I am changing a global variable value inside a method and trying to return it later .
In FetchData.kt ( The called class )
var homeFeed: HomeFeed? = null // the variable that needs to be changed
fun execute() {
val client = OkHttpClient()
val url =
"..."
val request = Request.Builder().url(url).build()
val res = client.newCall(request).enqueue(object : Callback {
override fun onFailure(call: Call, e: IOException) {
e.printStackTrace()
}
override fun onResponse(call: Call, response: Response) {
val ch = response?.body?.string()
val gson = GsonBuilder().create()
homeFeed= gson.fromJson(ch, HomeFeed::class.java) // where the change happens
}
})
}
fun GetData(): HomeFeed? {
return homeFeed
}
In MainActivity.kt
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
btn.setOnClickListener {
val destination = "destination"
val places = FetchData(destination)
places.execute()
val data = places.GetData()
}
}
The problem is that a Null value is assigned to "data" variable in MainActivity as if the returned "homeFeed" variable wasn't changed at all .
I debugged the code to get ensured that no errors occur with api call and the variable is changed inside the method ( but not outside it! )
I am really stuck with that , any help ?
You are calling getData() immediately after making the async request, so it hasn't had a chance to be updated. Asynchronous functions do not immediately return. Some background thread does something (a network request), and in this case calls onResponse when the result is returned some time in the future.
It's just like the listener on your button. The code in the listener isn't run immediately, but some time in the future when the user presses it.
Instead of using this member property, your function can take a callback parameter that it invokes when the result is ready:
fun execute(resultHandler: (HomeFeed) -> Unit) {
//... snip ...
client.newCall(request).enqueue(object : Callback {
// ... snip ...
override fun onResponse(call: Call, response: Response) {
val ch = response?.body?.string()
val gson = GsonBuilder().create()
resultHandler(gson.fromJson(ch, HomeFeed::class.java))
}
})
}
Then when you call it, you pass a lambda that will be called when the result is ready:
btn.setOnClickListener {
val destination = "destination"
val places = FetchData(destination)
places.execute { homeFeedData ->
// Do something with homeFeedData when it arrives
}
}
Related
I am new android developer, how can I get result form this snippet, what way does exist, because it doesn't return anything, because of I'm adding element inside onResponse, but using only kotlin module:
private fun foo(list: ArrayList<CurrencyModel> = ArrayList()): ArrayList<CurrencyModel> {
val request = Request.Builder().url(BASE_URL_YESTERDAY).build()
val client = OkHttpClient()
client.newCall(request).enqueue(object : Callback {
override fun onFailure(call: Call, e: IOException) {
}
override fun onResponse(call: Call, response: Response) {
val body = response.body?.string()
val data = Gson().fromJson(body, Currencies::class.java)
list.add(CurrencyModel("USD", data.rates.USD, 0.0))
list.add(CurrencyModel("SEK", data.rates.SEK, 0.0))
list.add(CurrencyModel("EUR", data.rates.EUR, 0.0))
}
})
return list
}
}
You can give your function a callback parameter that's called when the response is receieved. And you shouldn't have an input list in this case, because if you have multiple sources modifying it at unpredictable future moments, it will be difficult to track.
The function can look like this:
private fun getCurrencyModels(callback: (ArrayList<CurrencyModel>) {
val request = Request.Builder().url(BASE_URL_YESTERDAY).build()
val client = OkHttpClient()
client.newCall(request).enqueue(object : Callback {
override fun onFailure(call: Call, e: IOException) {
}
override fun onResponse(call: Call, response: Response) {
val body = response.body?.string()
val data = Gson().fromJson(body, Currencies::class.java)
val list = arrayListOf(
CurrencyModel("USD", data.rates.USD, 0.0)),
CurrencyModel("SEK", data.rates.SEK, 0.0)),
CurrencyModel("EUR", data.rates.EUR, 0.0))
)
callback(list)
}
})
}
And then to use it:
getCurrencyModels { modelsList ->
// do something with modelsList when it arrives
}
An alternative is to use coroutines, which allow you to do asynchronous actions without callbacks. Someone has already created a library that lets you use OkHttp requests in coroutines here. You could write your function as a suspend function like this:
private suspend fun getCurrencyModels(): ArrayList<CurrencyModel> {
val request = Request.Builder().url(BASE_URL_YESTERDAY).build()
val client = OkHttpClient()
val response = client.newCall(request).await()
val body = response.body?.string()
val data = Gson().fromJson(body, Currencies::class.java)
return arrayListOf(
CurrencyModel("USD", data.rates.USD, 0.0)),
CurrencyModel("SEK", data.rates.SEK, 0.0)),
CurrencyModel("EUR", data.rates.EUR, 0.0))
)
}
and then use it like this:
lifecycleScope.launch {
try {
val currencyModels = getCurrencyModels()
// do something with currencyModels
} catch (e: IOException) {
// request failed
}
}
Coroutines make it really easy to avoid leaking memory when your asynchronous calls outlive your Activity or Fragment. In this case, if your Activity closes while the request is going, it will be cancelled automatically and references to your Activity will be removed so the garbage collector can release your Activity.
The onResponse() function is only called when the HTTP response is successfully returned by the remote server. Since this response doesn't happen immediately, you can't use the result in your code immediately. What you could do is use a ViewModel and LiveData variable and add the values to that variable in onResponse(). Something like:
private fun foo(list: ArrayList<CurrencyModel> = ArrayList()) {
val request = Request.Builder().url(BASE_URL_YESTERDAY).build()
val client = OkHttpClient()
client.newCall(request).enqueue(object : Callback {
override fun onFailure(call: Call, e: IOException) {
}
override fun onResponse(call: Call, response: Response) {
val body = response.body?.string()
val data = Gson().fromJson(body, Currencies::class.java)
val list: ArrayList<CurrencyModel> = arrayListOf()
list.add(CurrencyModel("USD", data.rates.USD, 0.0))
list.add(CurrencyModel("SEK", data.rates.SEK, 0.0))
list.add(CurrencyModel("EUR", data.rates.EUR, 0.0))
viewModel.list.postValue(list)
}
})
}
How can I return a value after a callback in kotlin, I tried using Thread.sleep but it doesn't work
fun searchColorFromAPI(): Colors {
val service: RetrofitService = ServiceGenerator.createService(RetrofitService::class.java)
val result: MutableList<String> = arrayListOf()
val call: Call<Colors?>? = service.unityConverter(result)
call?.enqueue(object : Callback<Colors?> {
override fun onResponse(call: Call<Colors?>?, response: Response<Colors?>) {
//switchProgressVisibility()
if (response.isSuccessful) {
val serviceResponse: Colors? = response.body()
if (serviceResponse != null) {
mColors = serviceResponse
}
else {
//buildToast(getString(R.string.null_response))
}
}
else {
//buildToast(getString(R.string.response_unsuccessful))
val errorBody: ResponseBody = response.errorBody()
Log.e(TAG, errorBody.toString())
}
}
override fun onFailure(call: Call<Colors?>?, t: Throwable?) {
/* buildToast(getString(R.string.error_calling_service))
Log.e(TAG, t?.message)*/
}
})
return mColors
}
Always, the mColors is returned before the onFailure or onResponse because they're asynchronous. Before this code was in MainActivity but I was advised to take off, but now when I try get mColors I get the empty value before and after the onResponse is executed, please I'm still learning Kotlin and Android.
Your problem stems from the fact that Retrofit call is asynchronous, so as soon as you call searchColorFromAPI it returns you mColors but the API call may not have been made yet, so you get the mColors value before API call.
To solve this issue, you can do
Use callback, this will require little modification in your current setup, but the 2nd option is preferable over this. Using callback your function should look like this.
/* Now instead of returning a value, your function takes a function (named callback)
as parameter. when your api call finishes, you can call the callback function and
pass the api response.
*/
fun searchColorFromAPI(callback: (Colors?) -> Unit) {
val service: RetrofitService = ServiceGenerator.createService(RetrofitService::class.java)
val result: MutableList<String> = arrayListOf()
val call: Call<Colors?>? = service.unityConverter(result)
call?.enqueue(object : Callback<Colors?> {
override fun onResponse(call: Call<Colors?>?, response: Response<Colors?>) {
//switchProgressVisibility()
if (response.isSuccessful) {
val serviceResponse: Colors? = response.body()
/** pass API response to callback */
callback(serviceResponse)
}
else {
val errorBody: ResponseBody = response.errorBody()
Log.e(TAG, errorBody.toString())
callback(null)
}
}
override fun onFailure(call: Call<Colors?>?, t: Throwable?) {
callback(null)
}
})
}
And in your activity declare a function as follows.
// This function will be called when your api call finishes
// and it will give you the api response
fun apiCallback(colors: Colors?){
if(colors == null){
// API Call failed
}
else{
// use colors as returned by API
}
}
And now call to searchColorFromApi should look like this
searchColorFromApi(apiCallback)
Use Live Data, declare following field in your viewmodel, if you are not using viewmodel then declare it in the class which has searchColorFromApi function.
var colors: MutableLiveData<Colors> = MutableLiveData()
and modify your searchColorFromAPI function as follows
fun searchColorFromAPI() {
val service: RetrofitService = ServiceGenerator.createService(RetrofitService::class.java)
val result: MutableList<String> = arrayListOf()
val call: Call<Colors?>? = service.unityConverter(result)
call?.enqueue(object : Callback<Colors?> {
override fun onResponse(call: Call<Colors?>?, response: Response<Colors?>) {
//switchProgressVisibility()
if (response.isSuccessful) {
val serviceResponse: Colors? = response.body()
if (serviceResponse != null) {
colors.postValue(response.body)
}
}
else {
colors.postValue(null)
val errorBody: ResponseBody = response.errorBody()
Log.e(TAG, errorBody.toString())
}
}
override fun onFailure(call: Call<Colors?>?, t: Throwable?) {
colors.postValue(null)
}
})
}
and in your activity do following
fun setupObservers(){
yourApiCallingClass.colors.observe(this, Observer {
// this code is called when ever value of color field changes
})
}
You can use live data ,that gets updated once the callback receives ,the same live data is observed by the caller fragment/activity
You can use coroutines to return a value from function which has asyn calls in it.
You can use interface callbacks to activity/ fragment to trigger the updates received from retrofit calls.
I can't observe the LiveData<PagedList> change in activity, but the UI is updated(the list has grown in activty).
I can only observe it once when the livedata is initialized.
when the paging library call loadAfter method, the ui is updated, but didn't call pageList.observe{}
Firstly, I put the process of data request into the Kotlin Coroutines, I can't observe the data change, then I used asynchronous requests instead.It still didn't work.
Here is my code:
PlayActivity main code
private val commentAdapter =
object : BasePagedAdapter(diffCallback, this) {
// just bind recycleview item and corresponding view model. etc.
}
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = DataBindingUtil.setContentView(this, R.layout.activity_play)
binding.vm = vm
binding.lifecycleOwner = this
val workId = intent.getLongExtra(WORK_ID, 0)
vm.listComment(workId)
play_rv_comment.adapter = commentAdapter
/* herer is the problem*/
vm.commentList.observe(this, Observer {
/*only log once when called loadInitial*/
LogUtils.e("observe", it)
commentAdapter.submitList(it)
})
PlayViewModel
class PlayViewModel : BaseViewModel() {
var workId: Long = 0
// the data which I want to observe
lateinit var commentList: LiveData<PagedList<WorkComment>>
private val commentPageSize = 15
fun listComment(workId: Long) {
// init by DataSource.Factory in android paging library
commentList = BaseDataSourceFactory(workId).toLiveData(commentPageSize)
}
DataSource.Factory in Android paging
class BaseDataSourceFactory(
val workId: Long
) :
DataSource.Factory<Long, WorkComment>() {
override fun create(): DataSource<Long, WorkComment> {
return object : PageKeyedDataSource<Long, WorkComment>() {
override fun loadInitial(
params: LoadInitialParams<Long>,
callback: LoadInitialCallback<Long, WorkComment>
) {
try {
val res = RetrofitUtil.getInstanceWithJwt().create(WorkCommentApi::class.java)
.listComment(
workId, 1, params.requestedLoadSize
)
res.enqueue(object : retrofit2.Callback<TResult> {
override fun onFailure(call: Call<TResult>, t: Throwable) {
}
override fun onResponse(call: Call<TResult>, response: Response<TResult>) {
callback.onResult(
response.body()!!.toList(WorkComment::class.java),
null, 2)
}
})
} catch (e: SocketTimeoutException) {
ToastUtils.showShort("请稍候重试")
} catch (e: Exception) {
LogUtils.e(e.localizedMessage)
}
}
// called many times, but I can't observe the PagedList change
override fun loadAfter(
params: LoadParams<Long>,
callback: LoadCallback<Long, WorkComment>
) {
val res = RetrofitUtil.getInstanceWithJwt().create(WorkCommentApi::class.java)
.listComment(
workId, 1, params.requestedLoadSize
)
res.enqueue(object : retrofit2.Callback<TResult> {
override fun onFailure(call: Call<TResult>, t: Throwable) {
}
override fun onResponse(call: Call<TResult>, response: Response<TResult>) {
callback.onResult(
response.body()!!.toList(WorkComment::class.java),
params.key + 1
)
}
})
}
override fun loadBefore(
params: LoadParams<Long>,
callback: LoadCallback<Long, WorkComment>
) {
}
}
}
}
Retrofit Api
interface WorkCommentApi {
/**
* list comment
*/
#GET("public/work/comment")
fun listComment(#Query("workId") workId: Long, #Query("current") current: Long, #Query("size") size: Int): Call<TResult>
}
I want to know what should I do to observe the LiveData<PagedList> change
This is happening because each time you call vm.listComment(workId), the object you first bound in activity is killed and new object is created.
You can use Transformations with MediatorLiveData.
Activity:
viewModel.logout().observe(this, Observer {
// do here
})
ViewModel:
class RepackViewModel(app: Application) : BaseViewModel(app) {
// IMPORTANT - Mediator
val logout = MediatorLiveData<PagedList<WorkComment>>()
fun logout() : LiveData<PagedList<WorkComment>> = logout
init {
// IMPORTANT - passes repo update to activity
logout.addSource(repo.getLogoutResponse()) { logout.postValue(it) }
}
}
Repository:
class BaseRepository(val app: Application) {
private val logout = MutableLiveData<PagedList<WorkComment>>()
fun getLogoutResponse(): LiveData<PagedList<WorkComment>> = logout
override fun create(): DataSource<Long, WorkComment> {
//when you get your data
logout.value = // your value
}
You need to have your work id be mutable data to be observed by the transformation. so whenever you update your work, id, it will fetch comments. Like Thus...
ViewModel:
val workIdMutableLiveData: MutableLiveData<Int> = MutableLiveData(workId)
//This performs the meat of the work to display the items in the recyclerview
var commentsList = Transformations.switchMap(workIdMutableLiveData) { workId ->
val config = PagedList.Config.Builder()
.setEnablePlaceholders(false)
.setPageSize(pagingLimit)
.build()
val pagedListBuilder = LivePagedListBuilder<Long, WorkComment>(BaseDataSourceFactory(workId), config)
pagedListBuilder.build()
}
Then in your activity, observe
yourViewModel.commentsList.observe(this, Observer { list ->
list ?: return#Observer
adapter.submitList(list)
yourRecyclerView.adapter = adapter
})
Whenever you update the workIdMutableLiveData by doing a
workIdMutableLiveData.postValue(workId)
...The recyclerview will update. Your recyclerview must inherit from PagedListAdapter.
After testing, I knew list couldn't be observed when it has inner data change, like add(), remove(). etc.
It only be observed when its reference has been changed, like create or assignment operation:
list.value = null
So I couldn't observe the data change of LiveData<List>
I got a Product class, which is constructed with a code. This code is made to call the open food facts API to instanciate all the class variables. The fact is the API call is an asynchronous function. So in my main Thread, when i try to access my object parameters, it's empty. Since i cannot interrupt the main Thread, how am i suppose to make a callback on my object instanciation ?
Here is the code
Product.kt
class Product(code: Long) {
val client = OkHttpClient()
var name: String? = null
var imageUrl: String? = null
var packerCode: Int? = null
var packerCity: String? = null
var lat: Int? = null
var long: Int? = null
init {
run("https://fr.openfoodfacts.org/api/v0/produit/$code.json")
}
private fun run(url: String) {
val request = Request.Builder()
.url(url)
.build()
client.newCall(request).enqueue(object : Callback {
override fun onFailure(call: Call?, e: IOException) {}
override fun onResponse(call: Call?, response: Response){
val jsonData = response.body()?.string()
val Jobject = JSONObject(jsonData)
name = Jobject.getJSONObject("product").getString("product_name")
}
})
}
}
GameActivity.kt
class GameActivity : AppCompatActivity(){
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_game)
val textView: TextView = findViewById(R.id.productName) as TextView
val product = Product(3564700014677)
// Product.name empty
textView.text = product.name
}
First of all we assume you don't want to use MVVM or so architecture. But I really recommend you to read about android architecture components like ViewModel, LiveData stuffs to understand how data flow should be done in android applications.
Coming to basics(Not very clean way), We have to create a interface and pass the reference to Product class and on success you have use the reference to call the activity to update the textview.
Step 1: create interface
interface ProductListener
{
fun onSuccess()
}
Step 2: Implement ProductListener in your activity
class GameActivity : AppCompatActivity(),ProductListener {
...
...
...
override fun onSuccess() {
}
}
Step 3: Pass the listener/activity reference to the Product class
val product = Product(3564700014677, this) //inside your activity
class Product(code: Long, var listener: ProductListener) {
...
...
private fun run(url: String) {
val request = Request.Builder()
.url(url)
.build()
client.newCall(request).enqueue(object : Callback {
override fun onFailure(call: Call?, e: IOException) {}
override fun onResponse(call: Call?, response: Response){
val jsonData = response.body()?.string()
val Jobject = JSONObject(jsonData)
name = Jobject.getJSONObject("product").getString("product_name")
// invoke listener here to let activity know the response
listener.onSuccess()
}
})
}
}
Step 4: Update the textview inside the onSuccess() implementation of activity
class GameActivity : AppCompatActivity(),ProductListener {
...
...
...
override fun onSuccess() {
textView.text = product.name
}
}
This might be a little weird but I can't figure out how to pass data after an API call. I'm very new to object oriented programming.
fetchedTags is null after the fetchTags() call. How do I get the data?
For example:
class MainActivity : AppCompatActivity() {
var fetchedTags: List<Tags>? = null
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
fetchTags()
println(fetchedTags[0].name)
fetchBooks()
makeMapOutOfTagsAndBooks()
}
fun fetchTags () {
//some processing
val request = Request.Builder().url(url).build()
client.newCall(request).enqueue(object : Callback {
override fun onResponse(call: Call?, response: Response?) {
val jsonData = response?.body()?.string()
val gson = GsonBuilder().setPrettyPrinting().create()
val tagList: List<Tags> = gson.fromJson(jsonData, object : TypeToken<List<Tags>>() {}.type)
fetchedTags = tagList
}
}
fetchBooks()
println(fetchedTags[0].name)
Your are calling println(fetchedTags[0].name) right after making your http call, since it's asynchronous your list is null at this point.
call it on your onResponse function