Cocoa cola-Cocoa Framework Singleton Mode

Cocoa cola-Cocoa Framework Singleton Mode

Singleton (Singleton mode) is a common software design mode. When this mode is applied, the class of the singleton object must exist with only one instance. In many cases, the whole system only needs to have one global object, which is helpful for us to coordinate the overall behavior of the system. For example, in a server program, the configuration information of the modified server is stored in a file, and the configuration data is read by a single instance object in a unified manner, other objects in the service process then obtain the configuration information through the singleton object. This method simplifies Configuration Management in a complex environment.

The idea of implementing the singleton mode is that a class can return an instance of the object (always the same) and a method to obtain the instance (must be a static method, the getInstance name is usually used). When we call this method, if the Instance held by the class is not empty, this instance is returned. If the instance maintained by the class is empty, create an instance of this class and assign the instance to the instance of this class, so that you can obtain the unique instance of this class only through the static method provided by this class.

The Singleton mode must be used with caution when multiple threads are used. When the unique instance has not been created, if two threads call the creation method at the same time, they do not detect the existence of the unique instance, and thus create an instance at the same time, in this way, the two instances are constructed, which violates the unique principle of instances in the singleton mode. The solution to this problem is to provide a mutex lock (although this will reduce the efficiency) for the variables that have been instantiated for the tag class ).

To create a singleton method in Objective-C, follow these steps.

Step 1 declare a static instance for your Singleton class and initialize it with the value nil. Step 2: In the instance obtaining method (for example, getClassA In the example), a class instance is generated only when the static instance is nil. This instance is usually called a shared instance. Step 3 rewrite the allocWithZone method for determination: you cannot use other methods to create an instance of our class, so that you can only obtain the instance of this class by obtaining the instance method. Therefore, the shared class instance is directly returned in the allocWithZone method. Step 4: implement the basic protocol Methods copyWithZone, release, retain, retainCount, and autorelease to ensure that a single instance has a correct state. The last four methods are used for Memory Management Code and are not suitable for garbage collection code.

The sample code is as follows:

@implementation Singletonstatic Singleton *singleton=nil;+(Singleton*) getInstance{    if (singleton==nil) {        singleton=[[super allocWithZone:NULL]init];    }    return singleton;}@end