Positioning a dialog on a Facebook Canvas Page

As promised (or threatened) here is a post on some little Facebook development insight gained through some trial, error, blood, sweat, and yes tears.

A problem I was having was accurately determining the x, y position of a given control for popping up informational dialogs. Sure, it was easy enough to use the getAbsoluteTop and getAbsoluteLeft calls to get the absolute positioning (supposed), but after playing around with these calls it became apparent that “absolute” didn’t mean “absolute” in the sense that most web / windows developers would understand. It really is a relative offset – but to what?

After perusing the FBJS docs, and finding no mention of a viewport (which is really what I was trying to determine the offset from), I took a total flier and thought – “well, if I could jet get the root element of my document, and find ITS x and y, I should be home free.

Bingo. That was the trick. Use document.getRootElement() to get the container element.

The code below demonstrates the technique.

var argCtl = document.getElementById(“myForm”);
var newTop = argCtl.getAbsoluteTop();
var newLeft = argCtl.getAbsoluteLeft();
var scrollW = argCtl.getScrollWidth();
var root = document.getRootElement();
var cTop = root.getAbsoluteTop();
var cLeft = root.getAbsoluteLeft();

document.getElementById(“confirm_dialog1″).setStyle(“top”, (newTop – cTop) + “px”);
document.getElementById(“confirm_dialog1″).setStyle(“left”, (newLeft – cLeft) + “px”);

First, grab a reference to an object on the page that you wish to position relative to (in this case, I am using a form named “myForm”). Get its x and y coordinates.

Next, get the container x and y by getting its reference (document.getRootElement()) and then its x and y values (getAbsoluteTop and getAbsoluteLeft).

Finally, to accurately position a dialog where an element exists, take the difference between the container and the element being positioned:

document.getElementById(“confirm_dialog1″).setStyle(“top”, (newTop – cTop) + “px”);
document.getElementById(“confirm_dialog1″).setStyle(“left”, (newLeft – cLeft) + “px”);

Voila. You’ve just positioned a dialog over an element.

In practice, you would want some arbitrary additional offset from the control so that you don’t hide it when you pop up / display your dialog. I’ll leave that as an exercise for the reader.

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