Handlers and Loopers and Messages! O my!

We've just finished adding a playlist to our app, and 0:00 we're even making sure to download one song at a time. 0:03 What else could we want? 0:06 Well, unfortunately we really shouldn't be using the playlist 0:08 inside our download thread. 0:13 If this were a larger app that had more than one developer, it wouldn't be too 0:16 hard to imagine someone else modifying our playlist outside of our download thread. 0:20 And if our playlist changes for any reason, 0:25 all of a sudden we'd be downloading the wrong songs. 0:28 What we need is a queue for the songs we want to download. 0:32 Once one song finishes, the next song in the queue would start downloading. 0:36 We can implement this by adding something called a message queue to our thread. 0:41 And in order to use a message queue, 0:46 we'll also need to be aware of handlers, loopers, and messages. 0:49 It's a lot to cover, but the following examples should help elucidate 0:53 the role each of these objects plays in the bigger picture. 0:57 Imagine a fully stocked burrito truck. 1:01 This burrito truck represents our thread. 1:03 Just like a thread is a place where we can execute our code, 1:06 the burrito truck is a place where we can execute making burritos. 1:10 In this example a runnable would be a recipe. 1:14 Something like, take one tortilla add a quarter pound of shredded chicken and 1:18 a half cup of shredded cheese, mix and fold into the shape of a burrito. 1:23 So before, when we were just using a runnable and a thread, 1:28 that would be like if we just showed up at the burrito truck with our own recipe and 1:32 then we just went inside and made the burrito ourselves. 1:37 But now we'd like our thread, burrito truck, 1:41 to have a queue of what it should be working on. 1:44 The first change we're going to make, 1:47 is we're going to hire someone to work in the burrito truck. 1:49 We'll call him Tim, and he represents the handler object. 1:52 In Android each handler is associated with only one thread. 1:57 And that handler is responsible for sending and 2:01 processing messages and runnables for the thread. 2:05 So Tim, our handler, only works at this burrito truck, our thread. 2:09 And he's responsible for taking and preparing the orders, 2:15 messages and runnables. 2:19 We're already familiar with runnables, but what's a message? 2:22 A message is just a way for us to send some arbitrary data to our handler. 2:26 It's then up to the handler how it will handle the message. 2:31 If we wanted to order a burrito, we could give Tim a recipe, or 2:36 we could order off the menu. 2:40 One breakfast burrito please. 2:41 When we order off the menu, that's an example of using a message. 2:44 We don't need to know how to make a breakfast burrito 2:48 because Tim knows how to handle it. 2:51 Once Tim takes our order, he's going to put it at the end of the list of orders. 2:54 This is the message queue. 2:59 It contains all the runnables and messages that our thread still needs to handle. 3:00 Now Tim's daughter Looper is too young to help with most of the burrito truck work. 3:06 But she's still eager to help out. 3:10 So when Looper notices her dad is about to finish an order, 3:13 she grabs the next order from the message queue and gives it to her dad. 3:17 Way to help out Looper. 3:21 All right, let's walk through an example to see how all these pieces work together. 3:24 A man walks up to the burrito truck with the recipe for a burrito he'd like. 3:29 Tim takes the order and adds it to the message queue. 3:34 Then Tim gets back to working on a previous order. 3:37 When Tim finishes the previous order, Looper grabs the next 3:40 order from the message queue and Tim starts working on this new order. 3:44 Then another guy walks up and places an order for a breakfast burrito. 3:49 Tim takes the order and adds it to the message queue. 3:53 Then Tim gets back to working on a previous order. 3:56 When Tim finishes the previous order Looper grabs the next order from 3:59 the message queue and Tim starts working on this new order. 4:04 Looks like Tim's an awfully busy guy. 4:08 Let's take a short break and then we'll see how to implement this in code. 4:11