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，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，谢谢大家对的支持。