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00:00:00,860 --> 00:00:03,970
With the setter of the location
property being private,

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00:00:03,970 --> 00:00:07,810
the only way it can be set is
from within the invader class.

3
00:00:07,810 --> 00:00:11,480
Right now, we don't have anything
that sets the location property.

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00:00:11,480 --> 00:00:15,490
The default value of an uninitialized
map location is null.

5
00:00:15,490 --> 00:00:18,290
We don't want users of the invader
class getting null when they ask for

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00:00:18,290 --> 00:00:19,940
the invader's location.

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00:00:19,940 --> 00:00:23,780
We need to initialize it to some starting
value when the object's created.

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00:00:23,780 --> 00:00:25,850
That's the purpose of the constructor.

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00:00:25,850 --> 00:00:27,280
So let's add a constructor.

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00:00:28,380 --> 00:00:30,310
So we'll type public Invader.

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00:00:31,420 --> 00:00:33,970
The constructor will set
the location property to the first

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00:00:33,970 --> 00:00:35,150
location on the path.

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00:00:35,150 --> 00:00:38,090
So the constructor needs
to have the path passed in.

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00:00:38,090 --> 00:00:42,131
We'll get the GetLocation method of the
path object to get the first location on

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00:00:42,131 --> 00:00:43,957
the path and assign it to location.

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00:00:49,544 --> 00:00:53,145
So now, all invaders will start
on the first step of the path.

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00:00:53,145 --> 00:00:55,577
[SOUND] In the tree house defense game,

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00:00:55,577 --> 00:01:00,071
invaders move down the path while
towers attempt to neutralize them.

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00:01:00,071 --> 00:01:04,805
If the invader successfully reaches the
end of the path, then the game is over and

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00:01:04,805 --> 00:01:06,417
the player will have lost.

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00:01:06,417 --> 00:01:10,113
It would be the responsibility of each
invader object to keep track of how far

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00:01:10,113 --> 00:01:11,960
down the path they are.

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00:01:11,960 --> 00:01:16,140
So, we'll need to add another field
called _pathStep to keep track of this.

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00:01:16,140 --> 00:01:18,570
No other classes will need
to know this information.

25
00:01:18,570 --> 00:01:19,898
So we'll make it private.

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00:01:22,329 --> 00:01:24,530
This field will change over time.

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00:01:24,530 --> 00:01:27,100
So we don't want to make it read-only.

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00:01:27,100 --> 00:01:30,150
We'll also give this field
an initial value of 0.

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00:01:30,150 --> 00:01:33,530
Because all invaders will
start on step 0 of the path.

30
00:01:34,590 --> 00:01:38,280
In fact we can use this _pathStep
field in our call to GetLocationAt,

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00:01:38,280 --> 00:01:40,400
instead of using 0 twice.

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00:01:40,400 --> 00:01:43,560
The reason we can do this is
because the _pathStep field is

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00:01:43,560 --> 00:01:46,610
set to 0 before the Invader
constructor is called.

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00:01:46,610 --> 00:01:48,040
The fields of a class are always

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00:01:48,040 --> 00:01:50,890
initialized before
the constructor is called.

36
00:01:50,890 --> 00:01:54,340
Of course, the default value
of an integer is always 0,

37
00:01:54,340 --> 00:01:58,360
so initializing it to 0 here is redundant.

38
00:01:58,360 --> 00:02:01,200
But it's a good practice
to initialize it to 0.

39
00:02:01,200 --> 00:02:04,690
By doing this we're saying that we've
thought about what we want this initial

40
00:02:04,690 --> 00:02:06,805
value to be, and we want it to be 0.

41
00:02:08,360 --> 00:02:13,310
Now we'll create a method called move
that'll advance the invader down the path.

42
00:02:13,310 --> 00:02:18,070
This will be called by code outside the
invader class, so it needs to be public.

43
00:02:18,070 --> 00:02:21,230
We'll strictly use this method
to tell the invader to move, so

44
00:02:21,230 --> 00:02:23,530
we don't need anything back from it.

45
00:02:23,530 --> 00:02:25,000
Let's give it a void return type.

46
00:02:27,590 --> 00:02:29,090
When the move method is called,

47
00:02:29,090 --> 00:02:33,040
it will advance the invader
one step down the path.

48
00:02:33,040 --> 00:02:36,020
To do that, we just need to
increase the _pathStep by 1.

49
00:02:37,260 --> 00:02:40,030
Now that we've changed what
step the invader is on,

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00:02:40,030 --> 00:02:42,210
we need to update its location.

51
00:02:42,210 --> 00:02:46,120
We can do this using
path.GetLocationAt method.

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00:02:46,120 --> 00:02:50,380
The only problem is, we don't have
a path object here to refer to.

53
00:02:50,380 --> 00:02:54,490
It looks like we need to store an instance
of the path object in the invader object

54
00:02:54,490 --> 00:02:56,000
so that it can be accessed later.

55
00:02:57,020 --> 00:03:00,200
To do this, we just need to create
another field in the Invader class and

56
00:03:00,200 --> 00:03:02,570
assign it the path that was
passed into the constructor.

57
00:03:03,610 --> 00:03:06,830
The path object is just for
the Invader class to use.

58
00:03:06,830 --> 00:03:08,709
So we'll make it private readonly.

59
00:03:12,518 --> 00:03:17,310
Then in the constructor we'll set
it to the path that was passed in,

60
00:03:17,310 --> 00:03:20,690
and I'll change this to
use the field instead.

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00:03:23,560 --> 00:03:27,410
Now we can go back to the move method and
use the object's instance of the path

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00:03:27,410 --> 00:03:30,531
to update the location property
to the invader's new location.

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00:03:31,650 --> 00:03:41,146
So we'll say location =
_path.GetLocationAt(_pathStep).

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00:03:43,730 --> 00:03:45,030
There we go.

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00:03:45,030 --> 00:03:47,500
Finally, a note about terminology.

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00:03:47,500 --> 00:03:52,300
We've now created classes that have
fields, properties, methods, and

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00:03:52,300 --> 00:03:53,830
constructors.

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00:03:53,830 --> 00:03:56,630
Each of these things are called
members of the class.

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00:03:57,660 --> 00:04:02,470
We can refer to the path and
pathStep fields, the location property and

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00:04:02,470 --> 00:04:06,650
the Move method collectively as
members of the Invader class.

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00:04:07,710 --> 00:04:11,730
Members that are treated like variables
in the class, such as fields and

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00:04:11,730 --> 00:04:15,770
properties, are often referred
to as member variables.

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00:04:15,770 --> 00:04:19,450
In other courses we'll learn about other
types of members that a class can have.