Observer (Push) - Behavioral Design Patterns - MetaTrader 5 Library | MT5 EA Download - MetaTrader 5 Resources

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//+------------------------------------------------------------------+
//| 201111_091814.mq5 |
//| 2019-2020, Dimitri Pecherica|
//| 792112@gmail.com |
//+------------------------------------------------------------------+
//
// Observer (push) - behavioral design pattern
//
// From: Design Patterns: Elements of Reusable Object-Oriented Software
// Author: erich gamma, richard helm, ralph johnson, john vlissides
// Published in 1994
//
// Intent
//
// Define one-to-many dependencies between objects so that when an object
//Change the state and all its dependencies will be notified and updated automatically
//
// Applicability
//
// When an abstraction has two aspects, one dependent on the other.
//Encapsulating these aspects in separate objects allows you to change and reuse
//They are independent
// When changes to one object require changes to other objects and you don't
//Know how many objects need to be changed
// When an object should be able to notify other objects without
//Assumptions about who these objects are. In other words, you don't want
//These objects are tightly coupled
//
// structure
//
// Observer
// | Topic |--------------------->*|Observer|
// |----------------------| |--------|
// |Attach(Observer) | |Update()|
// | detach(observer) | ^
// |Notify() | |
// |All o in observer| |
// | o.Update() | |
// ^ |
// | | Concrete observer |
// | Topic |--------------------|
// |Specific topic |<----------------|Update() |
// |---------------------| |Observer status = |
// |GetState() | | Topic.GetState()|
// |Return subject.State()| |--------------------|
// |SetState() | |observer_state |
// |------------------------|
// |subject_state |
//
// participants
//
// theme
// Understand its observers. Any number of observer objects can observe
//a topic
// Provides an interface for attaching and detaching observer objects
// Observer
// Define the update interface of the object that should be notified
//Theme changes
// specific topic
//Storage the state that the specific observer object is interested in
//Send notification to observer when state changes
// specific observer
// Maintain a reference to the specific theme object
// Store state that should be consistent with the theme
// Implement the observer update interface to keep its state consistent
//with the theme
//
//  cooperate
//
// Whenever a change occurs, the concrete subject notifies its observers
//This may make its observer's state inconsistent with its own state.
// After learning that the specific topic has changed, the specific
//The observer object can query the subject for information. concrete observer
//Use this information to coordinate its state with the body's state.
//
// aConcrete aConcrete another
//    themeObserver ConcreteObserver // | | | // | Set status () | | // | |< -------------------------- | | | // | |Notification() | | | // | |---------------------+ | | // | | | | | -------------------- | -------------->| | // | | | GetStatus()| | // | | Notifications are // not always called by the subject. It can be called by an observer or // implemented by another object entirely. // #include  201111 _084634.mqh> // push observer #include  201111 _084512.mqh> //Push topic #include  201111 _091110.mqh> //Push specific topics #include  201111 _090802.mqh> //Push specific observers //+------------------------------------------------------------------+ //|Client | //+------------------------------------------------------------------+ Blank On startup ( blank ) { Concrete topic; topic.Attachment( new concrete observer(topic)); topic.Attachment( new concrete observer(topic)); topic.status( "new push status" ); topic.Notify(); } // // Output // // Topic status is set to: new push status // Push status to observer 2097152... // Observer 2097152 status is updated to: new push status // Push status to observer 3145728... // Observer 3145728 status is updated to: new push status // // Result // // The Observer pattern allows you to change topics and observers independently. //You can reuse topics without reusing their observers, and vice versa. //It allows you to add observers without modifying the subject or other observers. // Further advantages and disadvantages of the Observer pattern include // the following: // Abstract coupling between subjects and observers // Support for broadcast communication // Unexpected updates // // Execution // // Mapping subjects to observers // Observing multiple objects // Who triggers updates? // Dangling references to deleted topics // Ensure topic state is self-consistent before notifying // Avoid observer-specific update protocols: push and pull models // Explicitly specify modifications of interest // Encapsulate complex update semantics // Combine principal and observer classes // // Related patterns // // Mediators // Singletons // +--------------------------------------------------------------------------------+