--- title: "Become a Firebase Taskmaster! (Part 3: Wiring up your Tasks)" description: "Become a Firebase Taskmaster! (Part 3: Wiring up your Tasks)" published: "2016-09-29" last_updated: "2016-09-29" authors: - "Doug Stevenson (Developer Advocate)" categories: - "Android" - "Best Practices" - "Java" canonical_url: "https://firebase.blog/posts/2016/09/become-a-firebase-taskmaster-part-3_29" --- # Become a Firebase Taskmaster! (Part 3: Wiring up your Tasks) *Published September 29, 2016 by Doug Stevenson (Developer Advocate)* --- Alrighty! Thanks for joining us for part three of this blog series about the Play services Task API for Android. By now, you've seen the essentials of the API in [part one](https://firebase.googleblog.com/2016/09/become-a-firebase-taskmaster-part-1.html), and how to select the best style of listener in [part two](https://firebase.googleblog.com/2016/09/become-a-firebase-taskmaster-part-2.html). So, at this point, you probably have everything you need to know to make effective use of the Tasks generated by Firebase APIs. But, if you want to press into some advanced usage of Tasks, keep reading! ### Put Yourself to Task We know that some of the Firebase features for Android will do work for you and notify a Task upon completion. But, what if you want to create your own Tasks to perform threaded work? The Task API gives you the tools for this. If you want to work with the Task API without having to integrate Firebase into your app, you can get the library with a dependency in your build.gradle: ```java compile 'com.google.android.gms:play-services-tasks:9.6.1' ``` But, if you _are_ integrating Firebase, you'll get this library included for free, so no need to call it out specifically in that case. There is just one method (with two variants) you can use to kick off a new Task. You can use the static method named "call" on the Tasks utility class for this. The variants are as follows: ```java Task call(Callable callable) Task call(Executor executor, Callable callable) ``` Just like addOnSuccessListener(), you have a version of [call()](https://firebase.google.com/docs/reference/android/com/google/android/gms/tasks/Tasks.html#call(java.util.concurrent.Callable%3CTResult%3E)) that executes the work on the main thread and another that submits the work to an Executor. You specify the work to perform inside the passed [Callable](https://developer.android.com/reference/java/util/concurrent/Callable.html). A Java Callable is similar to a Runnable, except it's parameterized by some result type, and that type becomes the returned object type of its call() method. This result type then becomes the type of the Task returned by call(). Here's a really simple Callable that just returns a String: ```java public class CarlyCallable implements Callable { @Override public String call() throws Exception { return "Call me maybe"; } } ``` Notice that CarlyCallable is parameterized by String, which means its call() method must return a String. Now, you can create a Task out of it with a single line: ```java Task task = Tasks.call(new CarlyCallable()); ``` After this line executes, you can be certain that the call() method on the CarlyCallable will be invoked on the main thread, and you can add a listener to the Task to find the result (even though that result is totally predictable). More interesting Callables might actually load some data from a database or a network endpoint, and you'd want to have those blocking Callables run on an Executor using the second form of [call()](https://firebase.google.com/docs/reference/android/com/google/android/gms/tasks/Tasks.html#call(java.util.concurrent.Executor,%20java.util.concurrent.Callable%3CTResult%3E)) that accepts the Executor as the first argument. ### Working on the Chain (Gang) Let's say, for the sake of example, you want to process the String result of the CarlyCallable Task after it's been generated. Imagine that we're not so much interested in the text of the resulting String itself, and more interested in a List of individual words in the String. But, we don't necessarily want to modify CarlyCallable because it's doing exactly what it's supposed to, and it could be used in other places as it’s written now. Instead, we'd rather encapsulate the logic that splits words into its own class, and use that after the CarlyCallable returns its String. We can do this with a [Continuation](https://firebase.google.com/docs/reference/android/com/google/android/gms/tasks/Continuation). An implementation of the Continuation interface takes the output of one Task, does some processing on it, and returns a result object, not necessarily of the same type. Here's a Continuation that splits a string of words into an List of Strings with each word: ```java public class SeparateWays implements Continuation > { @Override public List < String > then(Task < String > task) throws Exception { return Arrays.asList(task.getResult().split(" +")); } } ``` Notice that the `Continuation` interface being implemented here is parameterized by two types, an input type (`String`) and an output type (`List`). The input and output types are used in the signature of the lone method `then()` to define what it's supposed to do. Of particular note is the parameter passed to `then()`. It's a `Task`, and the `String` there must match the input type of the `Continuation` interface. This is how the `Continuation` gets its input - it pulls the finished result out of the completed `Task`. Here's another `Continuation` that randomizes a List of Strings: ```java public class AllShookUp implements Continuation , List > { @Override public List < String > then(@NonNull Task > task) throws Exception { // Randomize a copy of the List, not the input List itself, since it could be immutable final ArrayList < String > shookUp = new ArrayList<>(task.getResult()); Collections.shuffle(shookUp); return shookUp; } } ``` And another one that joins a `List` of `Strings` into a single space-separated String: ```java private static class ComeTogether implements Continuation , String> { @Override public String then(@NonNull Task > task) throws Exception { StringBuilder sb = new StringBuilder(); for (String word: task.getResult()) { if (sb.length() > 0) { sb.append(' '); } sb.append(word); } return sb.toString(); } } ``` Maybe you can see where I'm going with this! Let's tie them all together into a chain of operations that randomizes the word order of a `String` from a starting `Task`, and generates a new `String` with that result: ```java Task playlist = Tasks.call(new CarlyCallable()) .continueWith(new SeparateWays()) .continueWith(new AllShookUp()) .continueWith(new ComeTogether()); playlist.addOnSuccessListener(new OnSuccessListener < String > () { @Override public void onSuccess(String message) { // The final String with all the words randomized is here } }); ``` The [`continueWith(`)](https://firebase.google.com/docs/reference/android/com/google/android/gms/tasks/Task#continueWith(com.google.android.gms.tasks.Continuation%3CTResult,%20TContinuationResult%3E)) method on Task returns a new Task that represents the computation of the prior Task after it’s been processed by the given `Continuation`. So, what we’re doing here is chaining calls to `continueWith()` to form a pipeline of operations that culminates in a final Task that waits for each stage to complete before completing. This chain of operations could be problematic if these they have to deal with large Strings, so let's modify it to do all the processing on other threads so we don't block up the main thread: ```java Executor executor = ... // you decide! Task playlist = Tasks.call(executor, new CarlyCallable()) .continueWith(executor, new SeparateWays()) .continueWith(executor, new AllShookUp()) .continueWith(executor, new ComeTogether()); playlist.addOnSuccessListener(executor, new OnSuccessListener() { @Override public void onSuccess(String message) { // Do something with the output of this playlist! } }); ``` Now, the `Callable`, all of the Continuations, and the `final Task` listener will _each_ run on some thread determined by the `Executor`, freeing up the main thread to deal with UI stuff while this happens. It should be totally jank-free. At first blush, it could seem a bit foolish to separate all these operations into all the different classes. You could just as easily write this as a few lines in a single method that do only what's required. So, keep in mind that this is a simplified example intended to highlight how Tasks can work for you. The benefit of chaining of Tasks and Continuations (even for relatively simple functions) becomes more evident when you consider the following: * How might you plug in new sources of input Strings? So, what if we also had a BlondieCallable? And a PaulSimonCallable? * What about different kinds of processing for the input Strings, such as a YouSpinMeRound continuation that rotated the order of the Strings in a List one position to the right (like a record)? * What if you wanted different components of the processing pipeline to be executed on different threads? Practically speaking, you're more likely to use `Task` continuations to perform a series of modular chain of filter, map, and reduce functions on a set of data, and keep those units of work off the main thread, if the collections can be large. But, I had fun with music theme here! ### What if the Playlist Breaks? One last thing to know about Continuations. If a runtime exception is thrown during processing at any stage along the way, that exception will normally propagate all the way down to the failure listeners on the final `Task` in the chain. You can check for this yourself in any Continuation by asking the input `Task` if it completed successfully with the `isSuccessful()` method. Or, you can just blindly call getResult() (as is the case in the above samples), and if there was previously a failure, it will get re-thrown and automatically end up in the next Continuation. The listeners on the final `Task` in the chain should always check for failure, though, if failure is an option. So, for example, if the `CarlyCallable` in the above chain returned null, that would cause the `SeparateWays` continuation to throw a `NullPointerException`, which would propagate to the end of the `Task`. And if we had an `OnFailureListener` registered, that would get invoked with the same exception instance. ### Pop Quiz! What's the _most efficient_ way, with the above chain, of finding out the number of words in the original string, without modifying any of the processing components? Take a moment to think about it before reading on! The answer is probably more simple than you'd imagine. The most obvious solution is to count the number of words in the final output string, since their order only got randomized. But there is one more trick. Each call to `continueWith()` returns a new `Task` instance, but those are all invisible here because we used a chaining syntax to assemble them into the final `Task`. So you can intercept any of those those tasks and add another listener to it, in addition to the next `Continuation`: ```java Task > split_task = Tasks.call(new CarlyCallable()) .continueWith(executor, new SeparateWays()); split_task = .continueWith(executor, new AllShookUp()) .continueWith(executor, new ComeTogether()); split_task.addOnCompleteListener(executor, new OnCompleteListener > () { @Override public void onComplete(@NonNull Task > task) { // Find the number of words just by checking the size of the List int size = task.getResult().size(); } }); playlist.addOnCompleteListener( /* as before... */ ); ``` When a `Task` finishes, it will trigger _both_ of the Continuations on it, as well as _all_ of the added listeners. All we've done here is intercept the `Task` that captures the output of the `SeparateWays` continuation, and listen to the output of that directly, without affecting the chain of continuations. With this intercepted task, we only have to call size() on the List to get the count. ### Wrapping Up (part 3 of this series) All joking aside, the `Task` API makes it relatively easy for you to express and execute a sequential pipeline of processing in a modular fashion, while giving you the ability to specify which Executor is used at each stage in the process. You can do this with or without/Firebase integrated into your app, using your own Tasks or those that come from Firebase APIs. For the next and final part to this series, we'll look at how Tasks can be used in parallel to kick off multiple units of work simultaneously. As usual, if you have any questions, consider using Twitter with the #AskFirebase hashtag or the [firebase-talk Google Group](https://groups.google.com/forum/#!forum/firebase-talk). We also have a dedicated Firebase Slack channel. And you can follow me [@CodingDoug](https://twitter.com/CodingDoug) on Twitter to get notified of the next post in this series. Lastly, if you're wondering about all the [songs](https://www.youtube.com/watch?v=dQw4w9WgXcQ) I referenced in this post, you can find them here: * [Call Me Maybe](https://www.youtube.com/watch?v=fWNaR-rxAic) * [Working on the Chain Gang](https://www.youtube.com/watch?v=f5lYNgCVwFo) * [Separate Ways](https://www.youtube.com/watch?v=LatorN4P9aA) * [All Shook Up](https://www.youtube.com/watch?v=3rQEbQJx5Bo) * [Come Together](https://www.youtube.com/watch?v=uSM5MpKSnqE) * [Call Me](https://www.youtube.com/watch?v=StKVS0eI85I) * [You Can Call Me Al](https://www.youtube.com/watch?v=ULjCSK0oOlI) * [You Spin Me Round](https://www.youtube.com/watch?v=PGNiXGX2nLU) Continue reading with [part 4](https://firebase.blog/posts/2016/10/become-a-firebase-taskmaster-part-4).