方法的ruby关键字参数

Question

How can I declare a method with keyword arguments just like rails do. some examples may be

Person.find(:all, :conditions => "..."). 

How can I use symbols to create methods similar to the above?

I am very new to ruby. Thanks in advance!

Answer 1

Ruby doesn't actually have keyword arguments. Rails is exploiting a feature of Ruby which lets you omit the braces around a hash. For example, with find , what we're really calling is:

Person.find(:all, { :conditions => "...", :offset => 10, :limit => 10 } )

But if the hash is the last argument of the method, you can leave out the braces and it will still be treated as a hash:

Person.find(:all, :conditions => "...", :offset => 10, :limit => 10)

You can use this in your own methods:

def explode(options={})
    defaults = { :message => "Kabloooie!", :timer => 10, :count => 1 }
    options = defaults.merge(options)

    options[:count].times do
        sleep options[:timer]
        puts options[:message]
    end
end

And then call it:

explode :message => "Meh.", :count => 3

Or call it without an argument, resulting in all default values being used:

explode

Answer 2

Since Ruby 2.0, ruby does have keyword arguments .

def my_method(arg1, name: 'defaultName', number: 0)
  puts arg1, name, number
end

Answer 3

I agree with accepted answer given by Samir Talwar and christopherwright. The only potential downside is that you get no warnings if you use an incorrect keyword symbol as an argument or when looking up an option, it just ends up ignored. If that's something you're concerned about, the gem hash_keyword_args addresses it. The idiom would be

def explode(opts={})
    opts = opts.keyword_args(:message => "Kabloooie!", :timer => 10, :count => 1)

    opts.count.times do
        sleep opts.timer
        puts opts.message
    end
end

Notice the use of accessor methods so you'll get a NoMethodError if you mistype a keyword. And the calling behavior is:

explode(:message => "Okay")   # works
explode(:msg => "Oops")       # raises ArgumentError

The gem also provides a few other features you might or might not care about, such as being able to indicate that a keyword is required. I've been using it happily for a while now.

(Disclaimer: I'm the author of the gem.)

Answer 4

You just need to define a method where one of the parameters is a hash. It's actually pretty simple.

def method(arg1, params)
  name = params[:name]
  number = params[:number]

And then call it like:

method(arg1, :name => 'Eric', :number => 2)

Two notes:

1. In Ruby, you don't need to surround the parameters hash in {} when you call the method in most cases, unless you have something complicated going on like passing multiple hashes. In that case, make sure you surround those parameters with {}
2. Ruby is dynamically typed, so you don't need to say that params is a hash when you define the method.

Answer 5

Since Ruby is typed dynamically, just do :

def my_method(arg1, arg2)
  #things
end

example:

my_method(:test, {:somehash => "yay"})

or

my_method :test, :somehash => "yay"

or

my_method(:test, :somehash => "yay")

Answer 6

Ruby 2.0 introduced real keyword arguments, and Ruby 2.1 added required keyword arguments.

There's a nice article up at https://chriszetter.com/blog/2012/11/02/keyword-arguments-in-ruby-2-dot-0/ on this, I've borrowed the examples from there:

Ruby 2.0+:

def exclaim(text, exclamation: '!', number: 7)
  text + exclamation * number
end

exclaim('hello', number: 4) #=> 'hello!!!!'

# equivalent:
exclaim('hello', {:number => 4}) #=> 'hello!!!!'

Ruby 2.1+:

def exclaim(text, exclamation: '!', number:)
  text + exclamation * number
end

exclaim('Yo', number: 5) # => 'Yo!!!!!'
exclaim('Yo') # raises: ArgumentError: missing keyword: number