与tokio的MQTT连接

Question

我正在尝试使用tokio和mqtt_v5机箱中提供的编解码器创建一个MQTT连接。我的代码不能编译，我不明白为什么。这是我到目前为止所写的代码，发送代码可能不正确。

use tokio::net::TcpStream;
use tokio_util::codec::Framed;
use tokio_util::codec::Decoder;
use std::net::SocketAddrV4;
use mqtt_v5::types::Packet as MqttPacket;
use mqtt_v5::codec::MqttCodec;
use futures_sink::Sink;
use futures_core::stream::Stream;

struct MqttConn {
    inner: Framed<TcpStream, MqttCodec>,
}

impl MqttConn {
    async fn new(addr: SocketAddrV4) -> MqttConn {
        let tcp = TcpStream::connect(addr).await.expect("cannot connect to mqtt");
        MqttConn { inner: MqttCodec::new().framed(tcp) } 
    }

    async fn handle(&self, handler: &dyn Fn(&MqttConn, MqttPacket) -> ()) {
        while let Some(p) = self.inner.next().await {
            handler(self, p)
        }
    }

    async fn send(&self, p: MqttPacket) {
        self.inner.start_send(p);
    }
}

我从编译器得到以下错误：

error[E0599]: no method named `framed` found for struct `MqttCodec` in the current scope
  --> src/mqtt.rs:17:44
   |
17 |         MqttConn { inner: MqttCodec::new().framed(tcp) } 
   |                                            ^^^^^^ method not found in `MqttCodec`
   |
   = help: items from traits can only be used if the trait is in scope
   = note: the following trait is implemented but not in scope; perhaps add a `use` for it:
           `use tokio_util::codec::decoder::Decoder;`

error[E0599]: no method named `next` found for struct `Framed<tokio::net::TcpStream, MqttCodec>` in the current scope
  --> src/mqtt.rs:21:40
   |
21 |         while let Some(p) = self.inner.next().await {
   |                                        ^^^^ method not found in `Framed<tokio::net::TcpStream, MqttCodec>`

error[E0599]: no method named `start_send` found for struct `Framed<tokio::net::TcpStream, MqttCodec>` in the current scope
  --> src/mqtt.rs:27:20
   |
27 |         self.inner.start_send(p);
   |                    ^^^^^^^^^^ method not found in `Framed<tokio::net::TcpStream, MqttCodec>`

warning: unused import: `tokio_util::codec::Decoder`
 --> src/mqtt.rs:3:5
  |
3 | use tokio_util::codec::Decoder;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^

warning: unused import: `futures_sink::Sink`
 --> src/mqtt.rs:7:5
  |
7 | use futures_sink::Sink;
  |     ^^^^^^^^^^^^^^^^^^

编译器说Decoder特征不在作用域中，但我使用了它。如果我尝试建议的导入，我发现模块tokio_util::codec::decoder是私有的。基于源，tokio_util::codec::decoder::Decoder被重新导出为tokio_util::codec::Decoder。另外，如果我没看错文档，Framed应该实现Sink和Stream，因此它应该有next和start_send方法。

来自Cargo.toml的相关行：

[dependencies]
tokio = { version = "1.0.1", features = ["full"] }
tokio-util = { version = "0.6", features = ["full"] }
futures-sink = "0.3.9"
futures-core = "0.3.9"

mqtt-v5 = "0.1.1"

我怎样才能编译这段代码？

Answer 1

您有许多库不兼容问题，它们导致了一些不太明显的错误消息。

mqtt-v5 depends on tokio-util^0.3，它是为tokio 0.2而不是1.0编写的。您需要回滚到tokio 0.2和tokio-util 0.3。这应该可以解决Decoder和Sink的问题。

此外，Stream特征只提供poll_next()，这是任务级的流方法。next()是由StreamExt特性提供的，以及其他类似于Iterator中的便捷方法。