Javascript/jQuery: How to detect img is fully downloaded or not?

问题

All I need to do is locate image at the center of its parent div.
First, I tried this.

$(document).ready(function() { 
    $('#image1').css({
    'top': 50%, 'left': 50%,
    'margin-top': (-$('#image1').height()/2) + 'px',
    'margin-left': (-$('#image1').width()/2) + 'px'
    }); 
});

It failed cause $('#image1').height() and width() gives 0 before it is fully downloaded.
So I tried to keep inspecting size of the image until it has specific width and height.
like,

function inspectSize() { return ($('#image1').width() != 0); }
function setPosition() {
    if(!inspectSize()) {
        setTimeout(setPosition, 1000);
        return;
    }
    $('#image1').css({
        'top': 50%, 'left': 50%,
        'margin-top': (-$('#image1').height()/2) + 'px',
        'margin-left': (-$('#image1').width()/2) + 'px'
    }); 
}
$(document).ready(setPosition);

Still it doesn't work.
because $('#image').width() returns 28px before it is downloaded in IE8 (by the way, IE7 was fine)

So I finally tried waitForImages jQuery plugin like this,

$(document).ready(function() {
    $('#image1').waitForImages(function() {
         setPosition(); // without inspectSize() 
    });
});

It works fine with cached images, but doesn't work with non-cached images.
The function is called before Image1 has width and height.

What should I do?

回答1:

You should be able to just bind your repositioning callback to the load event handler.

So in your last example:

$('#image1').load(function() {
     setPosition(); // without inspectSize() 
});

Where #image1 points to your image tag. Alternatively, as you mention the #image1 might just be a container then:-

$('#image1 img').load(function() {
     setPosition();
});

Update jfriend00 makes a good point below, in that 'load' will only fire when an image is added to the DOM, not if it already exists on the page at the time your JavaScript is run.

To cover both scenarios, you can do this:

var img = $('#image1');

if (img.prop('complete')) {
     setPosition();
} else {
     img.load(function() { setPosition(); });
}

The prop('complete') check will return true if the image already exists, and if it doesn't we bind the 'load' event handler.

回答2:

a simple example is to bind the load event to the image. I always do the following

<img data-src="image.jpg" />

note that i don't set the src attribute yet

$(function() {
    $("img").each(function() {
        $(this).load(function() {
            alert('ready to call $(this).width()');
        }).prop("src", $(this).prop("data-src"));
    });
});

the .prop("src", $(this).prop("data-src")); set's the new src after the load event is binded

回答3:

Try writing the same code inside.

$('window').load(function() {
    // Your code here..
});

来源：https://stackoverflow.com/questions/8613562/javascript-jquery-how-to-detect-img-is-fully-downloaded-or-not

标签

javascript

jquery

waitforimages