Document Objects Are Model Controllers

In the Model-View-Controller design pattern, document objects—that is, instances of subclasses of UIDocument—are model controllers. A document object manages the data associated with a document, specifically the model objects that internally represent what the user is viewing and editing. A document object, in turn, is typically managed by a view controller that presents a document to users.

Relevant Chapter: Designing a Document-Based Application

When Designing an Application, Consider Document-Data Format and Other Issues

Before you write a line of code you should consider aspects of design specific to document-based applications. Most importantly, what is the best format of document data for your application, and how can you make that format work for your application in iOS and Mac OS X? What is the most appropriate document type?

You also need to plan for the view controllers (and views) managing such tasks as opening documents, indicating errors, and moving selected documents to and from iCloud storage.

Relevant Chapters: Designing a Document-Based Application, Document-Based Application Preflight

Creating a Subclass of UIDocument Requires Two Method Overrides

The primary role of a document object is to be the “conduit” of data between a document file and the model objects that internally represent document data. It gives the UIDocument class the data to write to the document file and, after the document file is read, it initializes its model objects with the data that UIDocument gives it. To fulfill this role, your subclass of UIDocument must override the contentsForType:error: method and the loadFromContents:ofType:error: method, respectively.

Relevant Chapter: Creating a Custom Document Object

An Application Manages a Document Through Its Life Cycle

An application is responsible for managing the following events during a document’s lifetime:

• Creation of the document

• Opening and closing the document

• Monitoring changes in document state and responding to errors or version conflicts

• Moving documents to iCloud storage (and removing them from iCloud storage)

• Deletion of the document

Relevant Chapter: Managing the Life Cycle of a Document

An Application Stores Document Files in iCloud Upon User Request

Applications give their users the option for putting all document files in iCloud storage or all document files in the local sandbox. To move document files to iCloud, they compose a file URL locating the document in an iCloud container directory of the application and then call a specific method of the NSFileManager class, passing in the file URL. Moving document files from iCloud storage to the application sandbox follows a similar procedure.

Relevant Chapter: Managing the Life Cycle of a Document

An Application Ensures That Document Data is Saved Automatically

UIDocument follows the saveless model and automatically saves a document’s data at specific intervals. A user usually never has to save a document explicitly. However, your application must play its part in order for the saveless model to work, either by implementing undo and redo or by tracking changes to the document.

Relevant Chapter: Change Tracking and Undo Operations

An Application Resolves Conflicts Between Different Document Versions

When documents are stored in iCloud, conflicts between versions of a document can occur. When a conflict occurs, UIKit informs the application about it. The application must attempt to resolve the conflict itself or invite the user to pick the version he or she prefers.

Relevant Chapter: Resolving Document Version Conflicts