UITableView numberOfSectionsInTableView and number

问题:

I am making a REALLY BASIC UITableView project without any data source. I am just displaying "1" on first and only cell.

I am returning 1 in both numberOfSectionsInTableView and numberOfRowsInSection. I am also doing an NSLog on both methods simply stating the name of the respective methods.

On initial load, I get this:

SomeShit[27511:936083] number of sections
SomeShit[27511:936083] number of rows in section
SomeShit[27511:936083] number of sections
SomeShit[27511:936083] number of rows in section
SomeShit[27511:936083] number of sections
SomeShit[27511:936083] number of rows in section

Why are these methods calling thrice? Correct me if I am wrong but this is really weird. I thought they were supposed to call only once because of number of items in both being just 1.

EDIT:

What I am doing in viewDidLoad,

self.tableView = [[UITableView alloc] initWithFrame:CGRectMake(0, 0, self.view.frame.size.width, self.view.frame.size.height) style:UITableViewStylePlain];
[self.view addSubview:self.tableView];

self.tableView.dataSource = self;
self.tableView.delegate = self;

[self.tableView registerClass:[UITableViewCell class] forCellReuseIdentifier:cellIdentifier];

回答1:

The delegate and data source are there to service the table view implementation, and methods may be called multiple times. You should not concern yourself with the implementation details of table views, just make sure your answers are consistent (i.e. you can answer correctly at any point in time and answers do not change without calling reloadData).

回答2:

Nowhere in the documentation does it say these methods are only call once in a reload data cycle.

You've made the assumption that they only need to be called once, but as you have shown, that's a bad assumption.

After reviewing the documentation for UITableView and UITableViewDataSource, the only promise is this.

-[UITableView numberOfSections]:

Discussion

UITableView gets the value returned by this method from its data source and caches it.

This means you can't know how calls to -[UITableView numberOfSections] will correspond with calls to -[UITableViewDataSource numberOfSectionsInTableView:].

Worse, the number of times these methods are called can (and have) change between different versions of iOS.

---

In short, make these methods idempotent and performant, and forget about implementation details of UITableView.

回答3:

You could try to figure out what is causing these reloads. Go to breakpoints section (1), press on "plus" button (2). Then add symbolic breakpoint with -[UITableView reloadData] in "symbol" field.

Then run your app and see the stack trace of method calls. You will be able to figure out the reason of such behaviour.