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Now that we've declared our variables, it's time to start using them! In this video we'll see how we can assign these variables to the corresponded Views from our layout.
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Awesome, now it's time to assign
values to our new view variables.
0:00
Let's start by adding four new
lines to the bottom of onCreate.
0:04
Then on the second new
line let's add a comment//
0:11
Assign the Views from
the lab file to the layout
0:16
file to the corresponding variables.
0:21
It's important that we don't
assign the views until
0:27
after the call to setContentView.
0:29
If we try to access a view from our
layout before calling setContentView
0:32
it won't exist and we'll get an error.
0:37
On the next line let's start by typing
factTextView and then an equals sign.
0:40
And now we need a method that will take
in the ID that we set in the layout and
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return our TextView.
0:50
Fortunately, every activity has a method
for this and it's called findViewById.
0:52
Let's start by typing findViewById
after the equals sign.
0:57
And notice that as your typing,
1:03
the autocomplete feature
is filling it in below.
1:05
Android Studio analyzes
what we're typing and
1:08
offers suggestions that match
what we've typed so far.
1:10
Automatic code completion is awesome.
1:14
It's super convenient and it makes it so
we only need to remember the first few
1:17
letters of something and
Android Studio will take care of the rest.
1:21
Now we can either finish
typing this method or
1:26
we can just hit Enter to select
it from code completion.
1:28
And now we see a hint about
the parameter we need to use here.
1:33
The findViewById method requires
an ID as it's parameter.
1:36
But rather than just type
in the ID from the layout,
1:41
we must refer to it through
a generated resource class.
1:44
When we build our project,
Android automatically builds a class for
1:48
us called simply R,
which stands for resources.
1:52
This class contains all of the IDs
of our files in the res directory,
1:58
as well as a ton of other
default resource ID's.
2:02
If we open the project pane and
switch to the project perspective, and
2:06
then drill into app, build, generated,
2:12
source, r, debug, and
2:18
then our package name,
we can even open the r class.
2:22
You'll notice at the top
there's a warning saying files
2:28
under the build folder are generated and
should not be edited.
2:31
We're not in here to edit anything.
2:36
It's just nice to know where
the R class comes from.
2:37
Let's close this and
switch back to the Android perspective.
2:41
And I'll close the project pane as well.
2:48
Back in FunFactsActivity.kt,
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let's type a capital R followed by a dot.
2:55
Then we need to pick which type
of resource we're looking for.
2:59
Since we're looking for an ID,
let's type ID followed by another dot.
3:03
Then let's start typing the ID
from our layout, fact, text view.
3:10
And there it is, so
hit Enter to select it.
3:15
Now, depending on which version
of Android you're using,
3:19
you might see an error like I do or
3:22
if you're targeting a newer version of
Android, everything will be just fine.
3:25
So what happened?
3:29
Let's take a look at some code to see what
changed in the new version of Android.
3:30
Here we've got a column
project where we mocked out
3:35
some classes from the Android API.
3:38
We start with a view class and
then we create a button, and
3:41
text view classes that
extend from our view class.
3:45
Then we create an activity class and give
it a findViewById method that takes in
3:49
an integer and returns a view, just like
you'd find in the real activity class.
3:53
Inside this method we create a local view
variable and depending on whether the ID
4:00
is a one or a two,
we set it equal to either a new Button or
4:05
a new TextView,
before returning our view variable.
4:10
Finally, we got a MainActivity
class which extends from Activity.
4:14
And inside that class's constructor
we tried to set up two variables,
4:19
a button, and a text view.
4:23
This is the same error I'm
getting back in Android Studio.
4:26
Type mismatch, it's expecting a button,
but we're setting it equal to a view.
4:30
To fix this, since we know that
findViewById with a parameter of 1 will
4:35
indeed return a button,
4:41
we can just cast the returned value
to a button by adding as Button.
4:43
And we can do the same thing for
our TextView by hitting Alt Enter.
4:50
However, in Android Oreo,
they made a change to the way we use
4:57
findViewById that means we
no longer need the cast.
5:01
So let's undo the casts, and
5:06
then see what's been
changed in findViewById.
5:11
Up in thefFindViewById method,
instead of returning a view we
5:14
want to use something called generics
to let us return multiple types.
5:18
Don't worry too much if this
part doesn't make sense.
5:23
What's important is how it effects our
view variables, not how it's implemented.
5:26
Let's start by declaring
our generic type and
5:32
what types it's allowed to have inside
angle brackets before the function name.
5:34
Here we're declaring
a generic type named t, and
5:43
we're saying it can only be classes
that have view as a parent.
5:47
Then, instead of returning a view,
let's return our new generic type, T.
5:53
Finally, since we've
changed the return type,
6:00
we need to add a cast to our view
variable before we return it, as T.
6:03
Perfect, now we no longer need to add
a cast to our findViewById calls.
6:09
But since this change is fairly recent,
6:17
you're likely to encounter a lot of code
that still uses casts with findViewById.
6:20
So if you see any code
that's still using casts and
6:25
it's not giving you any issues,
feel free to ignore it.
6:28
You're just on a newer version
of Android than we are.
6:31
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