Listview的添加头部源码分析.基于Android 27
当我使用listView.addHeaderView(view);的时候.

    public void addHeaderView(View v) {
        //调用下面的代码
        addHeaderView(v, null, true);
    }

public void addHeaderView(View v, Object data, boolean isSelectable) {
        final FixedViewInfo info = new FixedViewInfo();
        //把头布局的View和数据封装到FixedViewInfo中
        info.view = v;
        info.data = data;
        info.isSelectable = isSelectable;
        //把FixedViewInfo添加到集合中
        mHeaderViewInfos.add(info);
        mAreAllItemsSelectable &= isSelectable;

        // Wrap the adapter if it wasn't already wrapped.
        //如果mAdapter不为空
        if (mAdapter != null) {
            if (!(mAdapter instanceof HeaderViewListAdapter)) {
                //看下面的方法调用
                wrapHeaderListAdapterInternal();
            }

            // In the case of re-adding a header view, or adding one later on,
            // we need to notify the observer.
            if (mDataSetObserver != null) {
                mDataSetObserver.onChanged();
            }
        }
    }

    /** @hide */
    protected void wrapHeaderListAdapterInternal() {
        //把mAdapter传值到wrapHeaderListAdapterInternal并赋值给mAdapter.
        mAdapter = wrapHeaderListAdapterInternal(mHeaderViewInfos, mFooterViewInfos, mAdapter);
    }

    /** @hide */
    protected HeaderViewListAdapter wrapHeaderListAdapterInternal(
            ArrayList<ListView.FixedViewInfo> headerViewInfos,
            ArrayList<ListView.FixedViewInfo> footerViewInfos,
            ListAdapter adapter) {
        //把HeaderView的集合,FooterView的集合和原来的Adapter作为参数传给HeaderViewListAdapter
        //最终new HeaderViewListAdapter 对象进行返回.^_^以前的版本没有这么多调用的.
        return new HeaderViewListAdapter(headerViewInfos, footerViewInfos, adapter);
    }

看到这里可以看出来.其实就是在原来的Adapter进行一层包装.下面分析HeaderViewListAdapter
Listview的Adapter最重要的几个方法:
getCount

    public int getCount() {
        if (mAdapter != null) {
            //count的为头布局的条目加上底布局的条目加上原来Adapter的条目
            return getFootersCount() + getHeadersCount() + mAdapter.getCount();
        } else {
            return getFootersCount() + getHeadersCount();
        }
    }

getView()

    public View getView(int position, View convertView, ViewGroup parent) {
        // Header (negative positions will throw an IndexOutOfBoundsException)
        //如果当期的位置小于头布局的条目.直接取头布局的View
        int numHeaders = getHeadersCount();
        if (position < numHeaders) {
            return mHeaderViewInfos.get(position).view;
        }

        // Adapter
        //当前的position - 头布局的数量 ,让数量重新从0开始计算.
        final int adjPosition = position - numHeaders;
        int adapterCount = 0;
        if (mAdapter != null) {
            adapterCount = mAdapter.getCount();
            if (adjPosition < adapterCount) {
                //从0开始计算的position.如果不从0开始,原来的Adapter的数据会发生错乱.
                return mAdapter.getView(adjPosition, convertView, parent);
            }
        }

        // Footer (off-limits positions will throw an IndexOutOfBoundsException)
        //adjPosition - adapterCount 还是重新从0开始计算.从不然就发生角标越界异常.
        return mFooterViewInfos.get(adjPosition - adapterCount).view;
    }

看到这里.可以看出Listview原本是不支持添加头部布局.只是系统给我写好了.并做好了处理.让我们无感知而已.