Yield a double to a Block in RSpec

TIL in rspec you can yield a double to a block with and_yield, similar to how you return a double with and_return.

With and_return you can write a test like this:

sftp = Net::SFTP.start(args)
sftp.upload!(content, path)

# Test
client = double
allow(Net::SFTP).to receive(:start).and_return(client)
expect(client).to receive(:upload!)

However, if your code has a block like below and_return won't work. Instead, you can use and_yield to yield the double to the block:

Net::SFTP.start(args) do |sftp|
  sftp.upload!(content, path)
end

# Test
client = double
allow(Net::SFTP).to receive(:start).and_yield(client)
expect(client).to receive(:upload!)

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