Showing posts with label MVC. Show all posts
Showing posts with label MVC. Show all posts

Friday, August 4, 2017

Friday Challenge #5: Fixed Gear or 3-speed?

When I first encountered an OO language (Java) there was a tutorial that used bicycles to explain polymorphism. All bikes have some behaviors in common - you pedal them to move forward, you turn one of the two wheels using the handlebars to steer. While there are exceptions - pedal-less bikes for kids, sit down bikes with levers to steer, pedal assist (actually a moped) - the common case serves as great fodder for object oriented programming. With that, let's start the next challenge!

You don't have to use Java to take on this challenge, there are many many object oriented languages and many that have support for classes. Use whatever you would like to use - learn a new lang maybe!


Given the following types of bikes:

3-speed cruiser, id=1, type=MultiSpeedBicycle
10-speed mountain bike, id=2, type=MultiSpeedBicycle
BMX - fixed gear, id=3, type=SingleSpeedBicycle


Write a method that takes in a bike id and returns the bike. The method signature should have the base/parent type as the return type.


Bicycle getBicycle(int id);

Bicycle is an abstract type.

You can either make a static method on Bicycle if your language allows it, or make a BicycleFactory to house the method.


The basic bike (the abstract Bicycle type) has 3 methods - pedal(int pressure), applyBrakes(int pressure), steer(float angle).


A bike with gears EXTENDEDS the basic bike with two additional methods: shiftUp(), shiftDown().

*If you'd like, you can add some additional methods like getCurrentGear(), getMaxGear()...or you could return a bool from shiftUp and shiftDown to say if it changed or not...or return the resultant gear of the action. The more the internals of the object are protected the better, the consumer should not know what gear the bike is in, only that they can shift gears up or down.

Whatever you use to call getBicycle will eventually want to check what type that bicycle is...as far as it knows it only has the basic bicycle methods. Until it does know, shifting isn't an option.

Let's say we have a UI for this bicycle. Actually we would want a different UI for each type since users will interact differently with each type. The shiftable bicycles will have some way to shift gears in the UI - maybe a certain keypress or tapping the screen a certain way...how about pressing shift levers?


Fetch the proper UI for the type inside a method that has this signature...

BikeView getView(Bicycle bike); 


Inside that method, interrogate the type of bicycle and return the view for that type. Put that method into a BicycleViewFactory - do you think the constructor for it takes in all the views or maybe something that knows how to load all the views? Just stub out something for now - create the various view classes and have the method return the right one for the type of bicycle. Maybe you really only need two types of bicycles, maybe you want three - that's entirely up to you and far you want to go with the details.


So that's the challenge...write up those classes and put it altogether with some way to exercise (pun intended) the methods and verify that the right View is returned in the end. The View doesn't actually need to have anything in it, neither do the methods on Bicycle...stubs will work just fine. The lesson here is in how getBicycle and getView are implemented.


Oh! I almost forgot! There's one more thing you can do - you'll want something to load the bike and return the proper views...a BicycleController! First it needs to load up an input view to receive the bike id (maybe list them all - listBicycles() and have the user select one if you'd like). Once an id is sent to getBicycle, that method should call your _bicycleFactory.getBicycle() method to load up the bike. Pass that instance to the ViewFactory to get the view...if you got back a BicycleView return it to the user - else maybe return an ErrorView...the BicycleController should take in both the BicycleFactory and the BicycleViewFactory and hold internal references to those...they should both be set up and ready to use before they are passed into the controller.

Feel free to implement the UI in any way you see fit - heck try going for multiple UIs! Everything should be re-usable at this point...the only things that should change based on the UI you want to implement are the Views!

Let's take that a bit further... Consider different UIs. Maybe the user has certain preferences. Or maybe it's a different view per screen type. Perhaps there's a VR view with actual pedals and steering wheel as input...a stationary bike! Got it!? Those would have different Views for different user interfaces.

Alright, have fun! Happy Coding!

This challenge is all about polymorphism, factory pattern, MVC pattern, encapsulation, Dependency Injection.