Lifecycle .NET ASPX page on postback

Question

Apologies if this is vague or too general, I know we are supposed to ask specific programming questions, but this is to try and understand how postbacks work in ASP.NET when something is rendered on page initially, and not touched on postback, yet still appears on screen after these postbacks.

Detail: I have an ASPX page - on initial page load it sets up all the labels and controls and a repeater with thumbnails in it.

I can follow the code through as it sets up all components, which will then be rendered.

On postback, if a value has been changes (status for example), the page makes updates to database, and then re initializes the screen (calls the init method with postback set to false), setting up everything again (I think the aim was to set up certain controls like radio buttons for the status), including the image repeater again (which I think is a waste of time).

I have tested skipping the code that sets up the image repeater etc. on a status update (postback), and the image repeater displays fine in the browser - however I am not sure why - ASP.NET is stateless, so on a postback, if the page does not render all the components on the page everytime how can they persist between postbacks?

I know there is a reason out there, just can't frame my question well enough in google to find it. Does the server send back and update which the browser merges with the existing displayed page? Leaving all unchanges components as is?

The reason I need to know this, is I have a screen with a lot of images on it, and I do not want to be setting them every time unless I have to.

Answer 1

This link - provided by Steve (thank you very much) - was what I was looking for goes into depth on ViewState - vaguely remember reading about that when I first stated doing .NET - have completely forgotten about it, but this explains it perfectly!

Understanding ASP.NET ViewState https://msdn.microsoft.com/en-us/library/ms972976.aspx