LayoutInflater.from(context).inflater(R.layout.main_activity,null);

public static LayoutInflater from(Context context) {
    LayoutInflater LayoutInflater =
        (LayoutInflater) context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    if (LayoutInflater == null) {
      throw new AssertionError("LayoutInflater not found.");
    }
    return LayoutInflater;
  }

@Override
  public Object getSystemService(String name) {
    return SystemServiceRegistry.getSystemService(this, name);
  }

  /**
   * Gets a system service from a given context.
   */
  public static Object getSystemService(ContextImpl ctx, String name) {
    ServiceFetcher<?> fetcher = SYSTEM_SERVICE_FETCHERS.get(name);
    return fetcher != null ? fetcher.getService(ctx) : null;
  }

registerService(Context.LAYOUT_INFLATER_SERVICE, LayoutInflater.class,
        new CachedServiceFetcher<LayoutInflater>() {
      @Override
      public LayoutInflater createService(ContextImpl ctx) {
        return new PhoneLayoutInflater(ctx.getOuterContext());
      }});

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root) {
    return inflate(resource, root, root != null);
  }

public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot) {
    final Resources res = getContext().getResources();
    if (DEBUG) {
      Log.d(TAG, "INFLATING from resource: \"" + res.getResourceName(resource) + "\" ("
          + Integer.toHexString(resource) + ")");
    }

    final XmlResourceParser parser = res.getLayout(resource);
    try {
      return inflate(parser, root, attachToRoot);
    } finally {
      parser.close();
    }
  }

public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot) {
    synchronized (mConstructorArgs) {
      final Context inflaterContext = mContext;
      //快看，View的构造函数中的attrs就是这个！！！
      final AttributeSet attrs = Xml.asAttributeSet(parser);

      //这个数组很重要，从名字就可以看出来，这是构造函数要用到的参数
      mConstructorArgs[0] = inflaterContext;
      View result = root;

      try {
        // 找到根节点，找到第一个START_TAG就跳出while循环，
        // 比如<TextView>是START_TAG，而</TextView>是END_TAG
        int type;
        while ((type = parser.next()) != XmlPullParser.START_TAG &&
            type != XmlPullParser.END_DOCUMENT) {
          // Empty
        }

        if (type != XmlPullParser.START_TAG) {
          throw new InflateException(parser.getPositionDescription()
              + ": No start tag found!");
        }
        //获取根节点的名称
        final String name = parser.getName();

        //判断是否用了merge标签
        if (TAG_MERGE.equals(name)) {
          if (root == null || !attachToRoot) {
            throw new InflateException("<merge /> can be used only with a valid "
                + "ViewGroup root and attachToRoot=true");
          }
          //解析
          rInflate(parser, root, inflaterContext, attrs, false);
        } else {
          // 这里需要调用到PhoneLayoutInflater中的方法，获取到根节点对应的View
          final View temp = createViewFromTag(root, name, inflaterContext, attrs);

          ViewGroup.LayoutParams params = null;
          //如果指定了parentView(root)，则生成layoutParams，
          //并且在后面会将temp添加到root中
          if (root != null) {
            params = root.generateLayoutParams(attrs);
            if (!attachToRoot) {
              temp.setLayoutParams(params);
            }
          }

          // 上面解析了根节点，这里解析根节点下面的子节点
          rInflateChildren(parser, temp, attrs, true);

          if (root != null && attachToRoot) {
            root.addView(temp, params);
          }

          if (root == null || !attachToRoot) {
            result = temp;
          }
        }

      } catch (Exception e) {

      } finally {
        // Don't retain static reference on context.
        mConstructorArgs[0] = lastContext;
        mConstructorArgs[1] = null;
      }

      return result;
    }
  }

View createViewFromTag(View parent, String name, Context context, AttributeSet attrs,
      boolean ignoreThemeAttr) {
      ......
      ......
    try {
      ......

      if (view == null) {
        final Object lastContext = mConstructorArgs[0];
        mConstructorArgs[0] = context;
        try {
          //不含“.” 说明是系统自带的控件
          if (-1 == name.indexOf('.')) {
            view = onCreateView(parent, name, attrs);
          } else {
            view = createView(name, null, attrs);
          }
        } finally {
          mConstructorArgs[0] = lastContext;
        }
      }

      return view;
    } catch (InflateException e) {
      throw e;
      ......
    }
  }

public final View createView(String name, String prefix, AttributeSet attrs)
      throws ClassNotFoundException, InflateException {
    //1.通过传入的类名，获取该类的构造器

    Constructor<? extends View> constructor = sConstructorMap.get(name);
    Class<? extends View> clazz = null;

    try {
      if (constructor == null) {

        clazz = mContext.getClassLoader().loadClass(
            prefix != null ? (prefix + name) : name).asSubclass(View.class);

        if (mFilter != null && clazz != null) {
          boolean allowed = mFilter.onLoadClass(clazz);
          if (!allowed) {
            failNotAllowed(name, prefix, attrs);
          }
        }
        constructor = clazz.getConstructor(mConstructorSignature);
        constructor.setAccessible(true);
        sConstructorMap.put(name, constructor);
      } else {

        if (mFilter != null) {
          Boolean allowedState = mFilterMap.get(name);
          if (allowedState == null) {     
            clazz = mContext.getClassLoader().loadClass(
                prefix != null ? (prefix + name) : name).asSubclass(View.class);    
            boolean allowed = clazz != null && mFilter.onLoadClass(clazz);
            mFilterMap.put(name, allowed);
            if (!allowed) {
              failNotAllowed(name, prefix, attrs);
            }
          } else if (allowedState.equals(Boolean.FALSE)) {
            failNotAllowed(name, prefix, attrs);
          }
        }
      }

      //2.通过获得的构造器，创建View实例
      Object[] args = mConstructorArgs;
      args[1] = attrs;

      final View view = constructor.newInstance(args);
      if (view instanceof ViewStub) {
        final ViewStub viewStub = (ViewStub) view;
        viewStub.setLayoutInflater(cloneInContext((Context) args[0]));
      }
      return view;

    } catch (NoSuchMethodException e) {
     ......
    } 
  }

static final Class<?>[] mConstructorSignature = new Class[] {
      Context.class, AttributeSet.class};