Kotlin如何直接使用控件ID原理详析

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.AppCompatActivity
import android.os.Bundle
import 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.Bundle
import android.support.v4.app.Fragment
import android.view.LayoutInflater
import android.view.View
import android.view.ViewGroup
import 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.LayoutContainer
import 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();
  }

}
}

来源：https://juejin.im/post/5c161975f265da612b137e83