最近斷斷續續地把專案的介面部分的程式碼由JAva改成了Kotlin編寫，並且如果應用了kotlin-android-extensions外掛,一個顯而易見的好處是再也不用寫 findViewById()來例項化你的控制元件物件了，直接操作你在佈局檔案裡的id即可，這一點我感覺比butterknife做的還簡潔友好。

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"
    }
}

複製程式碼
 

下面我們來解析下原理。因為kotlin也是一門JVM語言，最近也會和java一樣編譯成class位元組碼，所以我們直接來反編譯看看生成的java檔案。

選擇Decompile,解析出來的程式碼如下

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();
      }

   }
}
複製程式碼

可以很清楚看到最終還是呼叫了findViewById()，不過獲取View物件直接呼叫的是findCachedViewById,並且建立一個 HashMap 進行View物件的快取，避免每次呼叫 View 時都會重新呼叫findViewById()進行查詢。

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"
    }
}

複製程式碼

反編譯後代碼如下

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();
   }
}
複製程式碼

可以看到最終是通過呼叫getView().findViewById()來進行控制元件的例項化。 看下getView()原始碼

    @Nullable
    public View getView() {
        return this.mView;
    }
複製程式碼

再看下mView成員變數的賦值時機：

  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;
        }

    }

複製程式碼

可以看到mView其實就是onCreateView()的返回值，所以我們不能在onCreateView()方法裡操作控制元件ID的方式操作View物件，會產生空指標異常。建議在onViewCreated()方法裡使用。

其他

比較遺憾的是kotlin-android-extensions貌似還不支援各種Adapter等動態佈局內的直接使用控制元件ID方法。