How to wait until image is loaded in jquery

问题:

I have this example, when my form is submitted I have to load an image file and wait until the image has loaded to return true (in this case to submit the form)

e.g.

$('#myform').submit(function(){
  var image=document.createElement('image')  
  var src=document.createAttribute('src')
  image.value="http://example.com/image.jpg"
  image.setAttributeNode(src);
  document.body.appendChild(image);
})

Here if I return true the form will be submitted (the image won't load) and if false it won't (the image will load but the form will not be submitted). How can I make a code that will return true after the image has loaded.

回答1:

There is an important concept one must note here - When you add an Event Handler function to be executed when a particular event is fired, the value that the Event Handler function returns actually goes nowhere, and is passed to nothing. Event Listeners are created to invoke Event Handler functions at an unknown point in the future, but typical script execution is completely linear and happens in the order of the commands in the script - so it is best to define the functionality of your application in a way that you perform certain actions like this only when certain events take place.

It looks like in your question above, you are defining one event handler to listen for when the form is submitted initially, so I will take that as the initial event which gets everything started. Here is how I would handle the submission process you describe:

//wrap the form element in jQuery and cache it so we don't work to re-select it
var $myform = $('#myform');

//specify that we are listening for -when- the form is submit
$myform.on('submit', function(event){
    //prevents the form from actually submitting when submit is first clicked
    event.preventDefault();

    //Simpler image construction
    var image = new Image();
    document.body.appendChild(image);

    //remember to listen for the image's load event before assigning a source
    $(image).on('load', function(){
        //this function is invoked in the future,
        //when the image load event has been fired

        //this bit actually submits the form via GET or POST
        $myform.submit();
    });

    //an image's `src` attribute can be set directly via the`src` property
    image.src = "http://placekitten.com/320/240";
});

Example of this working on JSFiddle:

http://jsfiddle.net/Admiral/uweLe/

I would recommend reading up on jQuery's .on() method to gain some insight on the currently preferred methods of event binding - that should clear things up a bit.

http://api.jquery.com/on/

Good luck!

回答2:

You could use an onload or jquery load event on the image and submit the form on the callback. Then if you want to wait longer still you could also add a timeout in the callback.

Something like:

$('#myform .submitButton').click(function(e){
  e.preventDefault();

  var image=document.createElement('image');

  $(image).load(function(){

    //Submits form immediately after image loaded.
    $('#myForm').submit();  

    // Submits form 1 second after image has loaded
    setTimeout(function(){ $('#myForm').submit(); }, 1000); 
  });

  var src=document.createAttribute('src');
  image.value="http://example.com/image.jpg";
  image.setAttributeNode(src);
  document.body.appendChild(image);
})

Note: In my example I've changed the submit event to a click event. You could still use the submit if you want, it would probably still work, as long as you returned false or used preventDefault.

回答3:

I use imagesLoaded PACKAGED v3.1.8

You can find it on github: https://github.com/desandro/imagesloaded

Their official site: http://imagesloaded.desandro.com/