How flex itemRenderer works ? (their life cycle)

You may want to look into ClassFactory from the Flex APIs:

This allows you to set a prototype Object with arbitrary types / values each of which will be passed to the item renderer. From the sample:

var productRenderer:ClassFactory = new ClassFactory(ProductRenderer);
productRenderer.properties = { showProductImage: true };
myList.itemRenderer = productRenderer;

The above code assumed that "ProductRenderer" has a public property called "showProductImage" which will be set with a value of "true."

I could be wrong but I think the "data" property you're referencing in the code sample above is the "data" for the top-level Container in your view and not for that particular row of the DataGrid. A few other approaches come to mind:

1. Implement a single item renderer class that examines the data being passed to it and utilizes the proper item renderer for the type of data supplied.
2. Implement a function in the view of your DataGrid that examines your dataProvider and returns the proper item renderer class; call this inside the DataGridColumn.itemRenderer property using a binding expression.
3. Implement an subclass of DataGridColumn that has the logic baked into it to set the correct itemRenderer.

I would actually recommend against mixing "renderer data" that is View-specific with data from your Model. Unless you wrap the core model data with an object that exposes it along with the renderer (what some people call a ViewModel).