getJSON Synchronous

问题:

GOAL: What I'm after is to get data from database and refresh main.php (more evident through draw_polygon) every time something is added in database (after $.ajax to submit_to_db.php).

So basically I have a main.php that will ajax call another php to receive an array that will be saved to database, and a json call another php to return an array will be used by main.php.

$(document).ready(function() {
    get_from_db();
    $('#button_cancel').click(function(){
       $.ajax({
          url: 'submit_to_db.php',
          type: 'POST',
          data: {list_item: selected_from_list},

          success: function(result){
             ...
             get_from_db();
          }
       });
    });
    function get_from_db(){
         $.getJSON('get_from_db.php', function(data) {
             ...
             draw_polygon(data);
         });
    }
 });

In my case, what I did was a get_from_db function call for getJSON to actually get data from database, with the data to be used to draw_polygon. But is that how it should be done? I'm a complete newbie and this is my first time to try getJSON and ajax too to be honest. So my question: How does asynchronous work actually? Is there another workaround for this instead of having to call function get_from_db with getJSON (it isn't synchronous, is it? is that why it doesn't update the page when it isn't within a function?) All the time - like $.ajax with async: false (I couldn't get it to work by the way). My approach is working, but I thought maybe there are other better ways to do it. I'd love to learn how. Thanks in advance. I hope I'm making any sense.

To make it more clearer, here's what I want to achieve:

1. @start of page, get data from database (currently through getJSON)
2. Paint or draw in canvas using the data
3. When I click the done button it will update the database
4. I want to AUTOMATICALLY get the data again to repaint the changes in canvas.

回答1:

Asynchronusly does mean the Request is running in the background, and calls your function back when it got a response. This method is best if you want to have a result but allow to use your app within the request. If you want to have a direct response, take a look at a synchron request. this request will pause script execution until it got a response, and the user can not do anything until the response was recieved. You can toggle it via:

async: false,

So for example:

$.ajax({
    url: "myurl",
    async: false,
    ...
})

回答2:

Since $.getJSON() uses ajax configurations, just set the global ajax configs:

// Set the global configs to synchronous 
$.ajaxSetup({
    async: false
});

// Your $.getJSON() request is now synchronous...

// Set the global configs back to asynchronous 
$.ajaxSetup({
    async: true
});

回答3:

$.getJSON(), doesn't accept a configuration, as it says in the docs it's a shorthand version of:

$.ajax({
  dataType: "json",
  url: url,
  data: data,
  success: success
});

So just rewrite your request in terms of that and async:false will work just as you expect.

回答4:

$.getJSON() is a shorthand notation for $.ajax() which can be configured to be synchronous (see jQuery.getJSON and JQuery.ajax):

$.ajax({
  dataType: "json",
  url: url,
  data: data,
  async: false, 
  success: function(data) {
      ...
      draw_polygon(data);
  }
});

Try to avoid synchronous calls though. Quote from jQuery doc (see async prop):

Cross-domain requests and dataType: "jsonp" requests do not support synchronous operation. Note that synchronous requests may temporarily lock the browser, disabling any actions while the request is active.

You might want to try jQuery Deferreds like this:

var jqxhr = $.getJSON(url);
jqxhr.done(function(data) {
    ...
    draw_polygon(data);
});