ViewBinding原理分析

以下是在Activity下使用ViewBinding，布局文件中有兩個(gè)TextView，分別是tv1、tv2,

//Activity代碼
val binding = ActivityMainBinding.inflate(layoutInflater)//1
setContentView(binding.root)

//布局文件activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/tv1"
                ....../>

    <TextView
        android:id="@+id/tv2"
        ....../>

</androidx.constraintlayout.widget.ConstraintLayout>

• 注釋1：構(gòu)建與布局文件相關(guān)聯(lián)的ActivityMainBinding，文件是在編譯時(shí)生成的，路徑為build/generated/data_binding_base_class_source_out/com/yang/myapplication/databinding，其中com/yang/myapplication為我的項(xiàng)目名稱。

public final class ActivityMainBinding implements ViewBinding {//1
  @NonNull
  private final ConstraintLayout rootView;//2
  @NonNull
  public final TextView tv1;//3
  @NonNull
  public final TextView tv2;//4
  ......
  @Override
  @NonNull
  public ConstraintLayout getRoot() {/5
    return rootView;
  }

  @NonNull
  public static ActivityMainBinding inflate(@NonNull LayoutInflater inflater) {//6
    return inflate(inflater, null, false);
  }

  public static ActivityMainBinding inflate(@NonNull LayoutInflater inflater,
      @Nullable ViewGroup parent, boolean attachToParent) {
    View root = inflater.inflate(R.layout.activity_main, parent, false);//7
    if (attachToParent) {
      parent.addView(root);
    }
    return bind(root);
  }

• 注釋1：實(shí)現(xiàn)ViewBinding接口，該接口只定義了getRoot接口函數(shù);
• 注釋2：rootView是xml對(duì)應(yīng)的根view對(duì)象;
• 注釋3、4：tv1、tv2對(duì)應(yīng)xml中的兩個(gè)TextView控件;
• 注釋5：實(shí)現(xiàn)getRoot函數(shù);
• 注釋6：對(duì)應(yīng)ActivityMainBinding.inflate(layoutInflater)調(diào)用的函數(shù);
• 注釋7：將布局轉(zhuǎn)化成View;

ActivityMainBinding.inflate(layoutInflater)最終會(huì)走到以下代碼

 public static ActivityMainBinding bind(@NonNull View rootView) {
      int id;
      id = R.id.tv1;
      TextView tv1 = ViewBindings.findChildViewById(rootView, id);//1
      if (tv1 == null) {
        break missingId;
      }

      id = R.id.tv2;
      TextView tv2 = ViewBindings.findChildViewById(rootView, id);//2
      if (tv2 == null) {
        break missingId;
      }

      return new ActivityMainBinding((ConstraintLayout) rootView, tv1, tv2);//3
    }
  }

• 注釋1、2：獲取對(duì)應(yīng)id的控件對(duì)象，當(dāng)外部通過(guò)binding.tv1獲取的就是這里的對(duì)象;
• 注釋3：將控件對(duì)象維護(hù)在ActivityMainBinding中;

//ViewBindings.findChildViewById
    public static <T extends View> T findChildViewById(View rootView, @IdRes int id) {
        if (!(rootView instanceof ViewGroup)) {
            return null;
        }
        final ViewGroup rootViewGroup = (ViewGroup) rootView;
        final int childCount = rootViewGroup.getChildCount();
        for (int i = 0; i < childCount; i++) {
            final T view = rootViewGroup.getChildAt(i).findViewById(id);//1
            if (view != null) {
                return view;
            }
        }
        return null;
    }

• 注釋1：最終也是使用View的findViewById來(lái)獲取view對(duì)象，跟Activity.findViewById類似。

Viewbinding優(yōu)點(diǎn)

1. 對(duì)比kotlin-extension，可以控制訪問(wèn)作用域，kotlin-extension可以訪問(wèn)不是該布局下的view;
2. 對(duì)比f(wàn)indViewById，減少模板代碼；
3. 兼容Kotlin、Java；
4. 官方推薦。

Viewbinding缺點(diǎn)

1. 增加編譯時(shí)間，因?yàn)閂iwBinding是在編譯時(shí)生成的，而且會(huì)增加包的體積；
2. include的布局文件無(wú)法直接引用，需要給include給id值，然后間接引用；

以上分析有不對(duì)的地方，請(qǐng)指出，互相學(xué)習(xí)，謝謝哦！