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javascript - Should setting an image src to data URL be available immediately?

问题描述:

Consider the following (fragile) JavaScript code:

var img = new Image;

img.src = "data:image/png;base64,..."; // Assume valid data

// Danger(?) Attempting to use image immediately after setting src

console.log( img.width, img.height );

someCanvasContext.drawImage( img, 0, 0 );

// Danger(?) Setting onload after setting src

img.onload = function(){ console.log('I ran!'); };

The Question(s)

  • Should the image width and height be correct immediately?
  • Should the canvas draw work immediately?
  • Should the onload callback (set after the src was changed) be invoked?

Experimental Tests

I created a simple test page with similar code. In Safari my first tests, both by opening the HTML page locally (file:/// url) and loading it from my server, showed everything working: the height and width is correct, the canvas draw works, and the onload also fires.

In Firefox v3.6 (OS X), loading the page after launching the browser shows that the height/width is not correct immediately after setting, drawImage() fails. (The onload handler does fire, however.) Loading the page again, however, shows the width/height being correct immediately after setting, and drawImage() working. It would appear that Firefox caches the contents of the data URL as an image and has it available immediately when used in the same session.

In Chrome v8 (OS X) I see the same results as in Firefox: the image is not available immediately, but takes some time to asynchronously 'load' from the data URL.

In addition to experimental proof of what browsers the above does or does not work, I'd really love links to specs on how this is supposed to behave. So far my Google-fu has not been up to the task.

Playing It Safe

For those who don't understand why the above might be dangerous, know that you should use images like this to be safe:

// First create/find the image

var img = new Image;

// Then, set the onload handler

img.onload = function(){

// Use the for-sure-loaded img here

};

// THEN, set the src

img.src = '...';

网友答案:

As no one has yet found any specifications about how this is supposed to behave, we will have to be satisfied with how it does behave. Following are the results of my tests.

            | is image data usable right  | does onload callback set after src
            |     after setting src?      |   is changed still get invoked?
------------+-----------------------------+-------------------------------------
Safari  5   |             yes             |                  yes                
------------+-----------------------------+-------------------------------------
Chrome  8   | **no** (1st page load), but |                  yes                
FireFox 3.6 |  yes thereafter (cached)    |                                     
------------+-----------------------------+-------------------------------------
IE8         |    yes (32kB data limit)    |                  yes         
------------+-----------------------------+-------------------------------------
IE9b        |             yes             |                 **no**              
------------+-----------------------------+-------------------------------------

In summary:

  • You cannot assume that the image data will be available right after setting a data-uri; you must wait for the onload event.
  • Due to IE9, you cannot set the onload handler after setting the src and expect it to be invoked.
  • The recommendations in "Playing it Safe" (from the question above) are the only way to ensure correct behavior.

If anyone can find specs discussing this, I will happily accept their answer instead.

网友答案:

After giving control back to the browser's rendering engine, the test reports true in FF 4b9 and Chromium 8.0.552.237. I modified these parts:

img.onload = function(){   // put this above img.src = …
    document.getElementById('onload').innerHTML = 'yes';
};
img.src = img_src;
…
window.setTimeout(function () {   // put this inside setTimeout
    document.getElementById('wh').innerHTML = (img.height==img.width) && (img.height==128);
}, 0);

Update: Yes, I understand this 'answer' is more like a comment, but just wanted to point it out. And after testing I get reproducible results:

  • without setTimeout, in both browsers I get false the first time I open the page and true only after hitting F5. After explicitly clearing the cache both say false again after reloading.
  • with setTimeout the width/height test always evualuates to true.

In both cases the image doesn't show up the first time, only after reloading the page.

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