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Kotlin直接使用控件ID原理解析

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"
    }
}

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其中kotlinx.android.synthetic.main.activity_main.*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();
      }

   }
}
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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"
    }
}

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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();
   }
}
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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;
    }
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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;
        }

    }

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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ActivityFragment,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
}
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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
    }
}
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可以看到相比ActivityFragment,我们的ViewHolder需要多实现一个接口LayoutContainer。看下它的源码:

/**
 * A base interface for all view holders supporting Android Extensions-style view access.
 */
public interface LayoutContainer {
    /** Returns the root holder view. */
    public val containerView: View?
}
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只有一个对象,我们需要设置这个值,用来手动指定root holder view,也就是ViewHolderitemView。反编译看下ViewHolder生成的java代码就好理解了,此处的getContainerView作用相当于Fragment的getView(),只不过Fragment自带了这个获取布局根View的方法,而Adapter需要再去通过LayoutContainer接口实现而已。

   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();
         }

      }
   }
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