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Project: RSpec Expectations 2.3

Publisher: RSpec

expect change

Expect some code (wrapped in a proc) to change the state of some object.

Scenarios
expecting change
Given
a file named "expect_change_spec.rb" with: 
class Counter
  class << self
    def increment
      @count ||= 0
      @count += 1
    end
    
    def count
      @count ||= 0
    end
  end
end

describe Counter, "#increment" do
  it "should increment the count" do
    expect{Counter.increment}.to change{Counter.count}.from(0).to(1)
  end

  # deliberate failure
  it "should increment the count by 2" do
    expect{Counter.increment}.to change{Counter.count}.by(2)
  end
end
When
I run "rspec ./expect_change_spec.rb"
Then
the output should contain "2 examples, 1 failure"
Then
the output should contain "should have been changed by 2, but was changed by 1"
expecting no change
Given
a file named "expect_no_change_spec.rb" with: 
class Counter
  class << self
    def increment
      @count ||= 0
      @count += 1
    end
    
    def count
      @count ||= 0
    end
  end
end

describe Counter, "#increment" do
  it "should not increment the count by 2" do
    expect{Counter.increment}.to_not change{Counter.count}.from(0).to(2)
  end

  # deliberate failure
  it "should not increment the count by 1" do
    expect{Counter.increment}.to_not change{Counter.count}.by(1)
  end
end
When
I run "rspec ./expect_no_change_spec.rb"
Then
the output should contain "2 examples, 1 failure"
Then
the output should contain "should not have changed, but did change from 1 to 2"

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