- Accessor Methods 4:33
- Accessor Methods 2 objectives
- Properties 4:22
- Properties 1 objective
- Auto-Properties 4:12
- Auto-Properties 2 objectives
- Using Properties 4:20
- Using Properties 1 question
- Computed Properties 4:19
- Computed Properties 1 objective
- Expression Bodied Members 2:52
- Expression Bodied Members 2 objectives
- Property Initial Values 3:57
- Property Initial Values 1 objective
- Property Practice 5:16
- Properties 4 questions
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Using Properties
4:20Properties can be used the same way fields are, but they're really more like methods.
With the setter of the location
property being private,
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the only way it can be set is
from within the invader class.
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Right now, we don't have anything
that sets the location property.
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The default value of an uninitialized
map location is null.
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We don't want users of the invader
class getting null when they ask for
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the invader's location.
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We need to initialize it to some starting
value when the object's created.
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That's the purpose of the constructor.
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So let's add a constructor.
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So we'll type public Invader.
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The constructor will set
the location property to the first
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location on the path.
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So the constructor needs
to have the path passed in.
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We'll get the GetLocation method of the
path object to get the first location on
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the path and assign it to location.
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So now, all invaders will start
on the first step of the path.
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[SOUND] In the tree house defense game,
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invaders move down the path while
towers attempt to neutralize them.
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If the invader successfully reaches the
end of the path, then the game is over and
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the player will have lost.
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It would be the responsibility of each
invader object to keep track of how far
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down the path they are.
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So, we'll need to add another field
called _pathStep to keep track of this.
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No other classes will need
to know this information.
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So we'll make it private.
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This field will change over time.
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So we don't want to make it read-only.
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We'll also give this field
an initial value of 0.
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Because all invaders will
start on step 0 of the path.
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In fact we can use this _pathStep
field in our call to GetLocationAt,
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instead of using 0 twice.
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The reason we can do this is
because the _pathStep field is
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set to 0 before the Invader
constructor is called.
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The fields of a class are always
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initialized before
the constructor is called.
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Of course, the default value
of an integer is always 0,
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so initializing it to 0 here is redundant.
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But it's a good practice
to initialize it to 0.
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By doing this we're saying that we've
thought about what we want this initial
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value to be, and we want it to be 0.
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Now we'll create a method called move
that'll advance the invader down the path.
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This will be called by code outside the
invader class, so it needs to be public.
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We'll strictly use this method
to tell the invader to move, so
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we don't need anything back from it.
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Let's give it a void return type.
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When the move method is called,
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it will advance the invader
one step down the path.
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To do that, we just need to
increase the _pathStep by 1.
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Now that we've changed what
step the invader is on,
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we need to update its location.
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We can do this using
path.GetLocationAt method.
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The only problem is, we don't have
a path object here to refer to.
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It looks like we need to store an instance
of the path object in the invader object
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so that it can be accessed later.
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To do this, we just need to create
another field in the Invader class and
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assign it the path that was
passed into the constructor.
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The path object is just for
the Invader class to use.
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So we'll make it private readonly.
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Then in the constructor we'll set
it to the path that was passed in,
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and I'll change this to
use the field instead.
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Now we can go back to the move method and
use the object's instance of the path
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to update the location property
to the invader's new location.
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So we'll say location =
_path.GetLocationAt(_pathStep).
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There we go.
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Finally, a note about terminology.
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We've now created classes that have
fields, properties, methods, and
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constructors.
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Each of these things are called
members of the class.
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We can refer to the path and
pathStep fields, the location property and
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the Move method collectively as
members of the Invader class.
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Members that are treated like variables
in the class, such as fields and
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properties, are often referred
to as member variables.
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In other courses we'll learn about other
types of members that a class can have.
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