Provide shortcuts to simply the creation of nested default hashes. This pattern is useful, common practice, and unsightly when done manually.
Methods
Public Instance methods
Dynamically create a nested hash structure used to cache calls to method_name The cache method is named +#{method_name}_cache+ unless :as => :alternate_name is given.
The hash structure is created using nested Hash.new. For example:
def slow_method(a, b) a ** b end
can be cached using hash_cache :slow_method, which will define the method slow_method_cache. We can then find the result of a ** b using:
slow_method_cache[a][b]
The hash structure returned by slow_method_cache would look like this:
Hash.new do |as, a|
as[a] = Hash.new do |bs, b|
bs[b] = slow_method(a, b)
end
end
The generated code is actually compressed onto a single line to maintain sensible backtrace signatures.
[ show source ]
# File vendor/rails/activesupport/lib/active_support/caching_tools.rb, line 31
31: def hash_cache(method_name, options = {})
32: selector = options[:as] || "#{method_name}_cache"
33: method = self.instance_method(method_name)
34:
35: args = []
36: code = "def #{selector}(); @#{selector} ||= "
37:
38: (1..method.arity).each do |n|
39: args << "v#{n}"
40: code << "Hash.new {|h#{n}, v#{n}| h#{n}[v#{n}] = "
41: end
42:
43: # Add the method call with arguments, followed by closing braces and end.
44: code << "#{method_name}(#{args * ', '}) #{'}' * method.arity} end"
45:
46: # Extract the line number information from the caller. Exceptions arising
47: # in the generated code should point to the +hash_cache :...+ line.
48: if caller[0] && /^(.*):(\d+)$/ =~ caller[0]
49: file, line_number = $1, $2.to_i
50: else # We can't give good trackback info; fallback to this line:
51: file, line_number = __FILE__, __LINE__
52: end
53:
54: # We use eval rather than building proc's because it allows us to avoid
55: # linking the Hash's to this method's binding. Experience has shown that
56: # doing so can cause obtuse memory leaks.
57: class_eval code, file, line_number
58: end