I'm very impressed with the Google Web Toolkit, and I'm excited about the power that AJAX could bring to my web applications.
I have a general question about how AJAX handles the fetching of images from a database.
If you are storing images in a database, a standard way of doing it is to use a BLOB. The traditional web-app way of doing things is to build a static page on the server using something like php, and then transmit it back to the client's browser. Recreating an image from a BLOB with php is a snap:
header('application/JPEG') echo $imgdata
How do you acheive the same thing with AJAX? I don't want to have my server creating and destroying images all the time, just so they can be downloaded by the client browser.
Having done some research, there does not seem to be a JavaScript way of reinterpreting the BLOB data.
Is the only workaround to simply not store the images as BLOBs in the first place, and just provide a URL/path?
You don't want to get BLOB data back via AJAX. For images to load just update their SRC to point to the new image
If you want to change images without reloading your page all you need to do with AJAX is get back an SRC of the new image you wan,t then update the SRC of the old image. In some cases (like when you have only a small number of images to cycle through) you won't even need AJAX.
Your SRC should be something like request_image.php?img_id=4 whose sole purpose is to get the image data as you have above :
header('application/JPEG') echo $imgdata
Alternatively you could show the image in an iframe, just give the iframe the same request_image.php URL
What about situations where the programmer needs to grab a generated image from the backend where the data needed to generate the image is sizable enough that it requires it be done with POST and the back-end server doesn't offer an ability to cache or save the generated files.
I had someone on one of my LUG lists trying to figure that out less than a week ago.
> You don't want to get BLOB data back via AJAX. For images to load just > update their SRC to point to the new image
> If you want to change images without reloading your page all you need > to do with AJAX is get back an SRC of the new image you wan,t then > update the SRC of the old image. In some cases (like when you have only > a small number of images to cycle through) you won't even need AJAX.
> Your SRC should be something like request_image.php?img_id=4 whose sole > purpose is to get the image data as you have above :
> header('application/JPEG') > echo $imgdata
> Alternatively you could show the image in an iframe, just give the > iframe the same request_image.php URL
In that case, I'm not sure AJAX is the way to go, as you mentioned, I'm not aware of there being any way javascript can create the image from a binary stream. Also I'm not sure the responseStream xmlHttpRequest property is supported on all browsers and I think the Mozilla documentation says that some versions of their browsers only support the text/xml mime-type for an AJAX response.
You'll have to think of some ingenious way or requesting the image. Maybe posting from an iframe?
When SVG support is better across browsers, then AJAX will be perfect for the situation you describe, provided you want to use that format of course.
Probably so. I suggested since they couldn't write files to disk, to try keeping it in memory or db, and just passing back a reference that they could pass to a script when changing the src tag but that isn't really a very clean way of handling the problem. Not sure why he couldn't save any temp files to his webserver as that is probably the easiest method. SVG or canvas would be better but IE especially is lacking in support for these. I haven't tested to see if IE7 is going to support them or not but I can add that to my wishlist.
In that case, I'm not sure AJAX is the way to go, as you mentioned, I'm
> not aware of there being any way javascript can create the image from a > binary stream. Also I'm not sure the responseStream xmlHttpRequest > property is supported on all browsers and I think the Mozilla > documentation says that some versions of their browsers only support > the text/xml mime-type for an AJAX response.
> You'll have to think of some ingenious way or requesting the image. > Maybe posting from an iframe?
> When SVG support is better across browsers, then AJAX will be perfect > for the situation you describe, provided you want to use that format of > course.
1. Using a small php script in an iframe to display the image. I'm not sure how you could do that with GWT though - is there a way to make a widget populate itself with external content?
2. Use the server code which is extracting the blob to turn it into an actual file on the server, and pass back the URL to it.
All my server code is written in Java, and throwing php into the mix is pretty messy. On the other hand, the second option needs some fairly clever maintenance of the created files.
in that article Marcel Kratochvil saying that oracle works very well with blobs, but what about with mySQL db, especially myisam? any information about performance or benchmarks? any expirience about that?
It doesn't work in Internet Explorer though, and I believe the workaround involves passing all the image data back to the server, and then back again as the image. It'd be easier just to call a separate php script which fetches the image from the database on it's own.
Still, once IE7 is released this may become a more standardized way of passing image/binary data around.
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