Singleton mode

List of articles

Chapter one OC The singleton model
Chapter two OC Object initialization of
The third chapter OC And agreements
Chapter four OC And main Function operation and some basic concepts

Preface

Tips ： In this article, we talk about singleton mode

1. Concept
2. establish

One 、 Some concepts about singleton

Definition ：

If a class can always create only one instance , Then this class is called a singleton class

Why use singleton mode ？

Sometimes we need a global object , And we should ensure that there is only one copy of the overall situation , At this time, we need to use the singleton design pattern , We need to pay attention to ： Protect threads in a multithreaded environment .

The structure of singleton pattern ：

Singleton class ： A class that contains an instance and can create it by itself
Access class ： Classes that use singletons

Two 、static

Before creating the singleton pattern , So let's see static keyword

static Keywords can only modify local variables, global variables and functions
static Decorating a local variable means that the local variable is stored in the static area
Decorating a global variable means that the global variable can only be accessed in the current file
Decorating functions are used to restrict functions to be used only in the current source file

characteristic

Memory storage area ： The static memory storage area exists during the operation of the project engineering program

advantage ：

• Global variables

• Will not be remotely accessed by other branch files , modify

• And... Can be used in other documents static The same fields decorated by keywords , No conflict

• local variable

• Scope is still local , When the static local variable symbol or local function statement block is defined , The scope then ends 、

• function

• Static functions cannot be called by other files in the project

3、 ... and 、 establish

Common creation

Here is the definition of class

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface FKSingleton : NSObject
+ (id)instance;
@end

NS_ASSUME_NONNULL_END

test ：

#import<Foundation/Foundation.h>
#import"FKSingleton.h"
@implementation FKSingleton
static id instance = nil;
+ (id)instance {
    
    if (!instance) {
    
        instance = [[super alloc]init];
    }
    return instance;
}
@end
int main(){
    
    @autoreleasepool {
    
        if([FKSingleton instance] == [FKSingleton instance]){
    
            NSLog(@"1");
        } else{
    
            NSLog(@"0");
        }
    }
}

Code run results
return 1 It indicates that the instances created twice are equal
We control instance Class method to get FKSingleton When an instance , The program can only produce one instance at most , stay main When this instance is called in the function , You can see that the object generated twice is an object .

Use dispatch_once

The realization of the class

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface FKSingleton : NSObject
+ (id)instance;
@end

NS_ASSUME_NONNULL_END

test ：

#import<Foundation/Foundation.h>
#import"FKSingleton.h"
@implementation FKSingleton
static id instance = nil;
+ (id)instance2{
    
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken,^{
    
        instance = [[self alloc]init];
    });
    return instance;
}
@end
int main(){
    
    @autoreleasepool {
    
        if([FKSingleton instance2] == [FKSingleton instance2]){
    
            NSLog(@"1");
        } else{
    
            NSLog(@"0");
        }
    }
}

result ：