android - Kotlin Android Extensions 提供布局空指针

Question

有一个相当简单的场景给我带来了很多麻烦。我正在使用嵌入式 fragment 制作一个非常简单的 Activity 。这个 fragment 只是一个显示一些图像的 Gridview。当使用 Kotlin 扩展引用 Gridview 以直接引用 XML id 时，就会出现问题。 这里有什么问题？ kotlinx 对静态 fragment 不起作用吗？

错误:

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)

带有冒犯性代码行的 fragment

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 布局:

<?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"/>

父 Activity 布局

<?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"
    />

家长 Activity

class MainActivity: AppCompatActivity(){

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

Answer 1

要在 Fragment 中使用扩展，您需要使用您的 layoutView .这应该有效:layoutView.gridview_all_parts.adapter = MasterListAdapter(activity, AndroidImageAssets.getAll())

在这种情况下，您可以将 layoutView 设为全局。

更新说明它与 View 膨胀有关。就像在butterknife中一样，我们需要在fragment/recyclerView的情况下绑定(bind)膨胀 View ，在kotlin的情况下，我们需要那个膨胀 View 来访问xml中的 View 。

引自 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.