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Processes4:30 with Ben Deitch
In this video we’ll learn about what a Process is and explore how Android decides which Processes to kill in a low memory situation!
[MUSIC] 0:00 We've learned all about threads and services and we've already seen the word 0:04 process a few times, but we still don't know much about processes. 0:09 Let's fix that by taking a closer look at process in Android. 0:14 Let's start in the Android device monitor. 0:20 Everything over here on the left is a process, and 0:26 if we click to show the threads, say for our music machine app, 0:30 we can see the threads associated with this process. 0:35 So a good way to think about processes would be as a group of threads. 0:40 We already know that by default everything in an app runs on one thread, 0:45 the main thread. 0:49 Now let's add to that that by default, each app also runs in its own process. 0:51 So when we launch an app, all of the code execution will happen on the main thread 0:58 and this thread will be a part of our apps process. 1:03 When we create new threads, they're also a part of our apps process. 1:06 Remember when we were trying to download using just a thread and no service? 1:12 When we killed our apps process, the download stopped. 1:17 This is because the new thread we created was part of our app's process. 1:21 So when we killed the process, we killed the thread. 1:25 However, we are not limited to one process. 1:29 There are a few occasions where it might make sense for 1:34 an app to run in more than one process. 1:36 A good example of this is, not surprisingly, a music player. 1:39 In Android, each process is only allowed to use a certain amount of memory 1:44 based on the total amount of RAM on the device. 1:48 Using one process for our activity and a separate process for 1:51 our service gives both access to more memory. 1:55 But more than that, having two separate processes lets 2:00 Android reclaim the activities process when it's no longer needed. 2:04 If we've been listening to music in the background and 2:09 haven't interacted with the activity in a while, 2:11 then Android will be able to kill just the activities process. 2:14 This way, 2:19 we can free up system resources while making sure that our music keeps playing. 2:20 Cool, right? 2:24 [SOUND] Another important thing about processes is how they get killed. 2:25 We already know that when Android needs to reclaim memory, 2:31 it will start killing processes. 2:35 But how does Android pick which processes to kill? 2:37 Well luckily, Android has a method to determine which processes to kill 2:41 first and which to kill last. 2:46 It does this by grouping processes into five groups of varying importance. 2:49 The first group is foreground processes. 2:54 These are the most important processes and will be the last to be killed. 2:57 More specifically these are processes that a user is currently interacting with. 3:02 When we're clicking buttons in our music machine app, 3:07 our apps process would be considered a foreground process. 3:11 The second group is visible processes. 3:15 These are processes that don't have any foreground components, but 3:18 can still affect what the user sees on the screen. 3:22 An activity in the pause state would likely be running in a visible process. 3:25 The third group is service processes. 3:31 This is for processes which are running started services. 3:33 The importance level of a process running a bound service, on the other hand, 3:38 is determined by the importance level of what the service is bound to. 3:42 The fourth group is background processes. 3:47 If our activity isn't visible, 3:50 then our apps process is likely a background process. 3:52 The fifth, and least important group, is empty processes. 3:56 An empty process is what's left after all of our app's components 4:00 have been destroyed. 4:04 The only reason to keep an empty process around is for caching. 4:06 When Android needs more memory, these processes will be the first to go. 4:10 Hopefully that clears up some of the mystery surrounding processes and gives 4:16 you a good idea about how Android decides which processes can be safely killed. 4:19 In the next video, we'll see how to use two separate processes for 4:25 our music machine. 4:29
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