Create a new Cocoa application project called Expenses.

Open the main nib file.

Drag the table view object from the Tabulation Views palette as shown in Figure 9.3.

Figure 9-3. Adding a table view object to the interface

Resize the table view to fill the window.

Click the table view once to bring up the NSTableView Info window.

Select the Size pane and click the lines so they match those shown in Figure 9.4.

Figure 9-4. Changing the size attributes of an NSTableView

Select the NSTableView.

Set the attributes in the Attributes pane of the NSTableView Info window as shown in Figure 9.5.

Figure 9-5. NSTableView attributes

Make the columns’ width equal. Select the leftmost column (you may have to double-click), hold the cursor over the right edge of the column, and wait for the cursor to change to a pair of horizontally opposed arrows. Drag the column edge so the column view is divided into two equal parts. When you are done, Command-click on the selected column to deselect it.

Double-click the column header. Type Column 1 and press Return.

Repeat for the second column and name it Column 2.

One at a time, select the column headings and bring up the Table Column Info window. Select the Attributes pane and center the headings.

Create a subclass of NSObject and name it MyDataSource.

Instantiate the MyDataSource class.

Draw a connection from the table view object to the data source object in the Instances window. Make sure you have selected the table view and not its surrounding scroll view before you draw the connection. The table view will turn a darker shade of gray when selected.

Select the dataSource outlet in the Connections pane of the Info window and click Connect.

Create the files for MyDataSource, and add them to the Project Builder project.

Open MyDataSource.h in Project Builder and add the following declarations:

- (int)numberOfRowsInTableView:(NSTableView *)tableView;
- (id)tableView:(NSTableView *)tableView
        objectValueForTableColumn:(NSTableColumn *)tableColumn
        row:(int)row;

Open MyDataSource.m and add the following method implementations:

- (int)numberOfRowsInTableView:(NSTableView *)tableView
{
    return 10;
}

- (id)tableView:(NSTableView *)tableView
        objectValueForTableColumn:(NSTableColumn *)tableColumn
        row:(int)row
{
    return [NSCalendarDate date];
}

In Project Builder, open the Expenses example project (if it’s not already open).

Choose New File from the File menu.

An Assistant dialog box will appear. Choose Objective-C Class from the list.

Name the new file Expense.m and click Finish. Project Builder will create the header and implementation files, insert default templates, and add them to the project.

Open Expense.h and add the following instance variable declarations. Remember to insert these declarations between the curly braces.

NSCalendarDate *date;
NSString *category;
NSDecimalNumber *amount;

Add the following accessor method declarations. Remember to insert these declarations between the closing curly brace and the @end directive.

- (NSCalendarDate *)date;
- (void)setDate:(NSCalendarDate *)value;
- (NSString *)category;
- (void)setCategory:(NSString *)value;
- (NSDecimalNumber *)amount;
- (void)setAmount:(NSDecimalNumber *)value;

In Expense.m, add the next line between the #import and @implementation directives:

static NSString *defaultCategory = @"Food";

In the implementation section, add an init method:

- (id)init {
    self = [super init];
    [self setDate: [NSCalendarDate date]];
    [self setCategory: defaultCategory];
    [self setAmount: [NSDecimalNumber zero]];
    return self;
}

Add a corresponding dealloc method:

- (void)dealloc {
    [date release];
    [category release];
    [amount release];
    [super dealloc];
}

Finally, add the accessor method implementations:

- (NSCalendarDate *)date {
    return date;
}

- (void)setDate:(NSCalendarDate *)value {
    [date autorelease];
    date = [value copy];
}

- (NSString *)category {
    return category;
}

- (void)setCategory:(NSString *)value {
    [category autorelease];
    category = [value copy];
}

- (NSDecimalNumber *)amount {
    return amount;
}

- (void)setAmount:(NSDecimalNumber *)value {
    [amount autorelease];
    amount = [value copy];
}

Open the main nib file.

Select the NSTableView object, and in the Attributes pane of the Info window, change the number of columns to three.

Resize the three columns so that they are of equal width. The newly created third column will appear out of sight to the right of the second. You’ll have to select the second column and move its right edge to the left to reveal the third column.

Rename the three columns Date, Category, and Amount, respectively.

Use the Info window to center the third column’s label.

Select the Date column and drag a date formatter onto the Date column heading box. You’ll see the outline of the heading box highlight when the cursor enters it. In the Formatter pane of the Date column’s Info window, choose a date format that appeals to you. Also, select the Allow Natural Language box. This is a nice feature of the formatter that lets you use strings like “today,” “yesterday,” and “tomorrow” for entering dates.

Repeat step 3 for the Amount column, using a number formatter instead of a date formatter.

In Project Builder, open MyDataSource.h.

Add an instance variable for the array of Expense objects:

NSMutableArray *expenses;

Add method declarations for the accessor methods:

- (NSMutableArray *)expenses;
- (void)setExpenses:(NSMutableArray *)newExpenses;

Open MyDataSource.m and import Expense.h:

#import "Expense.h"

Add implementations for the accessor methods:

- (NSArray *)expenses {
    return expenses;
}

- (void)setExpenses:(NSMutableArray *)newExpenses {
    [expenses autorelease];
    expenses = [newExpenses retain];
}

Add implementations for the init and dealloc methods:

- (id)init {
    self = [super init];
    [self setExpenses: [NSMutableArray array]];
    return self;
}

- (void)dealloc {
    [expenses release];
    [super dealloc];
}

Change the implementation of numberOfRowsInTableView: so it returns the number of items in the array of Expense objects:

- (int)numberOfRowsInTableView:(NSTableView *)tableView
{
    return [expenses count];
}

Change the implementation for tableView:objectValueForTableColumn:row: so it retrieves a value from the array of Expense objects.

This method uses a column identifier to retrieve data from an Expense object. It accomplishes this in an especially clever way that bears some explanation. First, the column identifier is retrieved from the table column object that was sent as a message parameter. Next, an Expense object is retrieved from the array of expenses using a row number that was also sent as a message parameter.

Now for the tricky bit: since the table column identifier string you assigned in Interface Builder is the same as the name of an instance variable in the Expense object, you can use key-value coding to retrieve the proper attribute value from the Expense object in one step. Key-value coding is a kind of shorthand used for accessing instance variables. If an object has an instance variable, then you can use the variable name as a parameter of the message valueForKey: and get the value of that variable from the object in one simple step:

- (id)tableView:(NSTableView *)tableView
    objectValueForTableColumn: (NSTableColumn *)tableColumn
    row: (int)row 
{
    NSString *identifier = [tableColumn identifier];
    Expense *expense = [expenses objectAtIndex: row];

    return [expense valueForKey: identifier];
}

Key-value coding substantially simplifies the implementation of the data source method. Without this convenience, you would have to test the string value of the column identifier against a list in an elaborate if...then statement to decide which of the Expense object’s accessor methods to call. Instead of one line, you would have to do the following:

id value = nil;

    if ([identifier isEqual: @"date"])
        value = [expense date];
    else if ([identifier isEqual: @"category"])
        value = [expense category];
    else
        value = [expense amount];

    return value;

Add a method to MyDataSource.m that creates an array of Expense objects filled with test data and declare it in MyDataSource.h. Each object is created using the current date, the default expense category, and a dollar amount equal to the loop counter:

- (NSMutableArray *)generateTestData
{

    NSMutableArray *array = [NSMutableArray array];
    int index;

    for (index = 0; index < 15; index++) {
        Expense *exp = [[Expense alloc] init];
        [exp setAmount: 
                (NSDecimalNumber *)[NSDecimalNumber numberWithInt:index]];
        [array addObject: exp];
        [exp release];
    }

    return array;
}

Implement awakeFromNib for MyDataSource.m:

- (void)awakeFromNib
{
   [self setExpenses: [self generateTestData]];
}