Programming iOS 4

Table View Data

The structure and content of the actual data as portrayed in a table view comes from the data source, an object pointed to by the table view’s dataSource property and adopting the UITableViewDataSource protocol. The data source is thus the heart and soul of the table. What surprises beginners is that the data source operates not by setting the table view’s structure and content, but on demand. The data source, qua data source, consists of a set of methods that the table view will call when it needs information. This architecture has important consequences for how you write your code, which can be summarized by these simple guidelines:

Be ready

Your data source cannot know when or how often any of these methods will be called, so it must be prepared to answer any question at any time.

Be fast

The table view is asking for data in real time; the user is probably scrolling through the table right now. So you mustn’t gum up the works; you must be ready to supply responses just as fast as you possibly can. (If you can’t supply a piece of data fast enough, you may have to skip it, supply a placeholder, and insert the data into the table later. This, however, may involve you in threading issues that I don’t want to get into here. I’ll give an example in Chapter 37.)

Be consistent

There are multiple data source methods, and you cannot know which one will be called at a given moment. So you must make sure your responses are mutually consistent at any moment. For example, ...