This section describes the naming conventions for declared properties, instance variables, constants,
notifications, and
exceptions.
Declared Properties and Instance Variables
A declared property effectively declares accessor methods for a property, and so conventions for naming a declared property are broadly the same as those for naming accessor methods (see
“Accessor Methods”). If the property is expressed as a noun
or a verb, the format is:
@property (…)
type nounOrVerb;
For example:
@property (strong) NSString *title;
|
@property (assign) BOOL showsAlpha;
|
If the name of a declared property is expressed as an adjective, however, the property name omits the “is” prefix but specifies the conventional name for the get accessor, for example:
@property (assign, getter=isEditable) BOOL editable;
|
In many cases, when you use a declared property you also synthesize a corresponding instance variable.
Make sure the name of the instance variable concisely describes the attribute stored. Usually, you should not access instance variables directly; instead you should use
accessor methods (you do access instance variables directly in
init
and dealloc
methods). To help to signal this, prefix instance variable names with an underscore (_
), for example:
@implementation MyClass {
|
BOOL _showsTitle;
|
}
|
If you synthesize the instance variable using a declared property, specify the name of the instance variable in the
@synthesize
statement.
@implementation MyClass
|
@synthesize showsTitle=_showsTitle;
|
There are a few considerations to keep in mind when adding instance variables to a class:
-
Avoid explicitly declaring public instance variables.
Developers should concern themselves with an object’s interface, not with the details of how it stores its data. You can avoid declaring instance variables explicitly by using declared properties and synthesizing the corresponding instance variable.
-
If you need to declare an instance variable, explicitly declare it with either
@private
or @protected
.
If you expect that your class will be subclassed, and that these subclasses will require direct access to the data, use the
@protected
directive.
-
If an instance variable is to be an accessible attribute of instances of the class, make sure you write
accessor methods for it (when possible, use declared properties).
Constants
The rules for constants vary according to how the constant is created.
Enumerated constants
-
Use enumerations for groups of related constants that have integer values.
-
Enumerated constants and the typedef under which they are grouped follow the naming conventions for functions (see
“Naming Functions”). The following example comes from
NSMatrix.h
:
typedef enum _NSMatrixMode {
|
NSRadioModeMatrix = 0,
|
NSHighlightModeMatrix = 1,
|
NSListModeMatrix = 2,
|
NSTrackModeMatrix = 3
|
} NSMatrixMode;
|
Note that the typedef
tag (_NSMatrixMode
in the above example) is unnecessary.
-
You can create unnamed enumerations for things like bit masks, for example:
enum {
|
NSBorderlessWindowMask = 0,
|
NSTitledWindowMask = 1 << 0,
|
NSClosableWindowMask = 1 << 1,
|
NSMiniaturizableWindowMask = 1 << 2,
|
NSResizableWindowMask = 1 << 3
|
|
};
|
Constants created with const
-
Use const
to create constants for floating point values. You can use
const
to create an integer constant if the constant is unrelated to other constants; otherwise, use enumeration.
-
The format for const
constants is exemplified by the following declaration:
As with enumerated constants, the naming conventions are the same as for functions (see
“Naming Functions”).
Other types of constants
-
In general, don’t use the #define
preprocessor command to create constants. For integer constants, use enumerations, and for floating point constants use the
const
qualifier, as described above.
-
Use uppercase letters for symbols that the preprocessor evaluates in determining whether a block of code will be processed. For example:
-
Note that macros defined by the compiler have leading and trailing double underscore characters. For example:
-
Define constants for strings used for such purposes as notification names and dictionary keys. By using string constants, you are ensuring that the compiler verifies the proper value is specified (that is, it performs spell checking). The
Cocoa
frameworks provide many examples of string constants, such as:
APPKIT_EXTERN NSString *NSPrintCopies;
|
The actual NSString value is assigned to the constant in an implementation file. (Note that the
APPKIT_EXTERN
macro evaluates to extern
for
Objective-C.)
Notifications and Exceptions
The names for notifications and
exceptions follow similar rules. But both have their own recommended usage patterns.
Notifications
If a class has a delegate, most of its notifications will probably be received by the delegate through a defined delegate method. The names of these notifications should reflect the corresponding delegate
method. For example, a delegate of the global NSApplication
object is automatically registered to receive an
applicationDidBecomeActive:
message whenever the application posts an
NSApplicationDidBecomeActiveNotification
.
Notifications are identified by global NSString
objects whose names are composed in this way:
[Name of associated class] + [Did | Will] + [UniquePartOfName] + Notification
|
For example:
NSApplicationDidBecomeActiveNotification
|
NSWindowDidMiniaturizeNotification
|
NSTextViewDidChangeSelectionNotification
|
NSColorPanelColorDidChangeNotification
|
Exceptions
Although you are free to use exceptions (that is, the mechanisms offered by the
NSException
class and related functions) for any purpose you choose, Cocoa reserves exceptions for programming errors such an array index being out of bounds. Cocoa does
not use exceptions to handle regular, expected error conditions. For these cases, use returned values such as
nil
, NULL
, NO
, or error codes. For more details, see
Error Handling Programming Guide.
Exceptions are identified by global NSString
objects whose names are composed in this way:
[Prefix] + [UniquePartOfName] + Exception
|
The unique part of the name should run constituent words together and capitalize the first letter of each word. Here are some examples:
NSColorListIOException
|
NSColorListNotEditableException
|
NSDraggingException
|
NSFontUnavailableException
|
NSIllegalSelectorException
|
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