Hello
In I/O 17 Google made two major announcements
for Android app developers.
First one was of course Kotlin and second one was
Architecture components.
In fact Architecture components didn't get much buzz
because by the time it was announced
most of the developers had left to learn Kotlin :)
Let's first address the elephant in the room.
and the elephant is: Lifecycle.
The unique and challenging feature of the Android framework.
On desktop a user clicks an app, opens it
works on it and closes it. That is the lifecycle
but on Android an Activity
which is a representation of an app
might get created multiple times
during a single user experience.
Let's see what kind of challenges this brings to us.
Memory leak
There is an object which holds a reference of an
Activity and for some reason that
Activity gets recreated
so the older instance get destroyed but it can't be
garbage collected because the reference of that Activity
is being held by another object.
This leads to memory leak.
In fact this is such an issue that beginners
at early stage need to learn how to avoid them.
AsyncTask these days are mostly used
to teach us how memory leak happens :)
Data persistence
Suppose in your Activity in onCreate() method
You are fetching some data and for some reason
Activity gets recreated
Maybe user changed the orientation. The onCreate()
is called again and the data is fetched again which was not required.
User didn't ask for it so this
leads to inefficiency.
What has Google offered?
Google wants to be more opinionated
how apps should be developed
and more guidance to the developers.
It's not providing a solution but a tool
in the form of architecture components
using which we can create better apps.
Architecture components, there are two sections
One is lifecycle related which contains
classes like LifecycleOwner, LifecycleObserver
and then there are components related to data persistence
like LiveData, ViewModel and Room
Let's discuss them briefly one by one
What is LifecycleOwner?
LifecycleOwner is an interface
and any class which implements it becomes a LifecycleOwner
Currently Google provides us two
classes. LifecycleActivity and LifecycleFragment
which are LifecycleOwner
The important thing is LifecycleOwner
contains a LifecycleObject
which sends notification about Lifecycle events
to its Observer
LifecycleObserver
This is an interface and implemented by those classes
which want to receive lifecycle events
and those events are received
thru annotations
Let's discuss lifecycle classes in slightly more detail
Lifecycle is an abstract class.
It has two enum and few methods.
Enum State has values INITIALIZED, CREATED, STARTED etc.
This is something new
We used to get events which was for a moment
but now there are states which we can enquire
and find in which state a lifecycle is.
Event enum has values ON_CREATE, ON_START etc.
which we usually get in our Activity or Fragment
and then there are methods
to manage observers.
LifecycleRegistry class
it extends Lifecycle class. It manages observers
and pass on lifecycle events to those observers.
LifecycleOwner is an interface
it has only one method getLifecycle()
and it returns a Lifecycle object
LifecycleregistryOwner, it extends
LifecycleOwner interface
and it overrides getLifecycle() method
and returns LifecycleRegistry object.
and then finally comes
the LifecycleActivity which we are going to use
as a LifecycleOwner
It implements LifecycleRegistryOwner
It overrides the method getLifecycle()
and return an object mRegistry
of class LifecycleRegistry
Let's see how LifecycleObserver and LifecycleOwner works
MyLifecycleObserver
implements LifecycleObserver interface
LifecycleObserver interface doesn't contain any method
It's just there to mark this class as
as a LifecycleObserver
In its constructor we are passing LifecycleOwner object
and then we are calling getLifecycle() method
which returns a Lifecycle object
and then we call addObserver() method
of Lifecycle object and pass the instance of this
LifecycleObserver class
lifecycle events are received thru annotations
As you saw, no miracles happened.
We already get lifecycle events
in our Activity and Fragment
Now we can get it in LifecycleObserver classes
Not a big deal
but then it provides a better separation.
Our Activity and Fragment's OnStart()
onStop(), these methods wouldn't be cluttered
This separation
from architectural point of view is a good move
and it will lead to a clean code.
Now let's discuss data components
LiveData is a class with generic type
it's a wrapper to a data which can be observed
It's a lifecycle aware which means
it gets lifecycle events.
Here is an example.
We create an Observer object, MyObserver
then we call LiveData's observe() method
and we pass the LifecycleOwner object
as well as myObserver object
Now this LifecycleOwner object which we have passed
LiveData uses it to observe lifecycle events
that's how it knows
when to send us data and when not to.
The best thing about ViewModel class
is it survives configuration change. The way we get
an object of ViewModel class in our Fragment or Activity
is we call ViewModelProviders of() method
and then pass the instance of Fragment or Activity
then we call get() method and pass MyViewModel.class
ViewModel is perfect container
for LiveData as it persists
configuration change, so it solves our issue
of fetching the same data again and again.
Sharing data between fragments of an Activity
is easy.
In Fragment
instead of passing the instance of the
Fragment in of() method we pass the instance of
Activity using getActivity() so it returns the
ViewModel object of the Activity.
Don't put any references of Activity
or Fragment to avoid memory leak.
ViewModel persists configuration change
so it doesn't match the lifecyle of Activity or Fragment.
Room is an object mapping library for SQLite.
Those of us who fetch cursor from SQLite
know that we have to iterate thru the
cursor, extract the data, put them in POJOs
maybe create a List of POJOs.
The good thing is all this is done
by Room automatically
Room has less boilerplate code as
compared to what we use for SQLite.
Room is intelligent also.
so with these two line code
we are getting a list of users
matching a name
with a particular keyword.
Room also checks error at compile time
so this should help us find out bugs.
Observability
Room can also return LiveData
and this is how all these components connect.
So, we have LifecycleActivity / LifecycleFragment
which contains ViewModel
which contains LiveData returned by Room.
and then Room also supports RxJava2
Here is the overview of architecture components
We have Activity / Fragment
which observes LiveData
held inside ViewModel
ViewModel fetches it from repository
and repository fetches it either from
Room or remote data source.
Here repository is an abstraction layer.
We could have directly called Room from ViewModel
but using this abstraction layer, repository
we are being flexible with data sources
Road map
Architecture library is in alpha version right now,
hopefully soon we can get a stable version.
It requires Android Studio version 2.3 or higher.
Currently there are two new classes introduced
LifecycleActivity and LifecycleFragment
which are LifecycleOwner.
Later these classes will be merged respectively with
AppCompatActivity and Fragment
so we can use them normally as we do now.
What next?
Google has picked up popular practices and tried
to implement them in Android framework.
This shall reduce the learning curve and hopefully motivate
more developers to use them.
I would say again these are not solutions to a problem
but tools to improve our apps.
I would suggest to go ahead and read the architecture guide
read the documentation, try out the code labs.
I have put few links in the description box.
You might find them useful.
Thank you.
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