ios-4-Creating a singleton pattern

Singleton mode is used in the actual project development of a more than a design pattern, the design principle is that the whole system only produces an object instance, through a unified method to provide external use of this instance.

In Java, constructing a singleton typically declares the constructor of a class as private and then provides an instance object externally through a static method, so, in OC, see the complete code below for an example of how to implement a singleton.

@implementation Car

Declare a static object reference and assign nil

static Car *sharedinstance= Nil;

Declaring a class method (+ is a class method, that is, a static method in Java )

+ (Car *) sharedinstance

{

if (!sharedinstance)

{

Sharedinstance = [[Self alloc] init];

}

return sharedinstance;

}

@end

Override Allocwithzone: Method to prevent any class from creating a second instance. Use synchronized () to prevent multiple threads from executing the code at the same time (the thread lock)

+ (ID) Allocwithzone: (Nszone *) zone

{

@synchronized (self)

{

if (sharedinstance = = nil)

{

Sharedinstance = [Super Allocwithzone:zone];

return sharedinstance;

}

}

return sharedinstance;

}