Develop Reference

Errors with android:id fields in layouts not anymore recognized

Following an upgrade of android-studio, kotlin plugins and sourceCompatibility to Java 8 in an android project, many layout ids are not any more recognized (but strangely not all).
It looks like field name with underscores are not recognized in binding.
For instance:
whith the following declaration in the layout
<TextView
android:id="#+id/alert_hist_item__equipement"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="5px"
android:text="#string/Device"
android:textStyle="bold"
/>
in the code
alert_hist_item__equipement.text = "x"
is not recognized but
alertHistItemEquipement.text = "x"
is recognized.
If I change the field names in the layouts to equivalent camel case names (either only in the code, with the names in the layout still having underscores; or both in code and layout) the fields are recognized.
It is so painful for many reasons:
I have dozens of layouts all with id names containing underscore
I find names with underscore easier to read (and my team too)
it was never specified that android:id fields should follow certain rules
is this possible to change that behavior ? Could it be a bug ?

Please add following id 'kotlin-android-extensions' to you app level gradle.
plugins {
id 'kotlin-android-extensions'
}
According to Kotlin documentation they are deprecated but they are still working for me.
As and alternate they provide us with data and view binding. For you View Binding should work!
here is the example:
add following to your app level gradle:
buildFeatures{
viewBinding true
}
then in you activity declare as following (Assuming you activity name is ManuelYguel)
class ManuelYguel : AppCompatActivity() {
private lateinit var binding: ActivityManuelYguelBinding
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = ActivityManuelYguelBinding.inflate(layoutInflater)
setContentView(binding.root)
binding.alertHistItemEquipement.text = "x"
}
}

It is a sad and unfortunate constraint that comes from the new autogenerated binding class. This is specified in https://developer.android.com/topic/libraries/data-binding/expressions
it says:
Note: The binding class converts IDs to camel case.

Unresolved reference - activity does not recognize synthetic imports in android studio v4

last night I noticed I'm not able to change the attributes of elements in the layout from my main activity
so I built a new project and I had the same problem there too.
I could not find out what was wrong with my android studio so I'd appreciate it if someone with the same problem helps me out.
as you see in the picture when I call a defined view from the layout in my activity its not recognized
the error will be: Unresolved reference: txtHello

Kotlin Synthetic imports not working ?
Well, there's always the age-old alternative:
val foo: TextView = findViewById(R.id.your_id)
I believe synthetics have been deprecated and I guess support for it has just now been completely removed
Alternatively, you can make use of ViewBinding, which is another alternative.
Enable it in your build.gradle:
android {
...
buildFeatures {
viewBinding true
}
}
This generates a Binding object for your layout, so you can make use of it like this:
private lateinit var binding: YourLayoutNameBinding
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = YourLayoutNameBinding.inflate(layoutInflater)
val view = binding.root
setContentView(view)
}
then you have access to all views on the layout, through the binding:
binding.name.text = "foo"

An alternative you can look at is ViewBinding, a concept in Android that was introduced recently.
You should take a look for this
https://developer.android.com/topic/libraries/view-binding
You cannot set view id directly for your use in app, instead you need findViewById(R.id.idTextHello).setOnClickListener()
That's how views are bind in application.

import kotlinx.android.synthetic.main.activity_main.*
but dont forget to apply kotlin extension

You are trying to access your views via Kotlin Synthetics, which have been deprecated.
You can use ViewBinding instead.
Enable it in your module level build.gradle:
android {
...
buildFeatures {
viewBinding true
}
}
And then in your activity you can access views like :
private lateinit var _binding: ActivityNameBinding
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
_binding = ActivityNameBinding(layoutInflater)
val view = binding.root
setContentView(view)
}
and then you can access your views like this:
_binding.btn_start.setOnClickListener {
...
}
For detailed understanding of ViewBinding, you can look into this article:
https://medium.com/geekculture/android-viewbinding-over-findviewbyid-389401b41706

Should we use butterknife or findViewById() with Kotlin Android project as we can directly access views by id

I'm able to access my layout views(like button, TextView, EditText etc) directly inside the activity by their ids defined in layout xml file in Kotlin android project.
So, do we need to use findviewbyId(), or butterknife lib in kotlin android project?
StudentActivity.kt
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
val studentViewModel = getStudentViewModel()
updateButton.setOnClickListener {
val name = nameEditText.text.toString()
val age = ageEditText.text.toString()
val subject = subjectEditText.text.toString()
studentViewModel.updateStudentRecord(
Student(
name,
Integer.parseInt(age),
subject
)
)
}
}
}```

ButterKnife is an old solution for view binding. It has less boilerplate code than old findviewbyId way but because of annotation processors it impacts build time speed and doesn't provide Null safety and Type safety. A better solution is kotlinx.android.synthetic that you used in your example but it has some problems too. For example, if you set your content view to a layout, then type an id that only exists in a different layout, the IDE lets you autocomplete and add the new import statement. Unless the developer specifically checks to make sure their import statements only import the correct views, there is no safe way to verify that this won’t cause a runtime issue. As everything is global, one has to be careful to make sure that they only use views they are expecting and ignore the autocomplete. DataBinding and ViewBinding are the best solutions for now. They are similar at first look. Both generate binding classes that you can use to reference views directly with support Null safety and Type safety, but there are differences:
DataBinding approach needs you to add <layout> tag to your XML layout in order to enable the data binding process
ViewBinding doesn’t support layout variables or layout expressions, so it can’t be used to bind layouts with data in XML
ViewBinding is faster than DataBinding in build time because it does n't use annotation processors.

I think you will not use anymore, just if you want? But I believe that is not because of the Synthetic Accessors, it's because of the Data Bindings and the announced this year, View Binding

Nope, here is the the magic of kotlin. Just use your id from the layout file (xml) and directly use it. Like:
button.setOnClickListener {}
and so on. hope it will help.

Kotlin on Android alters library variable instead of defining a new variable

today I started a new Android project with Kotlin support. But as soon as I launch it on my phone, it is disconnected from Wi-Fi (Application has INTERNET and ACCESS_NETWORK_STATE permissons but no socket code yet). Here is the code:
package org.arch.cast
import android.os.Bundle
import android.support.v7.app.AppCompatActivity
class MainActivity : AppCompatActivity() {
internal var dummy = 10
internal var channel = 20
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
}
}
After experimenting a bit I noticed that one of my variables wasn't marked as "not used" even though it was not used.
This is the only class the project contains right now, no other service or activity exists so nothing is referring channel variable anywhere in the project. I also added a dummy variable to show that it is marked as not used. I noticed that the problem was its name and it was actually not defining it but altering another variable from a library I didn't import. And since it was related to Wi-Fi, phone was disconnecting.
Sure everything works when I change the variable name but this should not be the solution, it is only a temporary workaround. So the question is, how can I prevent Kotlin from altering the variable and make it actually define it in my class?

Kotlin Android View Binding: findViewById vs Butterknife vs Kotlin Android Extension

I'm trying to figure out the best way to do Android View Binding in Kotlin. It seems like there are a few of options out there:
findViewById
val button: Button by lazy { findViewById<Button>(R.id.button) }
Butterknife
https://github.com/JakeWharton/butterknife
#BindView(R.id.button) lateinit var button: Button
Kotlin Android Extensions
https://kotlinlang.org/docs/tutorials/android-plugin.html
import kotlinx.android.synthetic.main.activity_main.*
I'm pretty familiar with findViewById and Butterknife in java land, but what are the pros and cons of each view binding approach in Kotlin?
Does Kotlin Android Extensions play well with the RecyclerView + ViewHolder pattern?
Also how does Kotlin Android Extensions handle view binding for nested views via include?
ex: For an Activity using activity_main.xml, how would View custom1 be accessed?
activity_main.xml
<...>
<include layout="#layout/custom" android:id="#+id/custom" />
</>
custom.xml
<...>
<View android:id="#+id/custom1" ... />
<View android:id="#+id/custom2" ... />
</>

There are a lot of ways to access views in Android. A quick overview:
My advise would be:
findViewById: old school. Avoid.
ButterKnife: old school, but less boilerplate and some added functionality. Still lacks compile time safety. Avoid if possible.
Kotlin Synthetic: really a elegant cached version of findViewbyId. Better performance and way less boilerplate but still no (real) compile time safety. Will be no longer supported from Kotlin 1.8. Avoid if possible.
ViewBinding: Google's recommendation nowadays. It's faster than databinding and prevents logic errors inside XML (hard to debug). I use this option for all new projects.
Data Binding: most versatile option, since it allows code inside XML. Still used on a lot of existing projects. But can slow down build times (uses annotation processor just like ButterKnife) and a lot of logic inside XML has become a bit of anti pattern.
See also: https://www.youtube.com/watch?v=Qxj2eBmXLHg
Funny to note that Jake Wharton (original author of ButterKnife) has now joined Google and works on ViewBinding.

kotlin-android-extensions is better for Kotlin. ButterKnife is also good but kotlin-android-extensions is a better and smart choice here.
Reason : Kotlin uses synthetic properties and those are called on demand using caching function(Hence slight fast Activity/Fragment loading) while ButterKnife binds all view at a time on ButterKnife.bind()(that consumes slight more time). With Kotlin you don't even need to use annotation for binding the views.
Yes it also plays good with RecyclerView + ViewHolder pattern, you just need to import kotlinx.android.synthetic.main.layout_main.view.*(if layout_main.xml is Activity/Fragment layout file name).
You do not need to do any extra effort for layout imported using include. Just use id of imported views.
Have a look at following official documentation notes:
Kotlin Android Extensions is a plugin for the Kotlin compiler, and it does two things:
Adds a hidden caching function and a field inside each Kotlin Activity. The method is pretty small so it doesn't increase the size of APK much.
Replaces each synthetic property call with a function call.
How this works is that when invoking a synthetic property, where the receiver is a Kotlin Activity/Fragment class that is in module sources, the caching function is invoked. For instance, given
class MyActivity : Activity()
fun MyActivity.a() {
this.textView.setText(“”)
}
a hidden caching function is generated inside MyActivity, so we can use the caching mechanism.
However in the following case:
fun Activity.b() {
this.textView.setText(“”)
}
We wouldn't know if this function would be invoked on only Activities from our sources or on plain Java Activities also. As such, we don’t use caching there, even if MyActivity instance from the previous example is the receiver.
Link to above documentation page
I hope it helps.

I can't flag this question as a duplicate, as you're asking multiple things that have been answered / discussed under different questions.
What are the pros and cons of each view binding approach in Kotlin?
This has been discussed here.
How does Kotlin Android Extensions handle view binding for nested views via include? ex: For an Activity using activity_main.xml, how would View custom1 be accessed?
All Kotlin Android Extensions does is call findViewById for you. See here.
Does Kotlin Android Extensions play well with the RecyclerView + ViewHolder pattern?
Yes, it does. However, you have to use save the Views you get from it into properties, as there is no cache for them like in Activities or Fragments. See here.
If you still have unanswered questions, feel free to ask for clarification.

Take care of using
val button: Button by lazy { findViewById<Button>(R.id.button) }
I already confront the problem when the view is destroyed, and as the instance of your fragment survive(I think in the case of acitivities it doesn't apply), they hold the lazy property referencing to the old view.
Example:
You have an static value in the layout, let say android:text="foo"
//calling first time
override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
button.setText("bar")
// button is called for the first time,
// then button is the view created recently and shows "bar"
}
Then the fragment get destroyed because you replace it, but then ou comeback and it regenerated callin onCreateView again.
//calling second after destroyed
override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
button.setText(Date().time.toString())
//button is already set, then you are setting the value the to old view reference
// and in your new button the value won't be assigned
// The text showed in the button will be "foo"
}

Now there is a fourth option which is called View Binding, available with Android Studio 3.6 Carnary 11
Quoting from docs.
View Binding
View binding is a feature that allows you to more easily write code
that interacts with views. Once view binding is enabled in a module,
it generates a binding class for each XML layout file present in that
module. An instance of a binding class contains direct references to
all views that have an ID in the corresponding layout.
In most cases, view binding replaces findViewById.
Differences from findViewById
View binding has important advantages over using findViewById:
Null safety: Since view binding creates direct references to views, there's no risk of a null pointer exception due to an invalid
view ID. Additionally, when a view is only present in some
configurations of a layout, the field containing its reference in the
binding class is marked with #Nullable.
Type safety: The fields in each binding class have types matching the views they reference in the XML file. This means that
there's no risk of a class cast exception.
Differences from the data binding library
View binding and the data binding library both generate binding
classes that you can use to reference views directly. However, there
are notable differences:
The data binding library processes only data binding layouts created using the <layout> tag.
View binding doesn't support layout variables or layout expressions, so it can't be used to bind layouts with data in XML.
Usage
To take advantage of View binding in a module of your project, add the
following line to its build.gradle file:
android {
viewBinding.enabled = true
}
For example, given a layout file called result_profile.xml:
<LinearLayout ... >
<TextView android:id="#+id/name" />
<ImageView android:cropToPadding="true" />
<Button android:id="#+id/button"
android:background="#drawable/rounded_button" />
</LinearLayout>
In this example, you can call ResultProfileBinding.inflate() in an
activity:
private lateinit var binding: ResultProfileBinding
#Override
fun onCreate(savedInstanceState: Bundle) {
super.onCreate(savedInstanceState)
binding = ResultProfileBinding.inflate(layoutInflater)
setContentView(binding.root)
}
The instance of the binding class can now be used to reference any of
the views:
binding.name.text = viewModel.name
binding.button.setOnClickListener { viewModel.userClicked() }

if you using datainding library. you should databinding view binding.
because it is explict more then kotlin-extensions
p.s findviewbyid is very inconvenience and boilerplate code