Building Cocoa Applications: A Step by Step Guide

Loading from a File

Loading means that there is already a file on the computer’s disk that we want to read and use in an application. Writing the code to load a file is usually considerably easier than writing the code to save it, because the hard work of figuring out which instance variables to save and getting them on the disk has (presumably) already been done.

A user can load a file into the MathPaper application by doing any of the following:

• Choosing MathPaper’s File → Open menu command and specifying a file in the Open panel

• Choosing a file from MathPaper’s File → Open Recent submenu

• Double-clicking a MathPaper document file icon in the Finder

• Dragging a MathPaper document on top of the MathPaper.app application icon and dropping it

The NSDocument framework handles all of these file-opening techniques for us automatically! All we need to do is to implement a single method called loadDataRepresentation:ofType: .

1. Insert the following loadDataRepresentation:ofType method into the implementation of the MathController class in the file MathController.m:

                        - (BOOL)loadDataRepresentation:(NSData *)newHistory
                                                ofType:(NSString *)aType
                        {
                            if ([aType isEqualToString:@"MathPaper"]) {
                                MathDocument *temp;
                                temp = [NSUnarchiver unarchiveObjectWithData:newHistory];
                                if (temp) {
                                    [self setFrame:[temp frame] ];
                                    [self setHistory:[temp history] ];
                                    return YES;
                                }
                            }
                            return NO;
                        }

When any of the actions described at the beginning of this section take place, the NSDocument architecture will open the ...