Adding the Unary Minus Function to the Controller Class

We want the unary minus function (the button with the “+/-” on it) to change the sign of the number currently displayed in our Calculator’s numeric display area. One way to implement this function is to handle it with another case in the switch statement in the enterOp: method — we could give the “+/-” key its own tag and have the enterOp: method intercept it and perform the appropriate function. The problem with this approach is that the unary minus function has little in common with the other arithmetic functions: it takes one argument instead of two, and it operates immediately on the displayed value. A far better way to implement this function is to implement a new action method in the Controller class.

Using IB’s Read Files Commandwith a New Action Method

Adding new action methods to existing classes is slightly more difficult than creating the initial class definition. Early versions of IB simply replaced the existing class files (Controller.h and Controller.m, here) with new versions, wiping out any source code that you might have added. Current versions of IB detect that you have made changes in the class files and allow you to merge the changes using the Merge Files application. Unfortunately, this can be a painful and error-prone process. The safest way to add new outlets and actions to files after they have been created and edited is to add these items directly to the Objective-C interface files in a text editor, and then use IB’s Classes → Read Files menu command to inform IB about any new actions and outlets.

  1. Back in PB, insert the doUnaryMinus: action method definition, shown here in bold, into Controller.h. (You can tell that it’s an action method because of its IBAction typing and because sender is the only argument.) 

    ...
- (IBAction)clear:(id)sender;
- (IBAction)clearAll:(id)sender;
- (IBAction)enterDigit:(id)sender;
- (IBAction)enterOp:(id)sender;
- (void)displayX;
- (IBAction)doUnaryMinus:(id)sender;
@end

  2. Now insert the doUnaryMinus: method shown here into Controller.m: 

                            - (IBAction)doUnaryMinus:(id)sender
                        {
                           X = -X;
                           [self displayX];
                        }

  3. Choose PB’s File → Save All menu command to save the Controller class files.

It doesn’t matter where you put this method in Controller.m, as long as it’s between the directives @implementation and @end. However, for consistency, we suggest that you order the method implementations in the same way that the method declarations are ordered in the Controller.h class interface file.

Finally, we have to tell IB about the new doUnaryMinus: method and set up a connection between the on-screen unary minus button and the Controller:

  1. Activate IB by double-clicking MainMenu.nib in PB’s main window (MainMenu.nib is one of the Resources in your project).

  2. Click on the Classes tab in IB’s Nib File window to open the Classes pane.

  3. Select Controller in the class hierarchy in the Classes pane. Because it’s a subclass of NSObject, you may have to scroll to the left (or use the Search field).

  4. Choose IB’s Classes → Read Files menu command.[12]

IB will display the Read Files dialog, as shown in Figure 5-29.

Read Files dialog in IB

Figure 5-29. Read Files dialog in IB

This dialog tells us that the definition for Controller.h will be parsed from the edited file on disk.

  1. Make sure that Controller.h is selected, as shown in Figure 5-29, and click Parse in IB’s Read Files dialog to parse the saved Controller.h file on disk.

  2. Make sure that the Controller class is selected in the Nib File window and then type Command-1 to see the Controller’s attributes in the Info dialog.

The new doUnaryMinus: action method should appear in the Controller Info dialog, as in Figure 5-30, indicating that the Read Files command worked.

New doUnaryMinus: action method in IB after Read Files command

Figure 5-30. New doUnaryMinus: action method in IB after Read Files command

  1. Double-click the unary minus (“+/-”) button in the Calculator window. Make sure that the NSButton, not the NSMatrix as a whole, is selected.

  2. Connect the selected unary minus button to the Controller instance. Do this by Control-dragging from the button to the Controller instance in the Nib File window (note that the Instances tab view displays automatically) and then double-clicking the doUnaryMinus: action in the NSButtonCell Info dialog.

When a button in an NSMatrix object has its own target, as in the previous example, the button’s target overrides the target of the NSMatrix. Thus, when the user clicks on the unary minus button, the button will send the doUnaryMinus: message to its own target, rather than sending the enterOp: message to the target of the NSMatrix.

  1. Back in PB, build and run the program (note how PB nicely prompts to save files).

The unary minus function should now behave as expected (although the application isn’t perfect yet — it has some bugs).

You might be wondering why IB’s Read Files command didn’t bring in the definition of the displayX method in addition to the doUnaryMinus: method. The reason is that IB only looks for action methods when parsing a class interface (.h) file. An action method should always be declared in the form: 

- (IBAction)methodname:(id)sender;

with a single argument called sender. As we’ll see later, IB will also bring in outlet declarations when parsing a class interface file. These outlet declarations must be instance variables of the form: 

IBOutlet id outletname;

Action methods and outlets are the only types of information that IB learns about a class when it parses a class interface file.


[12] If you have the Controller class in IB’s Classes tab selected, you will have an additional menu item labeled “Read Controller.h”. As you might imagine, choosing this menu option causes the Controller.h file to be read directly, without forcing you to choose the file in the Read Files pane. This is a faster method of reading a class description.

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