Intent Extras and Results

Let's continue with an example of simple data. 0:00 We'll pass a song title to the detail activity and 0:02 then we'll pass back whether or not that song is a favorite for the user. 0:05 When we start an activity, 0:10 this intent object is actually passed to the new activity. 0:11 Because of this fact, we can add things to the intent and 0:14 use it as a little delivery person to carry data somewhere else. 0:17 Pretty neat, right? 0:20 Extra data like this is called, well, extras. 0:22 I love it when names make sense. 0:25 Let's add a new line here after we declare our intent and type intent.putExtra. 0:26 Now take a look at this. 0:33 This is a long list of different data types we can add as extras. 0:34 We can add lots of extras if we want, we just need to give each a unique name. 0:39 That's the first parameter you see in each of these methods' signatures. 0:43 We want to pass in some string data, so let's give it a name called SONG_TITLE, 0:47 in all caps, and then we can pass in the actual title. 0:52 I'll use my favorite Android song Gradle, Gradle, Gradle. 0:55 Now let's go over to the detail activity and see how to unpack this extra data. 1:01 We'll start by getting the intent used to start this activity. 1:06 So here at the bottom of our uncreate method, let's add a few lines and type 1:10 Intent intent = and then we can call a method from the activity class getIntent. 1:14 This gets us the specific intent that was used to start this activity. 1:21 Next, let's unpack the title into a string named songTitle. 1:27 The intent class includes get methods for each data type. 1:32 So, in our case we can call intent.getStringExtra and 1:35 pass in that same name, SONG_TITLE. 1:40 Now your code smell sense should be tingling right now. 1:44 We are repeating ourselves with this magic key value. 1:47 Let's convert it to something more useful. 1:50 Back in MainActivity, let's add at the top a new static constant. 1:53 So here I'm going to type public static final sString, and 1:58 let's call this EXTRA_TITLE. 2:03 Let's set it equal to the same value EXTRA_TITLE. 2:06 We can set it to whatever value we want, but I like to make mine match the keys. 2:09 Okay, so let's go down to the code in our testIntents method and 2:15 we can get rid of SONG_TITLE and just use EXTRA_TITLE. 2:18 And back over in DetailActivity we can make the same change. 2:24 Get rid of SONG_TITLE, and we need to preface it with MainActivity.EXTRA_TITLE. 2:26 There, that's better. 2:33 So with that we can use this value to update our title text view. 2:34 So it's called titleLabel.setText and then I'm gonna pass in songTitle. 2:37 A quick question. 2:45 What if we forgot to attach this extra to our intent? 2:47 We'd end up with a null value here. 2:50 We can't guarantee that this activity is lodged with an intent 2:52 that includes this specific extra piece of data. 2:56 That's the power of intents. 2:59 We can use them in any app, 3:00 we don't always need to start an activity the exact same way. 3:01 To protect against this, we need to add a simple if statement to check for 3:06 null from the intent before trying to do anything with it. 3:09 So we say, if intent.getStringExtra, 3:12 same thing, we want to use MainActivity.EXTRA_TITLE. 3:17 If it does not equal null, then we want to get it and set the value. 3:22 So let's cut this line, paste it in here, and 3:27 do the same thing for setting the text view. 3:32 All right, so we're all set. 3:36 Let's verify by running the app. 3:37 Okay so if we click on the download button, then we should see the title. 3:43 There it is, Gradle, Gradle, Gradle. 3:48 All right, looking good. 3:49 By the way, this same pattern applies to sending data to services too. 3:51 In fact, we already have that in this code from the services course. 3:55 Check out the downloadSongs method back in main activity. 3:59 This creates an explicit intent, but 4:04 it passes the class name of a service, not an activity. 4:06 We putExtra data in it, just like with activities. 4:10 And then we call startService, instead of startActivity. 4:13 Remember this as we go forward, because everything we see for 4:17 activities applies equally to services. 4:19 We'll also revisit this later in the course. 4:21 Let's take a short break and 4:24 then we'll look at passing data in the reverse direction. 4:25