在 Kotlin 无法“ findViewById”。出现错误“类型推断失败”

当我试图通过 id 查找 RecycleView时，出现了以下错误。

错误:- 类型推断失败: 没有足够的信息推断参数 < strong > T

Error

密码:

class FirstRecycleViewExample : AppCompatActivity() {
val data = arrayListOf<String>()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.first_recycleview)


val recycler_view =  findViewById(R.id.recycler_view) as RecyclerView ///IN THIS LINE I AM GETTING THE ERROR


data.add("First Data")
data.add("Second Data")
data.add("Third Data")
data.add("Forth Data")
data.add("Fifth Data")


//creating our adapter
val adapter = CustomRecycleAdapter(data)


//now adding the adapter to recyclerview
recycler_view.adapter = adapter
}
}
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最佳答案

Try something like:

val recyclerView = findViewById<RecyclerView>(R.id.recycler_view)

You can use Kotlin Android Extensions too for that. Check the doc here.
With it, you can call recycler_view directly in your code.

Kotlin Android Extensions:

  • In your app gradle.build add apply plugin: 'kotlin-android-extensions'
  • In your class add import for import kotlinx.android.synthetic.main.<layout>.* where <layout> is the filename of your layout.
  • That's it, you can call recycler_view directly in your code.

How does it work? The first time that you call recycler_view, a call to findViewById is done and cached.

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Usually Kotlin can infer your type using the info provided in the brackets.

In this case, it can't so you would have to specify it explicitly like this 

findViewById<RecyclerView>(R.id.recycler_view)

Not really sure about the type though but that is how you should specify it

I'd like to add that, using Anko, you can further simplify your code like this:

val recycler_view : RecyclerView =  find(R.id.recycler_view)
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You're on API level 26, where the return type of findViewById is now a generic T instead of View and can therefore be inferred. You can see the relevant changelog here.

So you should be able to do this:

val recycler_view = findViewById<RecyclerView>(R.id.recycler_view)

Or this:

val recycler_view: RecyclerView = findViewById(R.id.recycler_view)
小开

Try this:

Here bind generic function use to bind view. its general function it's can use for any view.

class MyActivity : AppCompatActivity() {
private lateinit var recycler_view : RecyclerView
override fun onCreate(savedInstanceState: Bundle?) {
...
recycler_view = bind(R.id.recycler_view)
}
}


fun <T : View> Activity.bind(@IdRes res : Int) : T {
@Suppress("UNCHECKED_CAST")
return findViewById(res) as T
}
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You can ignore giving ids to variables in kotlin. You just need to use a plugin

Paste this in your gradle

 apply plugin: 'kotlin-android-extensions'

and then you don't need to assign ids to var, you can just do it like

 recycler_view.setAdapter(youradapter);
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DEPRECATED

In Kotlin we can get the id of the view without the use of the findViewById syntax.

For example we are using the following the layout from that we will get the id of the view and perform operation

Layout

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">


<TextView
android:id="@+id/welcomeMessage"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:text="Hello World!"/>


</FrameLayout>

We are able to find the id of the view by using the following code

override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
 

welcomeMessage.text = "Hello Kotlin!" ////WE ARE GETTING THE IDS WITHOUT USING THE FINDVIEWBYID syntax
}

HOW?

To be able to use it, you need an special import (the one I write below), but the IDE is able to auto-import it. Couldn’t be easier!

import kotlinx.android.synthetic.main.YOUR_LAYOUT_NAME.*/// HERE "YOUR_LAYOUT_NAME" IS YOUR LAYOUT NAME WHICH U HAVE INFLATED IN onCreate()/onCreateView()

We need to import this property to get the id of the view without the use of the findviewbyid syntax.

For more information on this topic than please refer this link :- Click here

UPDATE

Now the kotlin-android-extensions is deprecated , instead of using the kotlin provided binding , we can use now the View-Binding and it works same as above solution, Just you need to use the below in your app gradle

android {
...
buildFeatures {
viewBinding true
}
}
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Add this line for your code as Whatever you are using controller.

val tvHello = findViewById<TextView>(R.id.tvHello)
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You don't even have to create an object to do things with a view. You can just use the id. For example, Kotlin

recycle_view.adapter = adapter

is same as, Java

RecycleView recycle_view = (RecycleView) findViewById(recycle_view);
recycle_view.setAdapter(adapter);
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Was unable to access the ui components like OP faced looks like below was needed inside app gradle:

plugins {
...
id 'kotlin-android-extensions'
}

Even though after adding this line android studio was still not able to auto resolve the imports for kotlin synthetics so you may need to invalidate cache and restart.

If still not working then import manually depending on views

  • activity /fragment view: import kotlinx.android.synthetic.main.<your_activity_view>.*
  • normal views : import kotlinx.android.synthetic.main.<your_layout_view>.view.*

Update - 2

If you are getting this info warning -

Warning: The 'kotlin-android-extensions' Gradle plugin is deprecated.

plugin 'kotlin-android-extensions' is going to be deprecated. So add below in your app gradle

android {
...
buildFeatures {
viewBinding true
}
}

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