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();
   }

  }
 }

总结

以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，如果有疑问大家可以留言交流，谢谢大家对我们的支持。