By default, a dialog box simply displays the word "Alert." To customize a dialog box, you can define message text and informative text. Message text appears in bold to catch the user's eye, while informative text appears underneath and provides additional explanation about the dialog box's message, as shown in Figure 20-2.

Figure 20-2. The appearance of message and informative text on a dialog box

After you've created a dialog box, you can use messageText and informativeText to define the text to display on your dialog box:

alert.messageText = @"Message Text";
alert.informativeText = @"Informative Text";

If you've ever used a new browser for the first time, you may have seen a dialog box pop up asking if you want to make the browser your default browser. This dialog box lets you click Yes or No, but it also includes a check box called a suppression check box, which you can check to make sure the dialog box doesn't keep popping up again and again, as shown in Figure 20-3.

Figure 20-3. The suppression check box displays "Do not show this message again."

To create a suppression check box, use suppressionBut-ton and set it to YES:

alert.showsSuppressionButton: = YES;

By default, a dialog box appears with only an OK button (see Figure 20-2). If you just need to alert the user about something noncritical, the user can acknowledge getting that message and remove the dialog box by clicking the OK button.

However, many times you may want to display more than one button and display custom text on each button. To do both, you need to use the addButtonWithTitle method and define the text that you want to appear on the button:

[alert addButtonWithTitle:@"OK"];

The first button you add to a dialog box appears on the far right. Each additional button you add appears to the left of the preceding button. Generally you don't want to use more than three or four buttons because then the dialog box starts to get cluttered and more confusing.

When the user clicks any button displayed on the dialog box, the dialog box immediately disappears. If you have two or more buttons displayed on the dialog box, you may want to know which button the user chose.

To determine the button that the user clicked, you can use a switch statement along with the NSAlertFirstButtonReturn, NSAlertSecondButtonReturn, and NSAlertThirdButtonReturn constants:

switch ([alert runModal])
    {
        case NSAlertFirstButtonReturn:
            NSLog (@"First button clicked");
            break;

        case NSAlertSecondButtonReturn:
            NSLog (@"Second clicked");
            break;

        case NSAlertThirdButtonReturn:
            NSLog (@"Third clicked");
            break;

        default:
            break;
    }

The [alert runModal] command displays the dialog box. The first button on the dialog box is the button on the far right, the second button is the button that appears to the left of the first button, and so on, as shown in Figure 20-4.

Figure 20-4. Identifying the first, second, and third buttons on a dialog box

If you have more than three buttons on a dialog box, you can identify each additional button by checking for NSAlertThirdButton+ x, where x is a number that represents an additional button beyond the third one.

To see how to create and display a dialog box, follow these steps:

The Open panel normally displays every file type on your computer. However, you can limit the Open panel to make available only certain types of files, such as PDF or Microsoft Word (.doc and .docx) files.

To define the types of files to allow the user to select, you must first create an array that lists all the file extensions that define all the files you want to allow the user to select, such as in this example:

NSArray *fileTypes = [NSArray arrayWithObjects:@"doc",  @"pdf", nil];

This code would force the Open panel to display only .doc or .pdf files. Any file with a different file extension, such as .tif or .jpg, would appear dimmed so the user can't select it. By limiting the types of files the user can select, you can ensure that the user won't select an inappropriate file that won't work in your program.

After you've defined a list of acceptable file extensions, the next step is to tell the Open panel which files are acceptable by using the allowedFilesTypes property like this:

NSArray *fileTypes = [NSArray arrayWithObjects:@"doc", @"jpg", @"pdf", nil];
myPanel.allowedFileTypes = fileTypes;

In most cases, you may want the user to select only a single file from the Open panel. However, you can let the user select multiple files, by holding down the

[myPanel setAllowsMultipleSelection:YES];

When you allow the user to select multiple files, you have no idea how many files the user might have selected. To retrieve each file name, you must use a loop that keeps retrieving each selected file name until it finds them all:

if ([myPanel runModal] == NSOKButton)
    {
        NSArray *filenamesArray = myPanel.URLs;
        for (NSURL *element in filenamesArray) {
            NSLog(@"File = %@", element);
        }
    }

The for loop keeps repeating until it retrieves all the file names from filenamesArray, which holds the list of the multiple files the user selected.

To see how to limit the file types the user can select and allow the user to select multiple files, follow these steps:

In this example, the user clicked two files, named Fig-3.jpg and Fig-5.jpg. Depending on which files you selected, you'll see those file names displayed instead.

The Save panel and the Open panel look and work similarly because the Save panel inherits features from the Open panel. As a result, the Save panel can also use the allowedFileTypes property to automatically add a specific extension to every file the user types in the Save panel.

To define a file extension, such as .xyz, you could use the following:

NSArray *fileTypes = [NSArray arrayWithObjects:@"xyz", nil];

If the user typed the file name "MyFiles" into the Save panel, the preceding code would tell your program to add the .xyz file extension so that the complete file name would be MyFiles.xyz. After you've defined a file extension, the next step is to tell the Save panel to use the allowedFilesTypes property:

NSArray *fileTypes = [NSArray arrayWithObjects:@"xyz", nil];
myPanel.allowedFileTypes = fileTypes;