In Java and Android, we can do this:
public static MyApplication extends Application {
private static Context appContext;
public void onCreate() {
appContext = this;
}
public static Context getAppContext() {
return appContext;
}
}
so that, somewhere else, we can do this:
appContext = MyApplication.getAppContext();
How do we do this in Kotlin? I've been going round in circles for the past hour or so.
Thanks in advance.
//Edit
Perhaps I should have been clearer. I meant how can we write the above in Kotlin and use it in Kotlin.
In Kotlin this is called the 'companion object':
class MyApplication: Application {
companion object {
var appContext: Context? = null
private set
}
}
The key element I was missing was the use of an init block to set the appContext that is inside the companion object (I had already tried the companion object path but was struggling to actually get appContext set).
See code below:
class MyApplication : Application() {
init {
appContext = this
}
companion object {
lateinit var appContext: Context
private set
}
}
This is then callable as usual via:
val testContext = MyApplication.appContext
Assumed you have some java code in android and you want to convert it to kotlin code:
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it's help me to convert java code I've found on internet and converting it to kotlin code,
may this answer not help you about your question, but it would help you to convert what you know in java that you don't know how-to-do in kotlin
you can use it this way
companion object{
//your static fields
}
to call it from kotlin ==> ClassName.FieldName
to call it from java ==> ClassName.Companion.getFieldName()
It seems like you want only one object of this class at runtime. This is called a singleton. There are recommendations to implement that properly in Java. Luckily Kotlin directly allows you to declare singleton objects on the top scope:
val o = object { your attributes and methods here}
Related
maybe I phrased my question a little strange, but something became interesting to me.
Let's imagine that I have some Extension function:
fun Int.foo() {
TODO()
}
Suppose that I need to pass the context of the Fragment from which I call it to this function, in which case I would do it like this:
fun Int.foo(context: Context) {
TODO()
}
Here we are explicitly passing the Context of our Fragment to the function. However, I'm interested in the question - is it possible to somehow change this function so (or can it be called in some other way) so that I do not have to explicitly pass the Context?
I understand that I could do like this:
fun Fragment.foo() {
var context = this.context
}
...however, I need an Extension function just above Int, so this method is not suitable.
Are there any ways how this can be done?
I guess you're looking for context-dependent declarations that let you combine multiple receiver scopes:
context(Fragment)
fun Int.foo() {
check(context != null) // context is actually Fragments context
}
Keep in mind however this feature is still in experimental state so it requires opt in by adding -Xcontext-receivers to your compiler options.
The Int class is just a class for op with numbers
It doesn't make sense to contain a Context object
It is not possible to get context without passing it to the function
There are other ways, which is to create a static object for the application class
for example
class App : Application() {
companion object {
var app: App? = null
}
init {
app = this;
}
}
and then
fun Int.foo(){
val context=App.app
...
}
I am seeing the following error
Platform declaration clash: The following declarations have the same
JVM signature (getHosts()Landroidx/lifecycle/MutableLiveData;):
private final fun <get-hosts>(): MutableLiveData<List> defined
in com.example.xx.viewmodel.HostsViewModel public final fun
getHosts(): MutableLiveData<List> defined in
com.example.xx.viewmodel.HostsViewModel
What am I doing wrong?
class HostsViewModel : ViewModel() {
private val hostsService = HostsService()
private val hosts: MutableLiveData<List<Host>> by lazy {
MutableLiveData<List<Host>>().also {
loadHosts()
}
}
fun getHosts(): MutableLiveData<List<Host>> {
return hosts
}
private fun loadHosts(){
hosts.value = hostsService.getHosts().body()
}
}
For every class property (val), Kotlin generates a getter called getHosts() and for var also a setter called setHosts(MutableLiveData<List<Host>> value) as per Java's convention. It hides it from the Kotlin user as getters and setters are usually just boilerplate code without offering much value. As such, your own getHosts() method clashes with the generated method at compilation. You have multiple possibilities to solve this issue:
Rename private val hosts to something else, e.g. private val internalHosts
Annotate the getHosts method with #JvmName("getHosts2"). If you do that though, consider the possibility that someone might call your code from Java and in that case, the caller would need to call getHosts2() in Java code, which might not be such nice API-design.
Reconsider your api design. In your case, you could simply make val hosts public and remove your getHosts() entirely, as the compiler will auto-generate getHosts() for you.
In addition to that, you might want to consider not exposing MutableLiveData in general as mentioned in the comments.
Edit:
Also, I would recommend that you do this:
val hosts: MutableLiveData<List<Host>> by lazy {
MutableLiveData<List<Host>>().also {
it.value = hostsService.getHosts().body()
}
}
and remove loadHosts to make your code more concise.
I'm not very clear about the best way to inject into a static methods helper class (lets say a Custom class).
I'm kinda new to Kotlin, and as I've learnt we can access a method statically in two ways:
Object class.
Class + companion object.
To start, I'm not sure which one is the most recommended one (if there is a best practice regarding this), but my "problem" arises when needing to inject dependencies into a static method class.
Let's go with a simple example:
I have a static methods class called AWUtils (not decided if it should be an object class or a class with companion object yet though, and this will most likely depend on the injection mechanism recommended) with the next method:
fun setAmpersand2Yellow(text2Replace: String, target: String): String {
return text2Replace.replace(
target, "<span style=\"color:" +
app.drawerFooterColor + ";\">" + target + "</span>"
)
}
Here, app is the instance of my AppSettings class which holds all app configuration so, as you see setAmpersand2Yellow needs AppSettings, and of course I would't pass it as a parameter by any means, so it's a AWUtils dependence.
Using AWUtils as a class with companion object for the static methods I cannot inject directly AppSettings into company object as far as I know (at least I cannot do constructor injection, let me know if I'm wrong) and if I inject into companion object parent class (AWUtils) constructor then I don't know how to access those dependences from the companion object itself (the child).
If I use fields injection in AWUtils as a class then it complains than lateinit field has not been initialised and I don't know how to deal with this, because as far as I know lateinit fields are initialised in onCreate, which does not exist in this kind of classes.
One other possibility is to use an object with fields and set the dependencies values from caller in a static way before calling the method, for example:
object AWUtils {
var app: AppSettings? = null
fun setAmpersand2Yellow(text2Replace: String, target: String): String {
return text2Replace.replace(
target, "<span style=\"color:" +
app.drawerFooterColor + ";\">" + target + "</span>"
)
}
}
#AndroidEntryPoint
class OtherClass
#Inject constructor(private val app: AppSettings) {
fun AnyFunction() {
var mystr = "whatever"
AWUtils.app = app
var yellowStr = AWUtils.setAmpersand2Yellow(myStr)
}
}
In the end, I'm not sure on how to supply dependencies to a static methods class and which form of "static" class should I choose.
Edit 1:
Apart from my ApSettings class, I need a context, like for example in this next isTablet method:
val isTablet: String
get() {
return ((context.resources.configuration.screenLayout
and Configuration.SCREENLAYOUT_SIZE_MASK)
>= Configuration.SCREENLAYOUT_SIZE_LARGE)
}
In the end, I need a context and my AppSettings (or any other custom classes) to be injected anyway in a class with static methods.
Edit 2:
I could do (from the activity):
AWUtils.context = this
AWUtils.app = app
var isTablet = AWUtils.isTablet
And it works, but rather to be in the need of assigning a value to two fields (or more) every time I need to call a static method, I would prefer the fields to be injected in any way.
That's what dependency injection is meant for, isn't it?
Edit 3: I'm starting to be fed up with Hilt, what is supposed would have been created to simplify our life, only makes our programming life much more complicated.
As you clarified in the comments, you want to have your utils class accessible in an easy way across your codebase, so this answer will focus on that and on your original questions.
I'm kinda new to Kotlin, and as I've learnt we can access a method statically in two ways: Object class or Class + companion object.
Kotlin does not have Java-style statics. One reasoning behind it was to encourage more maintainable coding practices. Static methods and static classes are also a nightmare for testing your code.
In Kotlin you would go with an object (but a class + companion object would work in the same way)
object AWUtils {
lateinit var appContext: Context
lateinit var appSettings: AppSettings
fun initialize(
appContext: Context,
appSettings: AppSettings,
// more dependencies go here
) {
this.appContext = appContext
this.appSettings = appSettings
// and initialize them here
}
val isTablet: Boolean
get() = ((appContext.resources.configuration.screenLayout
and Configuration.SCREENLAYOUT_SIZE_MASK)
>= Configuration.SCREENLAYOUT_SIZE_LARGE)
fun setAmpersand2Yellow(text2Replace: String, target: String): String {
return text2Replace.replace(
target, "<span style=\"color:" +
appSettings.drawerFooterColor + ";\">" + target + "</span>"
)
}
}
Since this object should be accessible across the whole application it should be initialized as soon as possible, so in Application.onCreate
#HiltAndroidApp
class Application : android.app.Application() {
// you can inject other application-wide dependencies here
// #Inject
// lateinit var someOtherDependency: SomeOtherDependency
override fun onCreate() {
super.onCreate()
// initialize the utils singleton object with dependencies
AWUtils.initialize(applicationContext, AppSettings())
}
Now anywhere in your app code you can use AWUtils and AppSettings
class OtherClass { // no need to inject AppSettings anymore
fun anyFunction() {
val mystr = "whatever"
val yellowStr = AWUtils.setAmpersand2Yellow(myStr)
// This also works
if (AWUtils.isTablet) {
// and this as well
val color = AWUtils.appSettings.drawerFooterColor
}
}
}
There is another way in Kotlin to write helper/util functions, called extension functions.
Your isTablet check might be written as an extension function like this
// This isTablet() can be called on any Configuration instance
// The this. part can also be omitted
fun Configuration.isTablet() = ((this.screenLayout
and Configuration.SCREENLAYOUT_SIZE_MASK)
>= Configuration.SCREENLAYOUT_SIZE_LARGE)
// This isTablet() can be called on any Resources instance
fun Resources.isTablet() = configuration.isTablet()
// This isTablet() can be called on any Context instance
fun Context.isTablet() = resources.isTablet()
With the above extension functions in place the implementation inside AWUtils would be simplified to
val isTablet: Boolean
get() = appContext.isTablet()
Inside (or on a reference of) any class that implements Context, such as Application, Activity, Service etc., you can then simply call isTablet()
class SomeActivity : Activity() {
fun someFunction() {
if (isTablet()) {
// ...
}
}
}
And elsewhere where Context or Resources are available in some way, you can simply call resources.isTablet()
class SomeFragment : Fragment() {
fun someFunction() {
if (resources.isTablet()) {
// ...
}
}
}
Edit 3: I'm starting to be fed up with Hilt, what is supposed would have been created to simplify our life, only makes our programming life much more complicated.
Yeah, Hilt is focusing on constructor injection and can only do field injection out-of-the-box in very limited cases, afaik only inside Android classes annotated with #AndroidEntryPoint and inside the class extending the Application class when annotated with #HiltAndroidApp.
Docs for #AndroidEntryPoint say
Marks an Android component class to be setup for injection with the standard Hilt Dagger Android components. Currently, this supports activities, fragments, views, services, and broadcast receivers.
If you feel that you need a lot of field injection, because you are working with "static"-like objects in Kotlin, consider using Koin instead of Hilt for your next project.
I have an interface WordsDataSource using which I have implemented two concrete classes namely WordsLocalDataSource that deals with local database and another WordsRemoteDataSource that deals with manipulating data online on the server. The problem is when I try to inject the two classes in repository class using abstract class name WordsDataSource like
DefaultWordsRepository(
private val wordsRemoteDataSource: WordsDataSource,
private val wordsLocalDataSource: WordsDataSource) {
And adding dependencies in Application class like
class WordsApplication : Application(), KodeinAware {
override val kodein = Kodein.lazy {
import(androidXModule(this#WordsApplication))
bind() from singleton { WordsDatabase.getInstance(instance()) }
bind<WordsDao>() with singleton { instance<WordsDatabase>().wordsDao() }
bind() from singleton { WordsLocalDataSource(instance()) }
bind() from singleton { WordsRemoteDataSource() }
bind<WordsRepository>() with singleton { DefaultWordsRepository(instance(), instance()) }
bind() from provider { ViewModelFactory(instance()) }
}
Then upon running the app I encounter the following issue in the logcat
org.kodein.di.Kodein$NotFoundException: 2 bindings found that match bind<WordsDataSource>() with ?<WordsFragment>().? { ? }:
bind<WordsLocalDataSource>() with singleton { WordsLocalDataSource }
bind<WordsRemoteDataSource>() with singleton { WordsRemoteDataSource }
I have tried the workaround for this by simply declaring the variables by their respective concrete class names like
DefaultWordsRepository(
private val wordsRemoteDataSource: WordsRemoteDataSource,
private val wordsLocalDataSource: WordsLocalDataSource) {
But still want to know whether or not is there any way to resolve the issue.
I am using the following dependencies for kodein
implementation "org.kodein.di:kodein-di-generic-jvm:6.3.3"
implementation "org.kodein.di:kodein-di-framework-android-x:6.3.3"
You have done it the right way by writing the explicit types:
DefaultWordsRepository(
private val wordsRemoteDataSource: WordsRemoteDataSource,
private val wordsLocalDataSource: WordsLocalDataSource)
When working with sub-types we cannot know what kind of implementation to choose. Writing
DefaultWordsRepository(
private val wordsRemoteDataSource: WordsDataSource,
private val wordsLocalDataSource: WordsDataSource)
Doesn't cannot find if you want both sub-types or twice the WordsRemoteDataSource or WordsLocalDataSource. Thus, you need to explicit define your types. Even, we could put WordsRemoteDataSource in the property wordsLocalDataSource, as we cannot rely on variable names.
I found in stack a lot of topic's about singleton. But in my opinion, singleton in Kotlin can be simpler:
class SingletonTwo private constructor(){
var myAge: Int = 16
companion object {
val ourInstance = SingletonTwo()
}
}
Now I can get access in other class to this variable myAge:
var abc: Int
abc = SingletonTwo.ourInstance.myAge
Something is wrong here ? This singleton is a little smaller than in other subjects on stack.
Please someone correct me if I'm wrong.
Declaring something with the object keyword instead of class makes it a singleton on its own:
object SingletonTwo {
var myAge: Int = 16
}
In this case, you can access the variable like so:
var abc: Int = SingletonTwo.myAge
You can see the docs about object declarations which also notes that they should be used to easily create singletons.