1
00:00:00,230 --> 00:00:02,110
The second pattern I want to discuss is the

2
00:00:02,110 --> 00:00:05,050
strategy pattern, which provides a way to configure a

3
00:00:05,050 --> 00:00:07,900
class with one of many behaviors. What does that

4
00:00:07,900 --> 00:00:11,040
mean? Well, more precisely, this pattern allows for defining

5
00:00:11,040 --> 00:00:15,330
a family of algorithms, encapsulating them into separate classes,

6
00:00:15,330 --> 00:00:17,900
so each algorithm in one class, and making these

7
00:00:17,900 --> 00:00:21,490
classes interchangeable, but providing a common interface for all

8
00:00:21,490 --> 00:00:25,350
the encapsulated algorithms. So in essence, the intent of

9
00:00:25,350 --> 00:00:29,250
a strategy pattern is to allow for switching between

10
00:00:29,250 --> 00:00:33,490
different algorithms for accomplishing a given task. For example, imagine

11
00:00:33,490 --> 00:00:36,610
having different sorting algorithms with different space or time

12
00:00:36,610 --> 00:00:38,800
tradeoffs. You might want to be able to have them

13
00:00:38,800 --> 00:00:42,670
all available and use different ones in different situations.

14
00:00:42,670 --> 00:00:44,820
And this pattern is applicable not only when we have

15
00:00:44,820 --> 00:00:47,260
different variants of an algorithm, but also when we

16
00:00:47,260 --> 00:00:51,690
have many related classes that differ only in their behavior.

17
00:00:51,690 --> 00:00:53,640
So let's get more concrete and see how this is

18
00:00:53,640 --> 00:00:55,960
done. And I'm going to do it as before, by

19
00:00:55,960 --> 00:00:59,700
discussing the structure and the participants for this strategy pattern.

20
00:00:59,700 --> 00:01:02,540
In this case, we have 3 types of participants for this

21
00:01:02,540 --> 00:01:07,210
pattern, the context, the algorithm, and the concrete strategies. There

22
00:01:07,210 --> 00:01:09,580
can be as many as the number of behaviors that

23
00:01:09,580 --> 00:01:12,300
I need to implement. So, let's see what those are.

24
00:01:12,300 --> 00:01:16,690
The context is the interface to the outside world. It maintains

25
00:01:16,690 --> 00:01:19,180
a reference to the current algorithm and allows for

26
00:01:19,180 --> 00:01:22,860
updating this reference at run time. So, basically the outside

27
00:01:22,860 --> 00:01:26,370
world will invoke the functionality provided by the different algorithms,

28
00:01:26,370 --> 00:01:29,170
by using this interface. And depending on which algorithm is

29
00:01:29,170 --> 00:01:31,640
currently selected, that's the one that will be executed when

30
00:01:31,640 --> 00:01:35,920
the functionality is involved. The algorithm, also called the strategy,

31
00:01:35,920 --> 00:01:37,970
so that's where the pattern gets its name, Is the

32
00:01:37,970 --> 00:01:42,130
common interface for the different algorithims. So all the algorithms

33
00:01:42,130 --> 00:01:46,690
implement this interface. Finally, the concrete strategies are the

34
00:01:46,690 --> 00:01:49,920
actual implementations of the algorithms. So if I have 10

35
00:01:49,920 --> 00:01:53,030
different variants of my algorithm, I will implement 10 different

36
00:01:53,030 --> 00:01:56,550
concrete strategies. They will all be implementations of this interface.