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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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。