Calculate layout delta to store on an item - sitecore

Question

I am trying to figure out how to calculate, then store the layout delta for a rendering programatically. The situation I'm in is that I have a rendering defined on my standard value. It's datasource is empty. I then have a process that creates an item based on that template, but I need to set the datasource on the rendering.
By default, the __Renderings field on the new item is blank (as is expected). So far, I've been able to get a RenderingReference to my rendering, detect that the datasource is blank, but I cannot for the life of me figure out how to set the datasource then store the correct delta in the __Renderings field on my item.
So far I have:
foreach (var device in new DeviceRecords(database).GetAll())
{
foreach (var rendering in myItem.Visualization.GetRenderings(device, false).Where(r => r.RenderingID == renderingId)
{
if (rendering.Settings.DataSource.IsNullOrEmpty())
{
var dataSourceItem = datasourceFolder.Add("Datasource name", dataSourceTemplate);
rendering.Settings.DataSource = dataSourceItem.ID.ToString();
using (new EditingContext(myItem)){
myItem[FieldIDs.LayoutField] == //????
}
}
}
}
My guess is I need to somehow invoke something in XmlDelta, but it looks like all of those methods want some Xml to work with, when all I have is the rendering item.

Answer 1

I wrote some code a while back that tried to extract data source information from Sitecore's XML deltas. I never tried updating it though, but this may work for you.
The class I used was Sitecore.Layouts.LayoutDefinition which is able to parse the XML and if I remember correctly it deals with the business of working out what the correct set of page controls is by combining the delta with the underlying template data. You can construct it like so:
string xml = LayoutField.GetFieldValue(item.Fields["__Renderings"]);
LayoutDefinition ld = LayoutDefinition.Parse(xml);
DeviceDefinition deviceDef = ld.GetDevice(deviceID);
foreach(RenderingDefinition renderingDef in deviceDef.GetRenderings(renderingID))
{
// do stuff with renderingDef.Datasource
}
So I think you can then use the API that LayoutDefinition, DeviceDefinition and RenderingDefinition provides to access the data. There's a bit more info on how I used this in the processImages() function in this blog post: https://jermdavis.wordpress.com/2014/05/19/custom-sitemap-filespart-three/
I think the missing step you're after is that you can modify the data this object stores (eg to set a data source for a particular rendering) and then use the ToXml() method to get back the revised data to store into your Renderings field?
You may be able to find more information by using something like Reflector or DotPeek to look inside the code for how something like the Layout Details dialog box modifies this data in the Sitecore UI.
-- Edited to add --
I did a bit more digging on this topic as I was interested in how to save the data again correctly. I wrote up what I discovered here: https://jermdavis.wordpress.com/2015/07/20/editing-layout-details/