Kotlin Android Extensions提供布局空指针

Question

There is a fairly simple scenario that is giving me quite a bit of trouble. I'm making a very simple Activity with an embedded fragment. This fragment is simply a Gridview that displays some images. The issue comes when referring to the Gridview using Kotlin extensions to refer directly to an XML id. What is the issue here? Does kotlinx not work on static fragments?

Error:

java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.android_me/com.example.android.android_me.ui.MainActivity}: java.lang.IllegalStateException: gridview_all_parts must not be null
 Caused by: java.lang.IllegalStateException: gridview_all_parts must not be null                                                                                  at com.example.android.android_me.ui.MasterListFragment.onActivityCreated(MasterListFragment.kt:22)

Fragment with offensive line of code

import kotlinx.android.synthetic.main.fragment_master_list.*

    class MasterListFragment: Fragment() {

        override fun onCreateView(inflater: LayoutInflater?, container: ViewGroup?, savedInstanceState: Bundle?): View? {
            val layoutView = inflater?.inflate(R.layout.fragment_master_list, container, false)
            return layoutView
        }

        override fun onActivityCreated(savedInstanceState: Bundle?) {
            //If this is removed, code runs
            gridview_all_parts.adapter = MasterListAdapter(activity, AndroidImageAssets.getAll())
            super.onActivityCreated(savedInstanceState)
        }
    }

Fragment Layout:

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

Parent Activity Layout

<?xml version="1.0" encoding="utf-8"?>
<!--have tried both class:= and android:name:=-->
<fragment xmlns:android="http://schemas.android.com/apk/res/android"
    class="com.example.android.android_me.ui.MasterListFragment"
    android:id="@+id/fragment_masterlist"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    />

Parent Activity

class MainActivity: AppCompatActivity(){

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

Answer 1

To use the extensions in Fragment, you need to use with your layoutView . This should work: layoutView.gridview_all_parts.adapter = MasterListAdapter(activity, AndroidImageAssets.getAll())

You can make your layoutView global in this case.

UPDATED EXPLANATION Its something to do with view inflating. Like in butterknife, we need to bind the inflated view in case of fragment/recyclerView, similarly in case of kotlin, we need that inflate view for accessing the views in the xml.

Quoting from official documentation ,

Importing synthetic properties It is convenient to import all widget properties for a specific layout in one go:

import kotlinx.android.synthetic.main.<layout>.*

Thus if the layout filename is activity_main.xml , we'd import

kotlinx.android.synthetic.main.activity_main.*.

If we want to call the synthetic properties on View, we should also import

kotlinx.android.synthetic.main.activity_main.view.*.

Once we do that, we can then invoke the corresponding extensions, which are properties named after the views in the XML file.

Answer 2

For everyone who found this problem in the other case.

The NPE from Android Extensions also occurs when we write the adapter for RecyclerView (especially: writing CustomViewHolder).

See LayoutContainer for more info and how to fix this issue.

1. Add this into your module level gradle

apply plugin: 'kotlin-android-extensions'

android {
    androidExtensions {
        experimental = true
    }
    // your config
    defaultConfig {}
}

2. Your adapter

class MainViewHolder(override val containerView: View) :
    RecyclerView.ViewHolder(containerView), 
    LayoutContainer { // Extends this
        fun bind(item: News) = containerView.apply {
            tv_item_title.text = item.title
        }
}