前言
最近断断续续地把项目的界面部分的代码由JAva改成了Kotlin编写，并且如果应用了kotlin-android-extensions插件,一个显而易见的好处是再也不用写 findViewById()来实例化你的控件对象了，直接操作你在布局文件里的id即可，这一点我感觉比butterknife做的还简洁友好。
Activity
import android.support.v7.app.AppCompatActivityimport android.os.Bundleimport kotlinx.android.synthetic.main.activity_main.*class MainActivity : AppCompatActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) setContentView(R.layout.activity_main) textview.text="hello world" }} 其中kotlinx.android.synthetic.main.activity_main.*是kotlin-android-extensions插件自动生成的。下面我们来解析下原理。因为kotlin也是一门JVM语言，最近也会和java一样编译成class字节码，所以我们直接来反编译看看生成的java文件。
选择Decompile,解析出来的代码如下
public final class MainActivity extends AppCompatActivity { private HashMap _$_findViewCache; protected void onCreate(@Nullable Bundle savedInstanceState) { super.onCreate(savedInstanceState); this.setContentView(2131296284); TextView var10000 = (TextView)this._$_findCachedViewById(id.textview); Intrinsics.checkExpressionValueIsNotNull(var10000, "textview"); var10000.setText((CharSequence)"hello world"); } public View _$_findCachedViewById(int var1) { if (this._$_findViewCache == null) { this._$_findViewCache = new HashMap(); } View var2 = (View)this._$_findViewCache.get(var1); if (var2 == null) { var2 = this.findViewById(var1); this._$_findViewCache.put(var1, var2); } return var2; } public void _$_clearFindViewByIdCache() { if (this._$_findViewCache != null) { this._$_findViewCache.clear(); } }} 可以很清楚看到最终还是调用了findViewById() ，不过获取View对象直接调用的是findCachedViewById,并且创建一个 HashMap 进行View对象的缓存，避免每次调用 View 时都会重新调用findViewById()进行查找。
Fragment
再来看下Fragment中的使用：
import android.os.Bundleimport android.support.v4.app.Fragmentimport android.view.LayoutInflaterimport android.view.Viewimport android.view.ViewGroupimport kotlinx.android.synthetic.main.fragment_blank.*class BlankFragment : Fragment() { override fun onCreateView(inflater: LayoutInflater, container: ViewGroup?, savedInstanceState: Bundle?): View? { return inflater.inflate(R.layout.fragment_blank, container, false) } override fun onViewCreated(view: View, savedInstanceState: Bundle?) { super.onViewCreated(view, savedInstanceState) textview_fra.text="hello world" }} 反编译后代码如下
public final class BlankFragment extends Fragment { private HashMap _$_findViewCache; @Nullable public View onCreateView(@NotNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { Intrinsics.checkParameterIsNotNull(inflater, "inflater"); return inflater.inflate(2131296285, container, false); } public void onViewCreated(@NotNull View view, @Nullable Bundle savedInstanceState) { Intrinsics.checkParameterIsNotNull(view, "view"); super.onViewCreated(view, savedInstanceState); TextView var10000 = (TextView)this._$_findCachedViewById(id.textview_fra); Intrinsics.checkExpressionValueIsNotNull(var10000, "textview_fra"); var10000.setText((CharSequence)"hello world"); } public View _$_findCachedViewById(int var1) { if (this._$_findViewCache == null) { this._$_findViewCache = new HashMap(); } View var2 = (View)this._$_findViewCache.get(var1); if (var2 == null) { View var10000 = this.getView(); if (var10000 == null) { return null; } var2 = var10000.findViewById(var1); this._$_findViewCache.put(var1, var2); } return var2; } public void _$_clearFindViewByIdCache() { if (this._$_findViewCache != null) { this._$_findViewCache.clear(); } } // $FF: synthetic method public void onDestroyView() { super.onDestroyView(); this._$_clearFindViewByIdCache(); }} 可以看到最终是通过调用getView().findViewById()来进行控件的实例化。
看下getView()源码
@Nullable public View getView() { return this.mView; } 再看下mView成员变量的赋值时机：
void performCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) { if (this.mChildFragmentManager != null) { this.mChildFragmentManager.noteStateNotSaved(); } this.mPerformedCreateView = true; this.mViewLifecycleOwner = new LifecycleOwner() { public Lifecycle getLifecycle() { if (Fragment.this.mViewLifecycleRegistry == null) { Fragment.this.mViewLifecycleRegistry = new LifecycleRegistry(Fragment.this.mViewLifecycleOwner); } return Fragment.this.mViewLifecycleRegistry; } }; this.mViewLifecycleRegistry = null; this.mView = this.onCreateView(inflater, container, savedInstanceState); if (this.mView != null) { this.mViewLifecycleOwner.getLifecycle(); this.mViewLifecycleOwnerLiveData.setValue(this.mViewLifecycleOwner); } else { if (this.mViewLifecycleRegistry != null) { throw new IllegalStateException("Called getViewLifecycleOwner() but onCreateView() returned null"); } this.mViewLifecycleOwner = null; } } 可以看到mView其实就是onCreateView()的返回值，所以我们不能在onCreateView()方法里操作控件ID的方式操作View对象，会产生空指针异常。建议在onViewCreated()方法里使用。
其他（动态布局）
除了Activity和Fragment，我们用的最多的UI布局当属Adapter了，kotlin-android-extensions也提供了对这一类动态布局的支持。因为这一功能是实现性质的，默认关闭，我们需要手动打开，在build.gradle中开启：
androidExtensions { experimental = true} 然后再recycler.adapter中使用如下：
import kotlinx.android.extensions.LayoutContainerimport kotlinx.android.synthetic.main.item_recyclerview.*class MyAdapter(val context: Context, val data: List<String>) : RecyclerView.Adapter<MyAdapter.ViewHolder>() { override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder { val view = LayoutInflater.from(context).inflate(R.layout.item_recyclerview, parent, false) return ViewHolder(view) } override fun onBindViewHolder(holder: ViewHolder, position: Int) { holder.name_tv.text = data[position] holder.itemView.setOnClickListener { Toast.makeText(context,"点击了第$position 项",Toast.LENGTH_SHORT).show() } } override fun getItemCount(): Int { return data.size } inner class ViewHolder(itemView: View) : RecyclerView.ViewHolder(itemView), LayoutContainer { override val containerView: View = itemView }} 可以看到相比Activity和Fragment，我们的ViewHolder需要多实现一个接口LayoutContainer。看下它的源码：
public final class ViewHolder extends android.support.v7.widget.RecyclerView.ViewHolder implements LayoutContainer { @NotNull private final View containerView; private HashMap _$_findViewCache; @NotNull public View getContainerView() { return this.containerView; } public ViewHolder(@NotNull View itemView) { Intrinsics.checkParameterIsNotNull(itemView, "itemView"); super(itemView); this.containerView = itemView; } public View _$_findCachedViewById(int var1) { if (this._$_findViewCache == null) { this._$_findViewCache = new HashMap(); } View var2 = (View)this._$_findViewCache.get(var1); if (var2 == null) { View var10000 = this.getContainerView(); if (var10000 == null) { return null; } var2 = var10000.findViewById(var1); this._$_findViewCache.put(var1, var2); } return var2; } public void _$_clearFindViewByIdCache() { if (this._$_findViewCache != null) { this._$_findViewCache.clear(); } } } 总结
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