RSpec - change の基本動作。 ( by / from / to の使い方 ) ref: http://qiita.com/Yinaura/items/62dc0d87f968f43e335b

file0.txt

x = 1
expect { x += 1 }.to change { x }     

# => true

file1.txt

x = 1
expect { x += 2 }.to change { x }

# => true

file2.txt

x = 1
expect { x += 1; x -= 1; }.to change { x }     

# => RSpec::Expectations::ExpectationNotMetError: result should have changed, but is still 1

file3.txt

x = 1
expect { x += 5 }.to change { x }.by(5)

# => true  

file4.txt

x = 1
expect { x -= 5 }.to change { x }.by(-5)     

# => true  

file5.txt

x = 3
expect { x += 4 }.to change { x }.by(7)

# => RSpec::Expectations::ExpectationNotMetError: result should have been changed by 7, but was changed by 4

file6.txt

x = 3
expect { x += 4 }.to change { x }.by(4)

# => true

file7.txt

x = 3
expect { x += 4 }.to change { x }.from(3).to(7)

# => true