Let add one more pattern into the
design pattern series by consider
Observer pattern. In observer pattern, there is a subject which will be observed by multiple observers. When an event happen with the subject, all observers who registered with the subject will get notify. One obvious example of this pattern is you and my blog. If you are subscribed to my blog, you will get notify when my blogs get updated. This relation between subject and observers provide a clear pattern. It's clear enough to be implemented as a module in ruby standard library, called
Observable.
In this post, I am going to use this observable module to implement a program that allow me follow stocks prices, and also calculate the average over time.
Assume that the stock market allow me to read current stock price with a function call "get_current_price(stock_symbol)". With this method, I can implement a simple class, SimpleStockWatcher, to read the current price and calculate the average as follow:
class SimpleStockWatcher
def initialize(symbol)
@symbol = symbol
@sum = 0
@count = 0
end
def update
price = get_current_price(@symbol)
@sum += price
@count += 1
show(price)
end
def show(price)
puts "Symbol #{@symbol}"
puts " Time: #{Time.now}"
puts " Current Price: #{price}"
puts " Average: #{@sum/@count}"
end
end
If I want to monitor Bank of America (BOA), and Google (GOOG), I implement the following loop:
stocks = [ SimpleStockWatcher("BOA"), SimpleStockWatcher("GOOG") ]
loop do
stocks.each(&:update)
sleep(1)
end
This loop will print the stocks price and average very second.
Now, I want to add more analysis, say 30 or 100 days moving average. No problem, I add more code into the SimpleStockWatcher. Then every second we get the stock prices, average, 30-day moving average, and 100-days moving average.
Let get a bit more complicate, I want to show only current price and 30-days moving average for stock "BOA". For "GOOG", I want to show current price, the average, and 100-days moving average. How do I satisfy this requirement?
Couple weeks ago, a little landed on my inbox. It's about "Biweekly Option" payment for mortgage.
