45. Databinding原理

setContentView

在Activity中使用的时候，先绑定布局文件，那么这个操作都做了什么？我们从源码中找答案

ActivityMainBinding binding= DataBindingUtil.setContentView(this,R.layout.activity_main)

在这个方法中，注意contentView是系统布局DecorView中的content节点，layoutId是当前Activity的布局id

    public static <T extends ViewDataBinding> T setContentView(@NonNull Activity activity,
            int layoutId, @Nullable DataBindingComponent bindingComponent) {
        activity.setContentView(layoutId);
        View decorView = activity.getWindow().getDecorView();
        ViewGroup contentView = (ViewGroup) decorView.findViewById(android.R.id.content);
        return bindToAddedViews(bindingComponent, contentView, 0, layoutId);
    }

这里childrenAdded正常情况下必须是1，因为parent是系统的content节点，而使用databing,我们自己写的布局的根结点需要是一个<layout>

    private static <T extends ViewDataBinding> T bindToAddedViews(DataBindingComponent component,
            ViewGroup parent, int startChildren, int layoutId) {
        final int endChildren = parent.getChildCount();
        final int childrenAdded = endChildren - startChildren;
        if (childrenAdded == 1) {
            final View childView = parent.getChildAt(endChildren - 1);
            return bind(component, childView, layoutId);
        } else {
            final View[] children = new View[childrenAdded];
            for (int i = 0; i < childrenAdded; i++) {
                children[i] = parent.getChildAt(i + startChildren);
            }
            return bind(component, children, layoutId);
        }
    }

继续往下就进入到了com.项目路径.DataBinderMapperImpl类中，在getDataBinder中可以看到,返回了ActivityMainBindingImpl

  @Override
  public ViewDataBinding getDataBinder(DataBindingComponent component, View view, int layoutId) {
    int localizedLayoutId = INTERNAL_LAYOUT_ID_LOOKUP.get(layoutId);
    if(localizedLayoutId > 0) {
      final Object tag = view.getTag();
      if(tag == null) {
        throw new RuntimeException("view must have a tag");
      }
      switch(localizedLayoutId) {
        case  LAYOUT_ACTIVITYMAIN: {
          if ("layout/activity_main_0".equals(tag)) {
            return new ActivityMainBindingImpl(component, view);
          }
          throw new IllegalArgumentException("The tag for activity_main is invalid. Received: " + tag);
        }
      }
    }
    return null;
  }

而在构造ActivityMainBindingImpl的时候可以看出，在mapBindings方法中，将布局中的子节点保存在了数组中，于是通过binding对象就可以取到这些节点了如：binding.tv1

    public ActivityMainBindingImpl(@Nullable androidx.databinding.DataBindingComponent bindingComponent, @NonNull View root) {
        this(bindingComponent, root, mapBindings(bindingComponent, root, 3, sIncludes, sViewsWithIds));
    }