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An application’s document types determine the file
formats that it can manipulate. For example, when you double-click on a
.txt file, OS X launches the TextEdit app (or a
different text editor if you’ve changed your default settings). In
addition, control-clicking on a
.txt file and hovering over the
Open With menu item lists every app capable of opening plaintext
files. This is all possible due to Cocoa’s document types
By default, document-based apps are
associated with a single document type. This is determined by the Document
Extension field when configuring a new project in Xcode. In the previous
chapter, we entered
txt as the Document Extension, so our
example app was automatically configured to manipulate
Understanding how to alter an app’s document types is a key component
of developing document-based applications. It lets you create custom file
formats that only your application can open, and it also makes it possible to
support multiple file formats in a single app. For instance, the
TextEdit app is able to open plaintext, XML, HTML, rich text format
.docx files, and more.
In this chapter, we’ll learn how to implement similar functionality by
adding multiple document types to an application. We’ll also explore how
this impacts the document architecture, including the use of multiple
NSDocument subclasses and custom
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