self super
当使用 self 调用方法时，会从当前类的方法列表中开始找，如果没有，就从父类中再找；而当使用 super 时，则从父类的方法列表中开始找。然后调用父类的这个方法。
Messages to self and super
Objective-C provides two terms that can be used within a method definition to refer to the object that performs the method—self and super.
Suppose, for example, that you define a reposition method that needs to change the coordinates of whatever object it acts on. It can invoke the setOrigin:: method to make the change. All it needs to do is send a setOrigin:: message to the same object that the reposition message itself was sent to. When you’re writing the reposition code, you can refer to that object as either self or super. The reposition method could read either:
- reposition |
{ |
... |
[self setOrigin:someX :someY]; |
... |
} |
or:
- reposition |
{ |
... |
[super setOrigin:someX :someY]; |
... |
} |
Here, self and super both refer to the object receiving a reposition message, whatever object that may happen to be. The two terms are quite different, however. self is one of the hidden parameters that the messaging routine passes to every method; it’s a local variable that can be used freely within a method implementation, just as the names of instance variables can be. super is a term that substitutes for self only as the receiver in a message expression. As receivers, the two terms differ principally in how they affect the messaging process:
-
selfsearches for the method implementation in the usual manner, starting in the dispatch table of the receiving object’s class. In the example above, it would begin with the class of the object receiving the reposition message. -
superis a flag that tells the compiler to search for the method implementation in a very different place. It begins in the superclass of the class that defines the method wheresuperappears. In the example above, it would begin with the superclass of the class where reposition is defined.
Wherever super receives a message, the compiler substitutes another messaging routine for the objc_msgSend function. The substitute routine looks directly to the superclass of the defining class—that is, to the superclass of the class sending the message to super—rather than to the class of the object receiving the message.
An Example: Using self and super
The difference between self and super becomes clear when using a hierarchy of three classes. Suppose, for example, that we create an object belonging to a class called Low. The superclass of Low is Mid; the superclass of Mid is High. All three classes define a method called negotiate, which each class uses for its own purpose. In addition, Mid defines an ambitious method called makeLastingPeace, which itself employs the negotiate method. The classes and those methods are illustrated in Figure 2-2.
Suppose that the implementation of makeLastingPeace (in the Mid class) uses self to indicate the object to send the negotiate message to:
- makeLastingPeace |
{ |
[self negotiate]; |
... |
} |
When a message is sent to a Low object to perform the makeLastingPeace method, makeLastingPeace sends a negotiate message to the same Low object. The messaging routine finds the version of negotiate defined in Low, the class of self.
However, if the implementation of makeLastingPeace instead uses super as the receiver,
- makeLastingPeace |
{ |
[super negotiate]; |
... |
} |
the messaging routine finds the version of negotiate defined in High. It ignores the class (Low) of the object that received the makeLastingPeace message and skips to the superclass of Mid, because Mid is where makeLastingPeace is defined. Neither implementation finds the Mid version of negotiate.
As this example illustrates, super provides a way to bypass a method that overrides another method. Here, the use of super enabled makeLastingPeace to bypass the Mid version of negotiate that redefined the High version of that method.
Not being able to reach the Mid version of negotiate, as just described, may seem like a flaw, but under the circumstances it’s intentional:
-
The author of the
Lowclass intentionally overrode theMidversion ofnegotiateso that instances ofLow(and its subclasses) would invoke the redefined version of the method instead. The designer ofLowdidn’t wantLowobjects to perform the inherited method. -
The author of the
MidmethodmakeLastingPeace, in sending thenegotiatemessage tosuper(as shown in the second implementation), intentionally skipped over theMidversion ofnegotiate(and over any versions that might be defined in classes likeLowthat inherit fromMid) to perform the version defined in theHighclass. The designer of the second implementation ofmakeLastingPeacewanted to use theHighversion ofnegotiateand no other.
The Mid version of negotiate could still be used, but it would take a direct message to a Mid instance to do so.
Using super
Messages to super allow method implementations to be distributed over more than one class. You can override an existing method to modify or add to it and still incorporate the original method in the modification:
- negotiate |
{ |
... |
return [super negotiate]; |
} |
For some tasks, each class in the inheritance hierarchy can implement a method that does part of the job and passes the message on to super for the rest. The init method, which initializes a newly allocated instance, is designed to work like this. Each init method has responsibility for initializing the instance variables defined in its class. But before doing so, it sends an init message to super to have the classes it inherits from initialize their instance variables. Each version of init follows this procedure, so classes initialize their instance variables in the order of inheritance:
- (id)init |
{ |
self = [super init]; |
if (self) { |
... |
} |
} |
It’s also possible to concentrate core functionality in one method defined in a superclass and have subclasses incorporate the method through messages to super. For example, every class method that creates an instance must allocate storage for the new object and initialize its isa variable to the class structure. Allocation is typically left to the alloc and allocWithZone: methods defined in the NSObject class. If another class overrides these methods (a rare case), it can still get the basic functionality by sending a message to super.
Redefining self
super is simply a flag to the compiler telling it where to begin searching for the method to perform; it’s used only as the receiver of a message. But self is a variable name that can be used in any number of ways, even assigned a new value.
There’s a tendency to do just that in definitions of class methods. Class methods are often concerned not with the class object, but with instances of the class. For example, many class methods combine allocation and initialization of an instance, often setting up instance variable values at the same time. In such a method, it might be tempting to send messages to the newly allocated instance and to call the instance self, just as in an instance method. But that would be an error. self and super both refer to the receiving object—the object that gets a message telling it to perform the method. Inside an instance method, self refers to the instance; but inside a class method, self refers to the class object. This is an example of what not to do:
+ (Rectangle *)rectangleOfColor:(NSColor *) color |
{ |
self = [[Rectangle alloc] init]; // BAD |
[self setColor:color]; |
return self; |
} |
To avoid confusion, it’s usually better to use a variable other than self to refer to an instance inside a class method:
+ (id)rectangleOfColor:(NSColor *)color |
{ |
id newInstance = [[Rectangle alloc] init]; // GOOD |
[newInstance setColor:color]; |
return newInstance; |
} |
In fact, rather than sending the alloc message to the class in a class method, it’s often better to send alloc to self. This way, if the class is subclassed, and the rectangleOfColor: message is received by a subclass, the instance returned is the same type as the subclass (for example, the array method of NSArray is inherited by NSMutableArray).
+ (id)rectangleOfColor:(NSColor *)color |
{ |
id newInstance = [[self alloc] init]; // EXCELLENT |
[newInstance setColor:color]; |
return newInstance; |
} |
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