box<dyn Stream>>而不是Box<dyn Stream>. 甚至有一种boxed()方法StreamExt可以在一次操作中对流进行固定和装箱；使用它的代码看起来像这样（操场）：
use futures::stream::{Stream, StreamExt};
use std::pin::Pin;
trait StreamProvidingTrait {
fn returnastream(&self) -> Pin<Box<dyn Stream<Item = i32>>>;
}
struct StreamProvider {}
impl StreamProvidingTrait for StreamProvider {
fn returnastream(&self) -> Pin<Box<dyn Stream<Item = i32>>> {
return tokio::stream::once(0).boxed();
}
}
fn main() {
let provider = StreamProvider {};
let stream = provider.returnastream();
let _fut = stream.into_future();
}