为什么ViewStub是默认不显示的？是懒加载？

关于为什么默认不显示，看源码：直接设置了gone。

 public ViewStub(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
        super(context);

        final TypedArray a = context.obtainStyledAttributes(attrs,
                R.styleable.ViewStub, defStyleAttr, defStyleRes);
        saveAttributeDataForStyleable(context, R.styleable.ViewStub, attrs, a, defStyleAttr,
                defStyleRes);

        mInflatedId = a.getResourceId(R.styleable.ViewStub_inflatedId, NO_ID);
        mLayoutResource = a.getResourceId(R.styleable.ViewStub_layout, 0);
        mID = a.getResourceId(R.styleable.ViewStub_id, NO_ID);
        a.recycle();

//---------------------这里------------------
        setVisibility(GONE);
        setWillNotDraw(true);
    }

 关于懒加载。

看on Measure的代码。-----设置了宽高为0.

  @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        setMeasuredDimension(0, 0);
    }

 包括他的 inflate() 代码中 inflateViewNoAdd(parent);会去判断代码id是否相等

 private View inflateViewNoAdd(ViewGroup parent) {
        final LayoutInflater factory;
        if (mInflater != null) {
            factory = mInflater;
        } else {
            factory = LayoutInflater.from(mContext);
        }
        final View view = factory.inflate(mLayoutResource, parent, false);

        if (mInflatedId != NO_ID) {
            view.setId(mInflatedId);
        }
        return view;
    }