hello, EGHDK!

how can I help you today?

go right ahead!

first, you mention closure a lot. Is that groovy specific for a block?

well, the term "closure" isn't unique to Groovy

and it's a trifle more involved than just a block

Okay. Just making sure. I just wasn't sure reading the chapter and you were mentioning this closure or that closure, and wasn't 100% on what you meant.

yeah, well, I probably should try to explain that term more in the book

though it's really a Groovy syntax thing

I guess in the context of that chapter, it was basically refering to it as a code block i guess right?

more or less

it's the stuff in { }

Okay, I just wasn't sure, because i hadn't heard it before really. I have heard of clojure and wasn't sure if it was the same or something.

I suspect that Clojure has closures, though I haven't really looked at it

heh. Okay. cool. Next question.

in the root build.gradle we include com.android.tools:gradle:verision#. Is that the gradle dependency number or the Android plugin for gradle version?

the latter

so most of my samples use 1.0.0

Because gradle has a version number and so does the android plugin for gradle correct?

correct

Where is gradle version set?

that depends

for most projects, it is in gradle/wrapper/gradle-wrapper.properties, as part of the distributionUrl property

hello, Raghu!

hi mark

EGHDK: let me take a question from Raghu, and I will be back with you shortly

Raghu: your turn! do you have a question?

yes

No problem. I'll start typing my other questions out.

as i told u yesturday i am using a intent service to start download and upload in a queue

maximum limit for a queue is 6

i have maintin queue of size 6

upload is happening one by one

like in google playstore

when we download updates for apps in google play store

they happen in a queue

when we want cancel a particular item we can cancel the download

i wanted to implement similar to google play stores way of downloading updates

for each app

OK... but what is your question?

wat is the best way to download in queue,maintain a queue and cancel or remove items from a queue

I would start by not using IntentService

ok

use a regular Service

ok

use a LinkedBlockingQueue or PriorityQueue, depending on whether you need priority-based sorting or something

ok

its easy to remove items in linkedblocking queue,,?

onStartCommand() would look at the Intent to see if it is a request to add a job or cancel an existing job

ohh ok

LinkedBlockingQueue has a remove() method

linked blocking queue should be used in service

for your purposes, yes

onStartCommand() would then either put a job in the queue or find a job (based on some criteria, like an ID) and remove it

ok

how do i update the ui

then, use an event bus (e.g., greenrobot's EventBus) to post events related to the jobs, such as completion status

ok

how do i send the request from the queue

iterate through the queue

service will start a thread..?

you use the LinkedBlockingQueue with a ThreadPoolExecutor, with just one thread (since you want only one download at a time)

yes exactly

if you need this queue to survive process termination, either use Square's Tape library or come up with your own persistence model

ok

and with that, let me take a question from EGHDK, and I will be back with you in a bit

EGHDK: your turn! do you have a question?

sure

just to piggyback here. How do you know of all the best collections for a specific task. just reading docs? or any good collections book?, second what if you didn't want to use an event bus?

I'm sure that I don't know all the best collections for a specific task

I have my go-to set of collection classes, ones that usually fit the way my brain works from a design standpoint

and you're either going to use an event bus or you are going to write your own event bus equivalent from scratch, which seems like a waste

would that revolve around broadcastrecivers?

there aren't great options for updating a UI from a service's thread without eventually getting to an event bus of some form

LocalBroadcastManager is one event bus implementation

it's not the one I'd recommend in general, but it does qualify

Oh. LocalBroadcastManager IS an event bus

yes

didn't know that.

we used to use system broadcasts for that role, which was bad bad bad bad bad

and so they gave us LocalBroadcastManager as a way to easily migrate from the bad stuff to a better approach

but, as event bus implementations go, it's not the most feature-rich and it's a bit clunky to code to

Interesting that when they built the platform that they didn't build LocalBroadcastManager then... huh?

or that they didn't put in a good way to update UI

there are lots of things that weren't in Android 1.0

True. That's why its a 1.0

oh, there are lower-level things, like the Handler/Messenger combination

Anyway. Good question Raghu

Okay back to my actual questions. Will I ever have to update the gradle version...? or only if the android gradle plugin updates and uses something that my current gradle version doesn't know about.

"Will I ever have to update the gradle version...?" -- someday, probably

"or only if the android gradle plugin updates and uses something that my current gradle version doesn't know about" -- yes

You can go back to raghu after that. I know I kinda took up my time with event bus/collections. I'll keep jotting down my other questions.

that has Google's plans for the relationship between Gradle version and Gradle for Android version

OK

Raghu: back to you -- do you have another question?

sorry i have more queries on first question

is that fine mark

sure

service processes one request(using a thread pool) and we use event bus

to update

how will i start the next download

1,2,3,4,5

1 completes

how do i move to 2

2 to 3

you put 1, 2, 3, 4, 5 in the queue

ok

everything else, we already discussed

you use ScheduledExecutorService with the LinkedBlockingQueue

it gives you control to do the download

ok

if you put 1, 2, 3, 4, 5 in the queue, it will give you control five times, to do 1, 2, 3, 4, 5

yes

activity will have to trigger in the response or scheduleexecutor knows that there are more items

to process

right

activity will call startService() to send the request to do the download to the service

service gets control in onStartCommand(), puts an item in the work queue

when the item comes to the head of the queue, the thread from the ScheduledExecutorService gives you control to do the work

and part of that work can be posting events on an event bus, or using a Messenger, or whatever, to update your UI

this thread is responsible for sending and handling the response

and also updating the Ui

this thread is responsible for doing whatever background work you wanted to have done via the queue

yes

and this thread is responsible for notifying the UI (event bus, Messenger, whatever) about anything the UI needs to know about

thanks mark

i am clear

let me take a question from EGHDK, and I will be back with you in a bit

EGHDK: back to you!

Okay, tell me if I'm correct. Maven Central is a remote repository that holds thousands of artifacts. Apache Maven is a build system. Maven is a type of way to set up an artifact.

the first two are solid

I'd phrase the third one more as "Maven Central implements what is known as a Maven-style repository for artifacts, with specific rules for metadata formats, directory structures, etc."

there are other artifact repository formats in the Java world (e.g., Apache Ivy), though Maven has largely won out in the marketplace of implementations

So can I replace "Maven Central" in that statement with JCenter?

yes

I'm just confused as of why https://bintray.com/bintray/jcenter shows a "Maven" logo right on that page.

JCenter is a Maven-style repository

"JCenter is the place to find and share popular Apache Maven packages for use by Maven, Gradle, Ivy, SBT, etc. "

an "Apache Maven package" is the combination of the actual artifact itself (e.g., a JAR) and some metadata about the artifact (a POM file)

okay, so maven central is crystal clear now... but is apache maven a build system that can build apache maven packages?

Apache Maven is a build system in general

same basic roles as Apache Ant, Gradle, etc.

they popularized the notion of artifacts and repositories in the Java space

their build system itself is reasonably popular, and much more powerful than is Ant

gotcha. okay. that helps a bit. thanks

that's one of the problems in this area: "Maven" is used for lots of related but disparate things

let me take another question from Raghu, and I will be back with you shortly

Raghu: your turn! do you have another question?

no mark thanks for the solution

OK

if you come up with another question before the chat ends, let me know

sure

EGHDK: over to you!

In your book you mention if you want to use maven central you can add it in your app module build.gradle. And your example does just that. But why wouldn't you just put that right under where jcenter() is entered by default?

you usually don't need both jcenter() and mavenCentral()

you need one or the other

in a new Android Studio project, it will be set up with jcenter()

that didn't use to be the case

earlier versions of Android Studio used mavenCentral()

they switched around the time of Android Studio 1.0, or perhaps a bit before

so you'll see bits of both in the book samples, but more mavenCentral(), due to the timing of when I did the work

gotcha. But lets say one dependency I want is on maven central and not bin tray... I could put that in just the root build.gradle then right?

yes

you might consider replacing jcenter() with mavenCentral(), though, rather than having both

cool.

Back to something that got my interest. event bus.

So I don't use services that often, so I don't know how I would communicate back to an activity, thats why Im asking these next questions.

I thought services could be bound or unbound (i was taking that as "bound" or "unbound" to the gui)

So you said localBroadCast is something you can use right now without any adiitional library right?

well, LocalBroadcastManager is in the Android Support package

so you would need support-v4 or support-v13

You said that before that you would use System Broadcasts?

back in the 2009-2011 timeframe, that was one approach we erroneously used

How did you use a system broadcast? aren't system broadcasts for like low battery and phone call and boot complete and stuff?

Why erroneous?

when I use the term "system broadcast", I mean methods like sendBroadcast() on Context, where the receivers could be in any app

in contrast to a "local broadcast", where sendBroadcast() is on LocalBroadcastManager

system broadcasts are a cross-process event bus

however, from a security and performance standpoint, never use a cross-process X when you can achieve the same results using an in-process X

and I, and others, weren't thinking of those implications back then

we were young(er) and stupid

Gotcha. So security and performance.

right

Makes sense. Don't want any app to catch your broadcast and do something you didn't want.

now, if you need a cross-process event bus, system broadcasts are your best option, but you just have to work out the security ramifications

right

can i ask a last question on linkedblockqueue mapping with the Ui(activity).

Raghu: go ahead

I wanted to cancel nth item(click on cancel from UI),how do I map the clicked item with the linked blocking queue in service so that i can remove it from the queue

you need some sort of identifier

yes

could be the URL of the download

or a generated ID

this url is part of queue

in a obect

object

I was under the impression that your background work involved downloads, which is why I suggested a URL

but it would be part of the work that you would put into the queue

yes exactly

ideally, those objects would be from some class (call it Job for now) that has a proper implementation of equals()

we can find the item from the identifier

that does the comparison by ID

then, you can use remove() on a Job with the same ID to remove the real actual Job instance that is in the queue

ok

job has equals and hashcode

remove(), contains(), etc. on LinkedBlockingQueue use equals(), rather than object identity, for finding things in the queue

right

so, the activity calls startService() with a "cancel this job" Intent, with the identifier

onStartCommand() creates the Job with that identifier and calls remove() on the LinkedBlockingQueue, thereby removing any Job for that identifier

EGHDK: if you have other questions, go ahead, as we are running out of chat time

You mentioned a lower level event bus? Message and Handler?

it's not really an "event bus", per se, but it is a way for disparate components to communicate

I know you have worked with Handler, based on prior chats

you can create a Messenger tied to a Handler

Messenger can go in things like an Intent extra

so a service's client (e.g., activity) can have a Handler, create a Messenger, and pass that Messenger to the service

the service can hold onto that Messenger and send messages back to the client

doing this right requires some amount of garbage-collection bookkeeping

Okay, so it is pretty low level. Is that what these other "good" event buses are built off of?

in the end, they all wind up using a Handler

Even localBroadcastReceiver?

actually, that one might not

no, no, it should

Otto is the one that might not

it comes back to threading

LocalBroadcastManager always delivers events on the main application thread, and so it is using a Handler tied to that thread

Okay, I'll look into it. I have to start using services. At this point, I don't see where I would use an event bus except service to activity interaction

BroadcastReceiver to UI interaction too

particularly for OS-initiated broadcasts that you have a manifest-registered receiver for

Main application thread has a single handler... or is it an Activity that has its own handler?

Handlers are tied to threads

or both... I guess?

there can be N Handler instances associated with the main application thread

an activity usually does not need more than one

and that's a wrap for today's chat

Gotcha. Okay, cool I will read about them. Super interesting stuff here.

the transcript will go up on http://commonsware.com/office-hours/ shortly

appreciate it mark. Till next chat! Thanks

the next chat is Tuesday, also at 7:30pm

have a pleasant day!