The nuances of UITableView - ios

Question

I have been using objective c for a few months now, using different foundation classes and generally playing around with the language.
In my own experience nothing has been more confusing than UITableView; Below is a bit of code that does not do much.
//the header file
#interface SLDataBankListTableViewController : UITableViewController <UITableViewDelegate, UITableViewDataSource>
#property (strong, nonatomic) IBOutlet UITableView *tableView;
#end
//implementation
#interface SLDataBankListTableViewController ()
#property (strong, nonatomic) SLDataSourceObject* dataSource;
#end
#implementation SLDataBankListTableViewController
- (void) viewWillAppear:(BOOL)animated {
[super viewWillAppear:YES];
_dataSource = [[SLDataSourceObject alloc] init];
}
#pragma mark - Table view data source
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView {
return 1;
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
return _dataSource.dataBankNames.count;
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
UITableViewCell* cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:#"reuse"];
cell.textLabel.text = [_dataSource.dataBankNames objectAtIndex:indexPath.row];
return cell;
}
#end
I have successfully used this class for over a dozen times and every time I make some stupid mistake like now. Finally I gathered some courage and decided to ask for help.
What are the subtle things, nuances of this class that I don't seem to grasp?
Edit: this is a part of a small program, other parts of it work fine, but the table displays nothing; Some changes have been recommended that are important but did not solve the problem.

Answer 1

It is a little hard to debug without known what is not working, but I see some things which might help you out.
UITableViewController has its own tableview but you seem to have another tableview wired up in a nib. Either use the UITableViewController tableview, or create your own, don't do both.
in cellForRowAtIndexPath you are creating a new cell every time instead of reusing the cells you have.
The delegate methods for the tableview can be called before viewWillAppear. You should create your datasource object earlier. I suggest viewDidLoad:. (Another reason viewWillAppear is a bad choice is that it can be call multiple times, and you can end up creating and destroying many datasource objects for no reason)
Hope that helps.

Answer 2

The big thing to remember about a table view is that it's a way for user to interact with an array of objects. The array is represented by a datasource, and the datasource methods describe what the view needs to know:
how many objects are in the array (called numberOfRowsInSection:)
how to display each one of the objects (called cellForRowAtIndexPath:)
To answer the latter question, the datasource must answer a view. That view's job -- like any view -- is to represent an object for the user. In row the table view uses a UITableViewCell.
The datasource array can be arbitrarily large, so directly mapping UITableViewCells to its elements can get arbitrarily expensive in memory terms. Rather than create a cell for every object in the array, the table view reuses cells.
When a cell scrolls off the top, the "new" one that appears at the bottom isn't new, it's the old cell handed back to the datasource to be reconfigured for the new row. To accomplish this, your datasource is expected to not allocate a new cell, as #JonRose correctly points out, but to ask the table view for a reused cell using dequeueReusableCellWithIdentifier.