The OnClientShow event receives the tooltip instance as its first argument: http://www.telerik.com/help/aspnet-ajax/tooltip-client-show.html. This example shows a RadToolTip, but the same applies for a RadToolTipManager.
There is also a common class that lets you get the currently active tooltip: http://www.telerik.com/help/aspnet-ajax/tooltip-client-side-overview.html.
You can base that decision on several factors, for example:
- the get_value() method will provide the Value contents. If this does not suffice, you can concatenate the desired title on the server, e.g. "desiredValue|desiredTitle" and split() the string according to the used symbol.
- the get_targetControl() method will return the DOM object of the tooltip target. You can use it to extract text or a custom attribute from it so you can set the title accordingly
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