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It is possible to request data from a second activity that will be passed back and used in the original activity. We need to specify this when we start the activity by using a different method called startActivityForResult(), and then we can return data to the original Activity in a few different ways.
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Sometimes we start a second activity with
the intention of getting some data to use
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in the original activity.
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We need to specify this when we start
the activity by using a different method,
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start activity for result.
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So back here., let's delete this line
where we are starting the activity,
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and instead call it
startActivityForResult.
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There we go.
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The first parameter like
before is our intent, but
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now we also need to
include a request code.
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Like with extras, it's a good practice
to define some reasonable constants.
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So let's call this REQUEST_FAVORITE and
then define it at the top of our class.
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So I'm going to scroll back up to the top
in here where we have our static strings.
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Let's do public static final and
in this case it's an int or
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request code has to be an int and
we called it REQUEST_FAVORITE and
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we can set this to really anything
we want let's just set it to zero.
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We'll just count up from
different requests values
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are useful when we request different
types of information from an activity.
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All right so now we're expecting a result
to come back here to main activity.
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How will we handle it?
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Using start activity for result requires
that we implement another method
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in the activity to handle the result.
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Let's scroll down all the way to
the bottom of our class and have it.
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I'm gonna add a few lines here just to
keep my code up higher in the screen, and
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now let's type onActivityResult and
hit Enter, and
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studio will drop in
the method signature for us.
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Check out the parameters of this method,
the first is the request code,
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if we have multiple types of request but
always come back to this method.
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So we can check this request code of
each to handle them in different ways.
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We only have one requestCode,
but still it's at a check,
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if requestCode equals and
then we call that REQUEST_FAVORITE.
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The second parameter is a resultCode,
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we are always going to want to
inspect the resultCode as well.
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So let's add, If resultCode equals,
and then start typing, in all caps,
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RESULT_ and the activity class
has some predefined result codes,
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CANCELED, FIRST_USER, and OK.
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You can check the activity API for
more info on these.
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RESULT_CANCELED is returned when the user
hits the back button or we can trigger it
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ourselves, we'll use RESULT_OK, so
let's select that and add a curly braces.
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All right let's follow the flow of
control over in DetailActivity.
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So I'm going to open that up and
now we're starting this activity and
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expecting a result.
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So we need to create one,
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let's do that when the user
taps on the favorite checkbox.
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So in here, create bottom
we're gonna add some lines and
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attach an oncheckedchanged listener.
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So we type,
favoriteCheckbox.setOnCheckedChangeList-
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ener and we wanna create a new one
just like we do for onclick listeners.
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So a new OnCheckedChanged listener,
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in here is the method that gets called and
in here we need another intent
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to hold the data that we want to
transfer back to the calling activity.
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So let's make one Intent, let's call
this result Intent = a new Intent.
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Let's add the value of this checkbox as
a boolean extra, resultIntent.put[Extra].
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And for the name, let's add another
static constant in main activity or
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call it extra favorite.
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We'll define it in a moment but here
we can use MainActivity.EXTRA_FAVORITE.
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Now this method passes in
a boolean is checked that will be
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true if the value is checked.
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So let's use that.
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And now we want to set the result
using the activities setResult method.
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We'll use like we said RESULT_OK.
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But we also want to attach this intent,
resultIntent, as a second parameter for
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this method.
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To finish this activity and
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return to the main one,
let's call the activities finish method.
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Now back in mainActivity let's go up and
add that constant we created.
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Public static final String and
we called it EXTRA_FAVORITE and
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let's set it equal to EXTRA_FAVORITE.
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Now we need to do something with the
result down in our method at the bottom.
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So back here where we have
our if statements, for
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the moment let's just log the result.
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So we'll set a boolean variable.
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Boolean result equals,
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now knows that the intent that's returned
is returned as a parameter called data so
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we can access it with
data.getBooleanExtra and
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we have the name EXTRA_FAVORITE.
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And let's log it with Log.i.
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We have a tag we can use
that will say "Is favorite?
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" and will append the result Boolean.
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And let's check it out.
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Whoops, I notice that
I have an error here.
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So this is happening because
the getBooleanExtra method
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requires a default value just in
case the Boolean isn't present.
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So we usually want to set
false as our default value.
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We also have an error in detail
activities, let's go take a look.
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And there, now it's resolved, it just need
to pick up the change from main activity.
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So now let's run this to see how it works.
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Okay, so first we need to tap
on the download button, and
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we come to the detail page,
we check the favorite box
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And it's gonna go back
to main activity and
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once it does I saw in logcat that the log
entry we added already showed up.
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So if I scroll up a little bit,
I can see here, is favorite?equals true.
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All right round trip
communication complete.
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