Lesson 4: Displaying data from the database

By default, the data source of a table view is its controller. In the case of this application, the RootViewController is currently the data source for the table view. In this lesson, you will implement the UITableViewDataSource protocol in the DataAccess class. This way, the DataAccess class will be responsible for providing the table view with the data it requires.

To begin, add the protocol to the DataAccess header.

@interface DataAccess : NSObject <UITableViewDataSource> {
    ULConnection *                    connection;
}

Getting the number of names in the database

The UITableViewDataSource protocol has two required methods. This section will implement one of those methods, namely, the tableView:numberOfRowsInSection: method. The table view has a single section: a list of names. Therefore, this method will simply return the count of the rows in the database. Use the following SQL statement:

    SELECT COUNT (*) FROM Names

Add the following method to get the count:

- (NSInteger)tableView:(UITableView *)tableView 
 numberOfRowsInSection:(NSInteger)section {
    const char *            COUNT = "SELECT COUNT (*) FROM Names";
    ULPreparedStatement *    prepStmt = connection->PrepareStatement(COUNT);
    
    if (prepStmt) {
        ULResultSet    *resultSet = prepStmt->ExecuteQuery();
        int            numberOfNames;
        
        resultSet->First();
        numberOfNames = resultSet->GetInt(1);
        resultSet->Close();
        prepStmt->Close();
        return numberOfNames;
    } else {
        NSLog(@"Couldn't prepare COUNT.");
    }
    
    return 0;
}

Creating the table cells

The second required method in the protocol is the tableView:cellForRowAtIndexPath: method. This method is responsible for creating the actual cell objects that the table view displays. The RootViewController defines a good template to reuse cells. Table cells are usually recycled using the pattern in the template to save memory and allow for fast scrolling. The only identifier the table view provides is a 0-based integer index. However, the database does not normally keep an ordered list of the names. In order to simulate an ordered list, the database uses an ordered index on the names and the application will use a select statement with a row limitation and an order by clause to select a given index. Although this method doesn't require the FOR UPDATE clause, this same query will be used to delete names in Lesson 5: Deleting data from the database. Add the following to DataAccess.mm just below the import statements:

#define SELECT_STMT @"SELECT TOP 1 START AT %d name FROM Names ORDER BY name FOR UPDATE"

To use this query in the method do the following in the DataAccess implementation: 

- (UITableViewCell *)tableView:(UITableView *)tableView 
         cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    static NSString *        CellIdentifier = @"Cell";
    UITableViewCell *        cell = 
     [tableView dequeueReusableCellWithIdentifier: CellIdentifier];
    ULPreparedStatement *    prepStmt;
    
    if (cell == nil) {
        cell = [[[UITableViewCell alloc] 
                 initWithStyle:UITableViewCellStyleDefault 
                 reuseIdentifier:CellIdentifier] 
                 autorelease];
    }
    
    // Make it so the cell cannot be selected. 
    cell.selectionStyle = UITableViewCellSelectionStyleNone;
    
    // +1 to the index since the DB uses a 1-based index rather than 0-based 
    prepStmt = connection->PrepareStatement(
        [[NSString stringWithFormat:SELECT_STMT, indexPath.row + 1] UTF8String]);
    if (prepStmt) {
        ULResultSet *    resultSet = prepStmt->ExecuteQuery();
        char            name[255];
        
        resultSet->First();
        resultSet->GetString("name", name, 255);
        resultSet->Close();
        prepStmt->Close();
        cell.textLabel.text = [NSString stringWithUTF8String:name];
    } else {
        NSLog(@"Couldn't prepare SELECT with index.");
    }
    
    return cell;
}

Setting the DataAccess object as the data source

Now that the DataAccess object is configured as a data source, you need to set the data source of the table. To do this, add the following at the end of the viewDidLoad method of the RootViewController class:

    // Set the data source. 
    [self.tableView setDataSource:[DataAccess sharedInstance]];

You also need to add the following to the viewWillAppear. Without it, changes are not picked up until the rows scroll out of view and back in:

    [self.tableView reloadData];

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