Operators and Schedulers

As data is emitted from an observable, we can optionally add steps to manipulate 0:00 the data before it reaches the observer's onNext method. 0:05 Manipulating data is done using operators. 0:09 Operators act on data from an observable before it reaches the observer. 0:12 RxJava contains many built in operators such as filter [SOUND], map [SOUND], or 0:17 skip [SOUND]. 0:22 Operators can also do more advanced things, 0:23 like combine two pieces of data into a single item to be emitted. 0:26 A helpful way to understand these operators is to visit RxMarbles. 0:31 This site provides interactive diagrams for common RxJava operators. 0:35 I've added the link in the teacher's notes. 0:40 Let's take a look at a couple to understand these diagrams. 0:43 Okay, open your browser and go to rxmarbles.com. 0:47 You can see here, there's a diagram in the middle and 0:52 there's a bunch of operators listed on the left hand side here. 0:54 Let's just go through a couple to see how it works. 0:58 We choose, first. 1:02 Here we can see the diagram, I can change the data being emitted by the observable. 1:04 Just like our diagram before, the observable is on the top, 1:09 the observer is on the bottom. 1:13 So here we can see that as the number 1 gets emitted, 1:15 it gets observed by the observer. 1:19 But also notice that as 2, 3, and 4 get emitted, 1:23 only the first item is still passed to the observer. 1:26 Let's go ahead and change one of these items being emitted. 1:30 Let's pass in 2 as the first item. 1:32 So if 2 is the first item emitted by the observable, 1:35 we can see that 2 is then the only item received by the observer. 1:38 So that's what first does, only emits the first item. 1:44 Okay, let's take a look at another one here. 1:49 How about map? 1:50 Here we can see this map is a function that takes in a value x and 1:54 multiplies it by 10. 1:58 So that means whatever value is emitted by the observable, 2:00 will be multiplied by 10 and that's the value the observer will see. 2:03 Here we can see that 1 is emitted and 10 is received, 2 is emitted, 20 is received. 2:08 We can change the order of these, it doesn't matter. 2:15 Map doesn't change the order, it simply adds a function 2:18 as an operator to the value and passes that result to the observer. 2:23 Let's go in and take another one. 2:28 How about merge. 2:30 So what does merge do? 2:31 Well, we can see that merge has two observables here, so 2:34 there's one observable on the top. 2:38 It's emitting 20, 40, 60, 80, 100. 2:40 And then we have another observable emitting 1 and 1. 2:43 Then we have an operator merge. 2:48 And then we have what the observer sees. 2:50 So if you take a look at this, 2:54 the observer sees the items from observable one up here on the top. 2:54 It also sees the items from the second observable. 2:59 And that's what merge does. 3:02 It takes the items from multiple observables, and 3:04 then emits them all to the observer. 3:06 So we can see this observer received the items from the first observable and 3:09 the second one as well. 3:12 We can change the order here and you can see that the order remains the same. 3:14 Whichever order they are emitted from either observable, 3:21 the observer sees them in that order as well. 3:24 Okay, and let's look at another one here. 3:28 How about combineLatest? 3:30 What does that do? 3:32 combineLatest here is a function, it takes in two values and 3:34 it concatenates them together. 3:38 So we can see we have again multiple observables, 3:41 the top observable emits numbers 1, 2, 3, 4, 5. 3:44 A second observable emits letters A, B, C, D. 3:48 So if a 1 and then an A comes along, and 3:53 then combineLatest takes the 1 and the A and it concatenates them and 3:56 it makes 1A, and that's what the observer sees. 4:01 So notice the observer doesn't get an item until there isn't two items emitted, 4:06 one from each observable. 4:12 Because the operator has to combine items, one from each observable. 4:13 So it needs to have an at least one item from each. 4:17 Notice that the next item emitted is 2A. 4:20 Why is that? 4:23 That's because 1's been emitted, then we have an A, then we have a 2. 4:24 And so it takes the latest two items. 4:30 That's gonna be the 2 and the A before it, cuz the B hasn't come yet. 4:33 Once the B comes then it will be 2B. 4:36 And if we change the order of these, you can see it makes sense, 4:39 it also changes the order and it changes what the observer sees. 4:43 Once the 1 and the B is emitted, we get 1B. 4:48 Then the 2 and the B are together combined to receive 2B in the observer. 4:52 Then once the A comes, the 2 and the A are combined together to receive 2A, and so 4:59 on and so forth, all of these. 5:03 And you can move these around and see how it changes what the observer sees by 5:04 combining the latest two values from two different observables. 5:08 Up until this point we haven't discussed threading. 5:13 As you know, threading is an important topic on Android to maintain a fluid and 5:16 fast application. 5:21 And there's a course listed in the teacher's notes if you wanna learn more. 5:22 However, handling multithreading has also been a major pain point for developers. 5:26 RxJava helps with this by providing a clean API to handle multithreading 5:32 called a scheduler. 5:36 A scheduler isn't a thread, but it describes a threading model underneath it. 5:38 This means RxJava is flexible to whatever type of threading model you are using. 5:43 By default, observables will emit items and 5:49 they will be passed to the subscriber on the thread the subscription is created on. 5:52 However, if you wanna change which thread items are received on, 5:56 you can use the method, observe on. 6:00 This ensures the calls to the observers on next., on completed and 6:02 on error are called on whatever scheduler you provide. 6:06 On Android, we need to do UI work on the main UI thread. 6:11 That's why the Rx Android companion library contains 6:15 the AndroidScheduler.mainThread class. 6:18 Using this scheduler ensures your observer receives OnNext on the main UI thread. 6:22 RxJava includes other scheduler's by default, 6:26 such as, schedulers.IO which is useful for performing long live calls. 6:31 So how would you move your long work off the UI thread? 6:37 The subscriber on operator specifies a scheduler on which the observable 6:40 should operate. 6:44 It's important to note that subscribe on can be called in 6:45 any order of your chain of operators. 6:48 The last call to subscribe on will determine which scheduler is used for 6:51 that observable. 6:55 Now that we've gone through the basics of the RxJava API, let's dive into an app, 6:57 and convert it to use what we just learned. 7:02