每 2 秒获取一次调用，但不希望请求堆积

Question

我正在尝试拨打 API 电话，我希望它每 2 秒重复一次。但是，我担心如果系统在 2 秒内没有收到请求，它将建立请求并继续尝试发送它们。我该如何防止这种情况？

这是我要执行的操作 fetch:

const getMachineAction = async () => {
    try {
        const response = await fetch( 'https://localhost:55620/api/machine/');
        if (response.status === 200) {
            console.log("Machine successfully found.");
            const myJson = await response.json(); //extract JSON from the http response
            console.log(myJson);               
        } else {
            console.log("not a 200");
        }
    } catch (err) {
        // catches errors both in fetch and response.json
        console.log(err);
    }
};

然后我用 setInterval 调用它。

function ping() {
    setInterval(
        getMachineAction(),
        2000
    );        
}

我曾想过在 setInterval 中做一些类似 promise 的结构，以确保提取已经完成并完成，但无法让它工作。

Answer 1

简单！只存储当前是否正在发出请求，并存储定时器是否已触发而不发送新请求。

let in_progress = false;
let missed_request = false;
const getMachineAction = async () => {
    if (in_progress) {
        missed_request = true;
        return;
    }
    in_progress = true;
    try {
        const response = await fetch('https://localhost:55620/api/machine/');
        if (missed_request) {
            missed_request = false;
            setTimeout(getMachineAction, 0);
        }
        if (response.status === 200) {
            console.log("Machine successfully found.");
            const myJson = await response.json(); //extract JSON from the http response
            console.log(myJson);               
        } else {
            console.log("not a 200");
        }
    } catch (err) {
        // catches errors both in fetch and response.json
        console.log(err);
    } finally {
        in_progress = false;
    }
};

要开始间隔，您需要省略 ():

setInterval(getMachineAction, 2000);

Answer 2

您可以使用 setTimeout 将 finally 添加到 try/catch，而不是使用 setInterval。

请注意，与使用 websockets 相比，像这样的长轮询会产生更多的服务器负载，后者本身更实时

const getMachineAction = async () => {
    try {
        const response = await fetch( 'https://localhost:55620/api/machine/');
        if (response.status === 200) {
            console.log("Machine successfully found.");
            const myJson = await response.json(); //extract JSON from the http response
            console.log(myJson);               
        } else {
            console.log("not a 200");
        }
    } catch (err) {
        // catches errors both in fetch and response.json
        console.log(err);
    } finally {
        // do it again in 2 seconds
        setTimeout(getMachineAction , 2000);
    }
};

getMachineAction()

Answer 3

The Promise.all() Solution

此解决方案可确保您不会错过 2 秒延迟要求，并且不会在另一个网络呼叫正在进行时触发呼叫。

function callme(){
//This promise will resolve when the network call succeeds
//Feel free to make a REST fetch using promises and assign it to networkPromise
var networkPromise = fetch('https://jsonplaceholder.typicode.com/todos/1');


//This promise will resolve when 2 seconds have passed
var timeOutPromise = new Promise(function(resolve, reject) {
  // 2 Second delay
  setTimeout(resolve, 2000, 'Timeout Done');
});

Promise.all(
[networkPromise, timeOutPromise]).then(function(values) {
  console.log("Atleast 2 secs + TTL (Network/server)");
  //Repeat
  callme();
});
}
callme();

注意：这会按照问题作者的要求处理不良案例定义：

"the "坏情况”（即需要超过 2 秒的时间）是我希望它跳过该请求，然后发送一个新请求。所以在 0 秒时请求发送。执行需要 3 秒，然后 2 秒后（在 5）它应该重新执行。所以它只是延长了发送时间。“

Answer 4

你可以试试这个，我得到了我正在寻找的确切结果

const getMachineAction = async () => {
    try {
        const response = await fetch( 'https://localhost:55620/api/machine/');
        if (response.status === 200) {
            console.log("Machine successfully found.");
            const myJson = await response.json(); //extract JSON from the http response
            console.log(myJson);               
        } else {
            console.log("not a 200");
        }
    } catch (err) {
        // catches errors both in fetch and response.json
        console.log(err);
    } finally {
        // do it again in 2 seconds
        setTimeout(getMachineAction , 2000);
    }
};

getMachineAction()

每 2 秒获取一次调用，但不希望请求堆积

Fetch call every 2 seconds, but don't want requests to stack up

javascript

asynchronous

promise

fetch-api