Problem Statement
I have a method which does something great, and is nice and small. It does everything i want, except some small feature that isn't core to the class. For example, I want to do some fudging for the input parameters and/or fudging the output parameters. I want to create another class to handle these orthogonal requirements.
TL;DR - Short summary
Wrap the original object in a proxy which looks like the original object to the outside world. Delegate calls to the original object eventually. This is an example of Aspect Oriented Programming.
Code Example
I'm building this example up as a series of requirements
Requirement 1: I want to be able to get my bank balance from the bank.
Requirement 2: I want to be able to log the fact someone has checked their bank balance
Requirement 3: I want to tell everyone that their bank balance is double of what it is really. But logging should log the correct value.
Requirement 4: I've changed my mind. It's hard for me to have plausible deniability if i log actual bank balances. Fudge that as well
Requirement 5: I want to cache the balance. I still expect logging and doubling.
A note on the example: Requirement 3 (the doubling filter) should probably go into the first class, it's core to the business. I just made it a proxy filter to illustrate the point about output fudging.
Conditions for a good proxy
- The constructor accepts the item it is proxying.
- The proxy passes requests to the original object in at least one branch of the code
- The proxy honors all methods of the original object with the same signature (in java world, implements the same interface)
- Proxies must be composable (this will follow from the point above): Proxy1.new(Proxy2.new(Proxy3.new(object)))
- That does not mean the order of proxies is immaterial
Practical Uses
- A proxy that caches the response from the inner method
- A proxy that runs the inner method in a single database transaction
- A proxy that adds logging before and after the method is called
- A proxy that validates inputs
DynamicProxy
In the examples above, we overrode a single method bank_balance. Now, imaging you want a proxy to overwrite 50 different methods of an object. Enter DynamicProxy for java, and DynamicProxy/SimpleProxy for .NET. These let you override all methods in a particular interface.
Of course, ruby doesn't need that thanks to method_missing!
Rack filters
Rack provides an easy way to add proxy pattern to the app called via filters (rails cleans this up as around filters). A filter which does nothing will be as follows (notice what the constructor accepts, and what methods the filter implements)
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Đang ngủ say thì Bạch Vũ chợt trở thân. Lưu Phong tiếp tục được nàng điều chỉnh góc độ cho mãn nhãn.
Bộ ngực lúc này bị che một chút nhưng hạ thể lại lộ ra nhiều hơn, cặp mông trắng nõn đã chui ra khỏi chăn, kín kín hở hở, thật là làm cho người ta máu huyết sôi trào. Đáng tiếc là vẫn bị áo ngủ che mất một chút.
“Nhanh lên, cao lên chút nữa…”
Ngay khi Lưu Phong đang cầu khẩn thượng đế thì Bạch Vũ tiếp tục cử động. Lớp áo ngủ che cặp mông khẽ bị vén lên cao một chút. Chỉ hận khí lực chưa đủ mà thôi.
“Thêm chút nữa…”
Ngay khi Lưu Phong đang tiếp tục cầu khẩn thì một thanh âm quen thuộc vang lên: “Ba ba, người làm gì vậy?”
tiểu Linh nhi? Lưu Phong đột nhiên như lò xo đứng thẳng dậy, nhìn quanh xem có Liễu Thanh Nghi không, cười ha ha nói: “Là tiểu Linh nhi. Để ba ba kể chuyện chú bé Hồ Lô cho ngươi nghe nhé.”
May mắn chỉ là tiểu Linh nhi, Lưu Phong tùy ý có thể lái sang chuyện khác. tiểu Linh nhi cũng không truy hỏi cặn kẽ nữa.
Kể chuyện chú bé Hồ Lô cho tiểu Linh nhi nghe xong, hắn giao lại cho Liễu Thanh Nghi. Còn mình thì quay về phòng, dặn dò Liễu Thanh Nghi không cho ai đến quấy rầy cả.
“Công tử muốn tu luyện sao?” Ở chung với nhau lâu như vậy, Liễu Thanh Nghi cũng biết nhiều chuyện về hắn. Mỗi lần hắn tu luyện thì nàng đều ở ngoài canh cửa. Không cho ai đến quấy nhiễu.
Đáng tiếc hôm nay nàng đã đoán sai. Lưu Phong hôm nay không tu luyện mà là cưỡng dâm tiểu khố của Bạch Vũ.
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