问题描述

我有一个网页，其中的不同部分都需要相同的后端数据.每个都是孤立的，因此它们最终都会对后端进行相同的调用.

I have a web page where different parts of it all need the same back-end data. Each is isolated, so they each end up eventually making the same calls to the back-end.

当调用已经在进行中并由同一网页上的不同代码段启动时，避免调用 Web 服务器的最佳方法是什么?

What is the best way to avoid making a call to the web server when one is already in progress and initiated by a different piece of code on the same web page?

这是一个例子.我将使用 setTimeout 来模拟异步调用.

Here's an example. I'll use setTimeout to simulate an asynchronous call.

假设有一个异步函数返回联系人列表，在这个例子中它基本上是一个简单的字符串数组:

Let's assume there's an async function that returns the list of contacts, which is basically a simple array of strings in this example:

var getContacts = function() {
  log('Calling back-end to get contact list.');
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      log('New data received from back-end.');
      resolve(["Mary","Frank","Klaus"]);
    }, 3000);
  });
};

现在，让我们创建三个单独的函数，每个函数都出于不同的目的调用上述函数.

Now, let's create three separate functions that each call the above function for different purposes.

转储联系人列表:

var dumpContacts = function() {
  getContacts().then(function(contacts) {
    for( var i = 0; i < contacts.length; i++ ) {
      log( "Contact " + (i + 1) + ": " + contacts[i] );
    }
  });
};

确定特定联系人是否在列表中:

Determine if a particular contact is in the list:

var contactExists = function(contactName) {
  return getContacts().then(function(contacts) {
    return contacts.indexOf(contactName) >= 0 ? true : false;
  });
};

获取第一个联系人的姓名:

Get the name of the first contact:

var getFirstContact = function() {
  return getContacts().then(function(contacts) {
    if ( contacts.length > 0 ) {
      return contacts[0];
    }
  });
};

下面是一些使用这三个函数的示例代码:

And here is some example code to use these three functions:

// Show all contacts
dumpContacts();

// Does contact 'Jane' exist?
contactExists("Jane").then(function(exists){
  log("Contact 'Jane' exist: " + exists);
});

getFirstContact().then(function(firstContact){
  log("first contact: " + firstContact);
});

上述例程使用全局 log() 函数.可以使用 console.log() 代替.上面的 log() 函数记录到浏览器窗口，实现如下:

The above routines make use of a global log() function. console.log() could be used instead. The above log() function log's to the browser window and is implemented as follows:

function log() {
  var args = Array.prototype.slice.call(arguments).join(", ");
  console.log(args);
  var output = document.getElementById('output');
  output.innerHTML += args + "<br/>";
}

并且在 html 中需要以下内容:

and requires the following in the html:

<div id='output'><br/></div>

当上面的代码运行时，你会看到:

When the above code is run, you will see:

Calling back-end to get contact list.

和

New data received from back-end.

三遍，没必要.

如何解决这个问题?

此示例在 Plunker 上可以执行:http://plnkr.co/edit/6ysbNTf1lSf5b7L3sJxQ?p=preview

This sample is on Plunker can be executed: http://plnkr.co/edit/6ysbNTf1lSf5b7L3sJxQ?p=preview

推荐答案

只需在调用的函数中缓存结果即可:

Just cache the result in the function making the call:

function cache(promiseReturningFn){
    var cachedVal = null;  // start without cached value
    function cached(){
        if(cachedVal) return cachedVal; // prefer cached result
        cachedVal = promiseReturningFn.apply(this, arguments); // delegate
        return cachedVal; // after we saved it, return it
    }
    cached.flush = function(){ cachedVal = undefined; };
    return cached;
}

这有一个警告，即对于 null 的实际结果会失败，否则它可以很好地完成工作.

This has the caveat of failing for actual results that are null but otherwise it gets the job done nicely.

您现在可以缓存任何 promise 返回函数 - 上面的版本只缓存忽略参数 - 但您可以构造一个类似的函数，它具有 Map 并基于不同的参数进行缓存 - 但让我们专注于您的用例.

You can now cache any promise returning function - the version above only caches ignoring arguments - but you can construct a similar one that has a Map and caches based on different arguments too - but let's focus on your use case.

var getContactsCached = cache(getContacts);

getContactsCached();
getContactsCached();
getContactsCached(); // only one async call ever made

cache 方法实际上甚至与 Promise 无关——它所做的只是获取一个函数并缓存它的结果——你可以将它用于任何事情.事实上，如果你正在使用像 underscore 这样的库，你可以使用 _.memoize 来为你做这件事.

The cache method is actually not even related to promises - all it does is take a function and cache its result - you can use it for anything. In fact if you're using a library like underscore you can use _.memoize to do it for you already.

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