Question

Using jQuery promises I do the following to manage a situation where I have multiple instances where I need to call for external resource, but I don't want to call for the same external resource more than once.

Initital REST calls gets me multiple data objects with references to additional information (in the example "department" property gives a reference to where I can find more info about the department).

{ "somedata": "...", "department": "https://api.../departments/1000", }

Already deep down in separate function calls for each data object I wish to coordinate all calls to the same resources. I don't want to fetch department name for "department 1000" multiple times.
I save promises OR a returned result in a dictionary and reuse them.

a. If it is a promise, I return the same promise to be reused by the caller.

b. If it is a fetched value, I resolve the promise asynchronous by using a timeout.

Here is the code:

var Departments = {};

function getDepartmentName(id){
   var dfd = new $.Deferred();
   var promise = dfd.promise();

   if(id in Departments){
        if(typeof Departments[id] == 'object'){
            return Departments[id];//reuse the promise
        }else{//department name is a string
            setTimeout(function(){ dfd.resolve(Departments[id]); }, 0);         
        }
   }

   else{
       Departments[id] = promise;//Set dictionary value to the promise
       var dPromise = FetchDepartment('https://api.../departments/'+id);
       $.when(dPromise).then(
           function(departmentName){
               Departments[id] = departmentName;//Re-set dictionary value to the fetched value
               dfd.resolve(departmentName);
           },
           function(err){
               delete Departments[id]; 
               dfd.reject('Call failed');
           }
       );   
   }
   return promise;

}

It seems to work but I am concerned whether this is such a great design. I have a few questions.

Is there some promise functionality I should use instead?
Are there any situations where things might happen in the wrong order?
Is it correct to reuse a promise instance like this? Multiple callers make a call to .then of the same promise object, and it seems to work well, but I don't know how jQuery chain all these recipients when the promise is resolved.

Answer 1

我将诺言或返回的结果保存在字典中，然后重用它们。如果它是一个获取的值，我将通过使用超时来异步解决Promise。

不要那么复杂。这的行为与您仅保留承诺并仅返回承诺的情况完全相同，而不是在最初的承诺完成后为呼叫创建新的承诺。

还有avoid deferreds，当您已经在处理承诺时。

var Departments = {};

function getDepartmentName(id) {
    if (id in Departments){
        return Departments[id]; //reuse the promise
    } else {
       var promise = FetchDepartment('https://api.../departments/'+id).then(null, function(err){
            delete Departments[id]; // allow retries
            throw err;
        });   
        return Departments[id] = promise;
    }
}

重用这样的一个Promise实例是否正确，在该实例中多个调用者都对同一个Promise对象的.then进行调用？

是的，这完全可以。他们的回调将按照与发起then的调用相同的顺序安排。

Javascript reusing promise to manage multiple calls

1 个答案: