Proxy Pattern#

The Proxy Pattern provides a surrogate or placeholder for another object to control access to it. It is a structural design pattern.

Description#

In some cases, it is undesirable or not possible to reference an object directly. In such instances, an intermediary known as a proxy can be used to achieve indirect referencing. The proxy object can act as a mediator between the client and the target object, and can filter out content and services that the client cannot see or add additional services that the client requires. Introducing a new object to manipulate the operations of the real object or to act as a stand-in for the real object constitutes the implementation mechanism of the proxy pattern.

Pattern Structure#

• Subject: Abstract Subject role.
• Proxy: Proxy Subject role.
• RealSubject: Real Subject role.

Advantages#

• The proxy pattern can coordinate the caller and the called party, thereby reducing the coupling of the system to a certain extent.
• Remote proxies enable clients to access objects on a remote machine, which may have better computational performance and processing speed, allowing for quick response and handling of client requests.
• Virtual proxies can reduce system resource consumption by using a small object to represent a large object, optimizing the system and improving runtime speed.
• Protecting proxies can control access permissions to the real object.

Disadvantages#

• Introducing a proxy object between the client and the real subject may cause a slowdown in request processing for certain types of proxy patterns.
• Implementing the proxy pattern requires additional work, and some implementations of proxy patterns can be quite complex.

Application Scenarios#

• Remote Proxy: Provides a local proxy object for an object located in a different address space, which could be within the same host or on another host. A remote proxy is also known as an ambassador.
• Virtual Proxy: If there is a need to create an object with high resource consumption, a relatively small object is created to represent it, and the real object is only created when needed.
• Copy-on-Write Proxy: This is a type of virtual proxy that defers the copying operation until it is actually needed by the client. Generally, deep cloning of objects is a costly operation, and the copy-on-write proxy can delay this operation until the object is actually used.
• Protect or Access Proxy: Controls access to an object and can provide different levels of access permissions to different users.
• Cache Proxy: Provides temporary storage space for the result of a specific operation, allowing multiple clients to share these results.
• Firewall Proxy: Protects the target from malicious users.
• Synchronization Proxy: Allows several users to use an object simultaneously without conflicts.
• Smart Reference Proxy: Provides additional operations when an object is referenced, such as recording the number of times this object is called.

Implementation#

class Subject{
    constructor(){
        this.name = null;
        this.__id = null;
    }
    say(){
        throw new Error("Abstract method cannot be called");
    }
}

class RealSubject extends Subject{
    constructor(){
        super();
        this.name = "real subject";
        this.__id = 1;
    }
    say(){
        console.log(this.name);
    }
}

class SubjectProxy{
    constructor(){
        this.instance = new RealSubject;
    }

    say(){
        this.instance.say();
    }

    getProperty(key){
        if(/^_{1,2}.*$/.test(key)) throw new Error("Properties beginning with _ or __ are not allowed to be accessed");
        return this.instance[key];
    }
}

(function() {
    var subject = new SubjectProxy();
    subject.say(); // real subject
    console.log(subject.getProperty("name")); // real subject
    // console.log(subject.getProperty("__id")); // Uncaught Error: Properties beginning with _ or __ are not allowed to be accessed
})();

Daily Question#

References#