Cover by David Flanagan, Yukihiro Matsumoto

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Custom Control Structures

Ruby’s use of blocks, coupled with its parentheses-optional syntax, make it very easy to define iterator methods that look like and behave like control structures. The loop method of Kernel is a simple example. In this section we develop three more examples. The examples here use Ruby’s threading API; you may need to read Threads and Concurrency to understand all the details.

Delaying and Repeating Execution: after and every

Example 8-1 defines global methods named after and every. Each takes a numeric argument that represents a number of seconds and should have a block associated with it. after creates a new thread and returns the Thread object immediately. The newly created thread sleeps for the specified number of seconds and then calls (with no arguments) the block you provided. every is similar, but it calls the block repeatedly, sleeping the specified number of seconds between calls. The second argument to every is a value to pass to the first invocation of the block. The return value of each invocation becomes the value passed for the next invocation. The block associated with every can use break to prevent any future invocations.

Here is some example code that uses after and every:

require 'afterevery' 1.upto(5) {|i| after i { puts i} } # Slowly print the numbers 1 to 5 sleep(5) # Wait five seconds every 1, 6 do |count| # Now slowly print 6 to 10 puts count break if count == 10 count + 1 # The next value of count end sleep(6) # Give the above time to ...

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