Let’s look at another simple yet powerful refactoring method, Replace Temp with Query. You can use this when you have a temporary variable holding the result of an expression. I will reach for this when temporary variables add too much to a method’s size/complexity, or when the result of an expression needs to be used in other methods. It's a fantastic next step after the last approach we look at: Extract Variable
Let’s use Replace Temp with Query in the Movie#total_price
method.
Before:
class Movie
TAX_PERCENTAGE = 0.5
def initialize(price:, discount_percentage: 0)
@price = price
@discount_percentage = discount_percentage
end
def total_price
# Temp variables holding expressions
tax = price * TAX_PERCENTAGE
discount = price * (discount_percentage / 100.0)
price + tax - discount
end
private
attr_reader :price, :discount_percentage
end
After:
class Movie
TAX_PERCENTAGE = 0.5
def initialize(price:, discount_percentage: 0)
@price = price
@discount_percentage = discount_percentage
end
def total_price
# Temp variables replaced with query methods
price + tax - discount
end
private
attr_reader :price, :discount_percentage
def tax
price * TAX_PERCENTAGE
end
def discount
price * (discount_percentage / 100.0)
end
The method before refactoring looks like:
def total_price
# Temp variables holding expressions
tax = price * TAX_PERCENTAGE
discount = price * (discount_percentage / 100.0)
price + tax - discount
end
The method after refactoring looks like:
def total_price
# Temp variables replaced with query methods
price + tax - discount
end
With the temp variables extracted to private query methods, there’s even less to grok when understanding the role of total_price
.
Let’s run through the exact steps to take when applying Replace Temp with Query:
Extract the expression into a private method. Try to use a private method as we don’t want to change the public API of an object if we don’t have to.
Replace all references to the temp with the method name. Additionally, use the new query methods in any other places that need them.
Reach for this when you need to share the result of an expression or to reduce the size/complexity of a method.
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