Hey all,
I have a question about this line of code:
within “#main-menu” do
find(“h6”, :text => menu).click if menu
click_link link
end
def within(selector, &blk)
new_blk = proc do
begin
scope_selectors.push(selector)
blk.call
ensure
scope_selectors.pop
end
end
super(selector.strip, &new_blk)
end
First, when within is called, it obviously passes the string
“#main-menu”, but does it also pass the block (the content between do
and end) as the second argument? The reason why I ask is because notice
within requires a second argument: &blk.
Yes it does. This is a standard feature/part of ruby. When defining a method, you may always put, as the last entry in the list of parameters, a parameter with the ampersand ‘&’ prefixed to its name. This “captures” (gives you direct access) any block passed to the method by making it available as a Proc instance. Otherwise, any block would still be passed in (if you called #block_given? it would return true) but the only way to “access” it is to call #yield.
So the #within method “captures” any block passed to it and it gets stored as a Proc instance, referenced by blk.
Second, we create a proc to convert a code block into an object and
store that object into new_blk local variable. Then we call the object
in the super argument list, which adds the selector (e.g. “#main-menu”)
into the scope_selectors array.
There is no “call” on the new_blk variable. The entire method definition is pretty much composed of the new Proc’s definition (all except the last line that calls #super). The “&new_blk” entry in #super’s argument list does the inverse of what the ampersand in #within’s method definition does.
In essence, an entry like: “&arg” in a method definition “captures” a block (as a proc) while a similar entry in a method call “sets it free” by making the proc appear just as if you’d passed a normal code block to the method.
Example:
def asdf(&arg)
arg.call
end
Is “effectively” equivalent to
def asdf
yield
end
And then:
asdf { puts “hi” }
is effectively equivalent to:
x = proc { puts “hi” }
asdf(&x)
Then it seems like we call the initial
code block (blk.call)
Yes, inside of the new proc stored in new_blk…
adjacent to within(). But the blk code block has
not been
converted to a proc,
This was done implicitly by the #within’s method definition parameter list’s “&blk” entry as described above.
so how is it possible to have a callable block
then?
Finally, super is only called on a parent class with a method of the
same name correct? Because in this case, within() is the only method in
entire application. super has another purpose?
I don’t know of any other purpose. Does the “super” line result in a NoMethodError being raised? If not, it’s finding some method up the chain to call. Is there some other gem or library that adds a #within method to Object or some other ruby magic like #method_missing somewhere up the chain?
