Chapter 3: Managing Dependencies

To collaborate, an object must know something about others. Knowing creates a dependency. If not managed carefully, these dependencies will strangle your application

Recognizing Dependencies

An object has a dependency when it knows:

• The name of another class.
  • Gear expects a class named Wheel to exist
• The name of a message that it intends to send to someone other than self.
  • Gear expects a Wheel instance to respond to diameter
• The arguments that a message requires.
  • Gear knows that Wheel.new requires a rim and a tire
• The order of those arguments.
  • Gear knows the first argument to Wheel.new should be rim and the second should be tire

Writing Loosely Coupled Code

Inject Dependencies

see 1_inject_dependencies.rb

• Referring to a class by its name inside of another class is bad.
• If the name of Wheel class changes, the gear_inches method must also change
• The bigger problem is that gear_inches is explicitly saying that it is only willing to calculate gear inches for instances of Wheel
• Gear will only collaborate with any other kind of object even if that object has a diameter and uses gears!

It's is not the class of the object that's important, it's the message you plan to send to it.

• Gear needs access to an object that can respond to diameter - a duck type
• We can use a technique called dependency injection to move the creation of a new Wheel instance outside of the class

Isolate Dependencies

see 2_isolate_dependencies.rb

Isolate Instance Creation

• Sometimes you can't break all unnecessary dependencies, but you can isolate them
• The first technique moves Wheel.new from gear_inches and into Gear's initialize method
• The next alternative isolates the creation of a Wheel into its own wheel method

Isolate Vulnerable External Messages

• gear_inches depends on Gear responding to wheel and wheel responding to diameter
• by creating a different diameter method to hold wheel.diameter, we remove the dependency within gear_inches

Remove Argument-Order Dependencies

see 3_remove_arg_orer_dependencies.rb

Use Hashes for Initialization Arguments

• arguments of our initialize method must be passed in the correct order. we can pass an object instead to remove this dependency

Explicitly Define Defaults

• we can use the fetch method to set defaults when using hashes in our initialize method
• fetch expects the key you're fetching to be in the hash and supplies several options for handling missing keys
• fetch will only set the default if the key is not found in the hash

Managing Dependency Direction

• All examples thus far have shown Gear depending on Wheel or diameter - but the code could have easily been written so that Wheel depends on Gear or ratio

Choosing Dependency Direction

Depend on things that change less often

John’s pick - Pick Krisp -https://krisp.ai

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