如何将同步ajax转换为异步？

Question

I am trying to check the server time and compare it against preset closing time to prevent user from clicking on expired sale items (and display the item has expired) but I am hitting a snag on how to accomplish this when it's mixed in with synchronous call

I know async:false is a very bad idea and it won't work in Firefox(works in Chrome and IE) but it does exactly what I wanted. I am having a hard time figuring out how to convert my code to be asynchronous. If I change part of my code asynchronous then all my other parts of code that depends on it falls apart because the timing is off.

- All I would call if sale_closed()

@get_server_time: (url) ->
$.ajax(
  url: url + '/servertime'
  type: 'GET'
  async: false
)

@sale_closed: () ->
   if not @is_valid_user
     return true
   if Config.override_bid_closed?
     return false
   if @get_server_time(url) > sale_end
     return true
   return false

What I tried to do but that's not working

@get_server_time: (url) ->
$.ajax(
  url: url + '/servertime'
  type: 'GET'
)

@sale_closed: () ->
  @get_server_time(url).done (data) ->
   if not @is_valid_user
     return true
   if Config.override_bid_closed?
    return false
   if data > sale_end
     return true
   return false

Same code in

function get_server_time (url) {
  return $.ajax({
    url: url + '/servertime',
    type: 'GET'
  });
};

function sale_closed() {
    get_server_time(url).done(function(data) {
     if (!this.is_valid_user()) {
      return true;
    }
    if (Config.override_sale_closed != null) {
      return false;
    }
    if (data > sale_end) {
      return true;
    }
    return false;
  });
};

Is the expectation to change all my sale_close code from

foo: () ->
  if sale_closed()
    /* do something */

to

do_something: () ->
 /* do something */

foo: () - >
  sale_closed(do_something)

That's a giant nightmare because I have to change a lot of code and would make them all asynchronous which may break all other functions that call 'foo', 'foo2', etc. (which were synchronous before).

Is there anything guarantee the variables do_something read will be in context and have the same value when it execute?

Thanks for your help as I try to wrap my head around this

---

Answer 1

Try doing some logging and if the problem is that you don't have access to the function @is_valid_user then try changing this

  @get_server_time(url).done (data) ->

to this

  @get_server_time(url).done (data) =>

the fat arrow binds the callback function on done to the current context of this therefore preserving the ability to call this.is_valid_user . Hard to tell exactly if this will work for you without a larger code context, but I might try that first.

Answer 2

Is this correct?

Not really. You cannot return from asynchronous callbacks as like as you would in synchronous code. Instead, you will want to use Promises:

@get_server_time: (url) ->
  $.ajax(
    url: url + '/servertime'
    type: 'GET'
  )

@sale_closed: () ->
  if not @is_valid_user
    return $.when true
  if Config.override_bid_closed?
    return $.when false
  @get_server_time(url).then (data) ->
    data > sale_end

Is the expectation to change all my code that calls sale_close?

Yes, you will need to change everything to callback style. With promises, the changes won't be very large though, and the flow stays linear.

Is there anything guarantee the variables do_something read will be in context and have the same value when it execute?

You should declare do_something locally inside the foo function (or just use a function expression), which will guarantee that all the variables you need are in scope. If you need the right this context, use fat arrow functions.