In this prototype, we change the spreadsheetobserver into a real spreadsheeetobserver. To run source code, simple issue the command: python txtinterface.py. In this prototype, we build our own simple text-interface. Run it as prototype 1, we can get the same result. With slight modification of spreadsheetsubject class, we can have our prototype 2. The spreadsheet class is also an aggregate of Data objects. In prototype 1, the spreadsheet class is added so that a spreadsheetsubject instance can have a single spreadsheet instance. I simply use the python interactive interface to instantiate class objects and change the state of subject. Since there is no interface for editing the subject. data class: implements the corresponding class in UML class diagram.plotobserver and spreadsheetobserver classes: simply print the changed value of data on the screen.subject class: containing only one single Data object.But the frame of observer pattern is there. No Gui observer, no spreadsheet subject, no text interface for changing the state of subject. In this prototype, we build a simplest version. Based on this pattern, I design the first class diagram in UML, which is shown in the following diagram. So, the observer pattern is quit suitable for this purpose. When the state of the spreadsheet is changed, all the views are updated. In our project, we have a spreadsheet which should be observed by the different views. implements the Observer updating interface to keep its state consistent with the subject's.stores state that should stay consistent with the subject's.maintains a reference to a ConcreteSubject object.sends a notification to its observers when its state changes.stores state of interest to ConcreteObserver objects.defines an updating interface for objects that should be notified of changes in a subject.provides an interface for attaching and detaching Observer objects.Any number of Observer objects may observe a subject. Please see page 293-299 of this book for more details.ĭefine a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically. The following simple description of observer pattern is from the book, Design Patterns. When the spreadsheet is updated, all the view instance should also updated. In this text interface, a user can change a cell's value (only numeric).Implement a text interface that allows user to select any parts of the spreadsheet for plot or spreadsheet viewing.Implement a spreadsheet view class that can show any part of the spreadsheet.Implement a plot view class that can plot any part of the spreadsheet.Implement a spreadsheet class that store all cell values.In this phase, the following requirment should be fulfilled:
![observer pattern observer pattern](http://www.doorapps.com/wp-content/uploads/2012/07/ObserverPattern.jpg)
Even for this simplified requirment, I use several phases for development. This spreadsheet should support all the requirements except the distributed and multi-user requirement. So developing a non-distributed, single-user spreadsheet is my present concern. So I don't need to consider any network communication in my early phases of development. In the Project Risk Analysis, I know that if I can develop a spreadsheet that can be access by a single user with multiple views, then I can use Pyro and my method to extend it to a distributed, multi-user spreadsheet. Hopefully this article helped you out.Observer Pattern Observer Pattern Starting Point Analysis Classic! Sometimes “Publication / Subscription” is used interchangeably to describe this pattern, although there are some minor differences between them and Addy points them all out in his book. If you are looking for a detailed explanation of it, I can’t recommend enough “Learning JavaScript Design Patterns” by Addy Osmani. This very simplified version of the observer pattern can save you from downloading some costly frameworks like Vue or React. See the Pen XaVRyY by Pawel Grzybek ( on CodePen.
![observer pattern observer pattern](https://i.stack.imgur.com/1vxRT.png)
With a little help of a few control buttons (added to make the demo more interactive), it gives you the power to do cool things like this with just a few lines of code. subscribe ( updateP3 ) // notify all observers about new data on event textContent = text // instantiate new Observer classĬonst headingsObserver = new Observable () // subscribe to some observers textContent = text const updateP3 = text => p3. textContent = text const updateP2 = text => p2. querySelector ( '.js-p3' ) // some actions to add to the observers arrayĬonst updateP1 = text => p1. querySelector ( '.js-p2' ) const p3 = document. querySelector ( '.js-p1' ) const p2 = document. querySelector ( '.js-input' ) const p1 = document.