Android - 在 Kotlin 中声明和使用视图

Question

I am trying to use kotlin for android and trying to declare Linearlayout like this:

internal var linlay_SuccessfulPayment : LinearLayout = null!!
internal var linlay_failPayment : LinearLayout

linlay_SuccessfulPayment = findViewById(R.id.linlay_SuccessfulPayment) as LinearLayout
linlay_failPayment = findViewById(R.id.linlay_failPayment) as LinearLayout

But in log I am getting this :

Caused by: kotlin.KotlinNullPointerException
                                                                         at com.example.activities.PaymentResult.<init>(Result.kt:14)
                                                                         at java.lang.Class.newInstance(Native Method)
                                                                         at android.app.Instrumentation.newActivity(Instrumentation.java:1096)
                                                                         at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3122)
                                                                         at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415) 
                                                                         at android.app.ActivityThread.access$1100(ActivityThread.java:229) 
                                                                         at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821) 
                                                                         at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                         at android.os.Looper.loop(Looper.java:148) 
                                                                         at android.app.ActivityThread.main(ActivityThread.java:7329) 
                                                                         at java.lang.reflect.Method.invoke(Native Method) 
                                                                         at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) 
                                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 

Please help me out.

Answer 1

Your problem is with nullability and this would be a good place to use the lateinit keyword (documentation) :

private lateinit var linlay_SuccessfulPayment: LinearLayout
private lateinit var linlay_failPayment: LinearLayout

This way you define a non-nullable var but delay the initialisation, which you can do in onCreate().
You do have to initialise it before accessing it or you will get a PropertyNotInitialisedException .

A second option is lazy initialisation usingproperty delegation :

private var linlay_SuccessfulPayment: LinearLayout by Delegates.lazy { findViewById(R.id.linlay_SuccessfulPayment) as LinearLayout }

This way the view is initialised only the first time it is used and you have everything in one place.

Answer 2

Using the !! operator checks if the value it's applied to is null , and if it is, it throws a KotlinNullPointerException ; otherwise it returns the value with a non-nullable type. Writing down null!! is basically shorthand for throw KotlinNullPointerException(...) .

For Android Views (and other cases where an object is initialized in a special init method instead of the constructor), you should use the lateinit keyword:

internal lateinit var linlay_SuccessfulPayment: LinearLayout
internal lateinit var linlay_failPayment: LinearLayout

This lets you have non-nullable properties in your Activity that you don't initialize when the constructor is called, but only later, in the onCreate method. You do however, in this case, take responsibility for initializing the variables before using them the first time, otherwise you'll get an exception at runtime.

Answer 3

只需在 gradle.build 中添加 kotlin 扩展，您就不需要初始化参数：

apply plugin: 'kotlin-android-extensions'

Answer 4

Initialize like below

internal var linlay_SuccessfulPayment : LinearLayout ?= null
internal var linlay_failPayment : LinearLayout ?= null

and use it like below

linlay_SuccessfulPayment = findViewById<LinearLayout>(R.id.linlay_SuccessfulPayment)
linlay_failPayment = findViewById<LinearLayout>(R.id.linlay_failPayment)

Answer 5

I'm just starting with Kotlin and trying to learn it. I bind views like this:

var myBtn: Button = findViewById<Button>(R.id.btnMyButton) as Button

or

var btnLogin: Button = findViewById<Button>(R.id.btnLogin)

Answer 6

You can use like below:-

 var linlay_SuccessfulPayment : LinearLayout? = null
 var linlay_failPayment : LinearLayout? = null


linlay_SuccessfulPayment = findViewById(R.id.linlay_SuccessfulPayment) as LinearLayout
linlay_failPayment = findViewById(R.id.linlay_failPayment) as LinearLayout

Answer 7

You can use apply plugin: 'kotlin-android-extensions' in your app level build.gradle for binding view without using the findViewById() boilerplate code.
eg linlay_failPayment.setParams() // any thing you want with only view_id
For more information check this kotlin-view-binding