From: Tim Hunter <cyclists@...>
Date: 2004-12-04T09:52:40+09:00
Subject: Re: Using yield

Joe Van Dyk wrote:

> I come from a heavy C++ background, discovered Ruby a few months ago and
> love it.
> 
> I've found that using blocks is a very natural thing.  However, I have not
> once used 'yield'.  I'm sure that there are events when using yield would
> be helpful, but I have no clue when it would be appropriate to use.
> 
> Thoughts?  When do you use the 'yield' statement in code?
> 
> Joe

I use it with methods that create resources that need to be cleaned up
after, the way File.open closes the file for you after the block
terminates. If you're coming from C++, think destructors.

Here's an example from RMagick. The Image#view method extracts a rectangle
of pixels from an image and yields to a block (if present). Within the
block you can address the pixels using [i][j] indexes. When the block ends,
if any of the pixels have been modified all the pixels are stored
("sync'd") back to the image. The code looks like this:

    def view(x, y, width, height)
        view = View.new(self, x, y, width, height)
        if block_given?
            begin
                yield(view)
            ensure
                view.sync
            end
            return nil
        else
            return view
        end
    end

You use #view like this:

image.view(5, 10, 20, 20) do |view|
        view[5][7] = 'yellow'
        # other useful stuff...
end