Kotlin直接使用控件ID原理解析

最近斷斷續續地把項目的界面部分的代碼由JAva改爲了Kotlin編寫，而且若是應用了kotlin-android-extensions插件,一個顯而易見的好處是不再用寫 findViewById()來實例化你的控件對象了，直接操做你在佈局文件裏的id便可，這一點我感受比butterknife作的還簡潔友好。

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"
    }
}

複製代碼

其中kotlinx.android.synthetic.main.activity_main.*是kotlin-android-extensions插件自動生成的。下面咱們來解析下原理。由於kotlin也是一門JVM語言，最近也會和java同樣編譯成class字節碼，因此咱們直接來反編譯看看生成的java文件。

選擇Decompile,解析出來的代碼以下

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();
      }

   }
}
複製代碼

能夠很清楚看到最終仍是調用了findViewById()，不過獲取View對象直接調用的是findCachedViewById,而且建立一個 HashMap 進行View對象的緩存，避免每次調用 View 時都會從新調用findViewById()進行查找。

Fragment

再來看下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"
    }
}

複製代碼

反編譯後代碼以下

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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能夠看到最終是經過調用getView().findViewById()來進行控件的實例化。 看下getView()源碼

@Nullable
    public View getView() {
        return this.mView;
    }
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再看下mView成員變量的賦值時機：

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;
        }

    }

複製代碼

能夠看到mView其實就是onCreateView()的返回值，因此咱們不能在onCreateView()方法裏操做控件ID的方式操做View對象，會產生空指針異常。建議在onViewCreated()方法裏使用。

其餘（動態佈局）

除了Activity和Fragment，咱們用的最多的UI佈局當屬Adapter了，kotlin-android-extensions也提供了對這一類動態佈局的支持。由於這一功能是實現性質的，默認關閉，咱們須要手動打開，在build.gradle中開啓：

androidExtensions {
    experimental = true
}
複製代碼

而後再recycler.adapter中使用以下：

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,"點擊了第$position 項",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
    }
}
複製代碼

能夠看到相比Activity和Fragment，咱們的ViewHolder須要多實現一個接口LayoutContainer。看下它的源碼：

/** * 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?
}
複製代碼

只有一個對象，咱們須要設置這個值，用來手動指定root holder view，也就是ViewHolder的itemView。反編譯看下ViewHolder生成的java代碼就好理解了，此處的getContainerView做用至關於Fragment的getView()，只不過Fragment自帶了這個獲取佈局根View的方法，而Adapter須要再去經過LayoutContainer接口實現而已。

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();
         }

      }
   }
複製代碼