Android源码分析：HeaderViewListAdapter

     对于手机开发，我一直坚持的是“用iPhone的方式开发iPhone应用，用Android的方式开发Android应用”。但如何去把握iPhone或者Android的方式呢？在iPhone开发时可能需要看大量Apple文档。而Android在看文档的同时还可以看看源码。这源码中有时可以发现一些内部实现加深理解，有时可以受到某些可以在应用中使用的启发。总之源码是一个需要去发掘的“矿场”，时而不时地都会发现一些“金子”。

 

    在Android开发的时候经常会有一些列表数据分组显示的需求，对于iPhone来说修改下模型数据并设置下风格就可以轻松搞定（如下图）。但对于Android这种需求就会有些痛苦了。因为系统提供的ListAdapter无法提供这种支持，因此只能自定义Adapter。为了实现该功能，Adapter通常就开始充斥很多特殊处理以及各种硬编码。如何用Android的方式设计和实现一个自定义Adapter呢？源码中的HeaderViewListAdapter可能会提供些答案或者启发。

            

 

图1：Group分隔的iPhone表格

注释版本的HeaderViewListAdapter.java如下：

/*如果List需要有标题就会使用该ListAdapter。该ListAdapter包装了另一个ListAdapter并保存着
标题视图和相关数据对象。该类是作为基类，在代码中可能不需要直接使用。
*/
// 类名有点不确切，其实是支持Header和Footter的
public class HeaderViewListAdapter implements WrapperListAdapter, Filterable {

    private final ListAdapter mAdapter;

    // These two ArrayList are assumed to NOT be null.
    // They are indeed created when declared in ListView and then shared.
    ArrayList<ListView.FixedViewInfo> mHeaderViewInfos;
    ArrayList<ListView.FixedViewInfo> mFooterViewInfos;

    // Used as a placeholder in case the provided info views are indeed null.
    // Currently only used by some CTS tests, which may be removed.
    static final ArrayList<ListView.FixedViewInfo> EMPTY_INFO_LIST =
        new ArrayList<ListView.FixedViewInfo>();

    boolean mAreAllFixedViewsSelectable;

    /// 这是个很有趣的概念，之后我们会一起了解
    private final boolean mIsFilterable;

    public HeaderViewListAdapter(ArrayList<ListView.FixedViewInfo> headerViewInfos,
                                 ArrayList<ListView.FixedViewInfo> footerViewInfos,
                                 ListAdapter adapter) {
        mAdapter = adapter;
        mIsFilterable = adapter instanceof Filterable;
        /// 很好的处理，这里实际是使用了一种Null对象模式
        if (headerViewInfos == null) {
            mHeaderViewInfos = EMPTY_INFO_LIST;
        } else {
            mHeaderViewInfos = headerViewInfos;
        }
        if (footerViewInfos == null) {
            mFooterViewInfos = EMPTY_INFO_LIST;
        } else {
            mFooterViewInfos = footerViewInfos;
        }

        mAreAllFixedViewsSelectable =
                areAllListInfosSelectable(mHeaderViewInfos)
                && areAllListInfosSelectable(mFooterViewInfos);
    }

 

    /// 支持多个标题栏哦
    public int getHeadersCount() {
        return mHeaderViewInfos.size();
    }

    /// 支持多个尾注栏哦
    public int getFootersCount() {
        return mFooterViewInfos.size();
    }

    public boolean isEmpty() {
        return mAdapter == null || mAdapter.isEmpty();
    }

    /// FixedViewInfo名字很贴切，这是相对于包装Adapter中的可变数据命名的
    private boolean areAllListInfosSelectable(ArrayList<ListView.FixedViewInfo> infos) {
        if (infos != null) {
            for (ListView.FixedViewInfo info : infos) {
                if (!info.isSelectable) {
                    return false;
                }
            }
        }
        return true;
    }

    /// 支持标题删减
    public boolean removeHeader(View v) {
        for (int i = 0; i < mHeaderViewInfos.size(); i++) {
            ListView.FixedViewInfo info = mHeaderViewInfos.get(i);
            if (info.view == v) {
                mHeaderViewInfos.remove(i);
                mAreAllFixedViewsSelectable =
                        areAllListInfosSelectable(mHeaderViewInfos)
                        && areAllListInfosSelectable(mFooterViewInfos);
                return true;
            }
        }

        return false;
    }

    /// 支持尾注删减
    public boolean removeFooter(View v) {
        for (int i = 0; i &lt; mFooterViewInfos.size(); i++) {
            ListView.FixedViewInfo info = mFooterViewInfos.get(i);
            if (info.view == v) {
                mFooterViewInfos.remove(i);

                mAreAllFixedViewsSelectable =
                        areAllListInfosSelectable(mHeaderViewInfos)
                        && areAllListInfosSelectable(mFooterViewInfos);

                return true;
            }
        }

        return false;
    }

    /// 标题视图、尾注视图和普通的数据视图，一视同仁
    public int getCount() {
        if (mAdapter != null) {
            return getFootersCount() + getHeadersCount() + mAdapter.getCount();
        } else {
            return getFootersCount() + getHeadersCount();
        }
    }

    public boolean areAllItemsEnabled() {
        if (mAdapter != null) {
            return mAreAllFixedViewsSelectable && mAdapter.areAllItemsEnabled();
        } else {
            return true;
        }
    }

    public boolean isEnabled(int position) {
        // Header (negative positions will throw an ArrayIndexOutOfBoundsException)
        int numHeaders = getHeadersCount();
        if (position &lt; numHeaders) {
            return mHeaderViewInfos.get(position).isSelectable;
        }

        // Adapter
        final int adjPosition = position - numHeaders;
        int adapterCount = 0;
        if (mAdapter != null) {
            adapterCount = mAdapter.getCount();
            if (adjPosition &lt; adapterCount) {
                return mAdapter.isEnabled(adjPosition);
            }
        }

        // Footer (off-limits positions will throw an ArrayIndexOutOfBoundsException)
        return mFooterViewInfos.get(adjPosition - adapterCount).isSelectable;
    }

    /// 需要分段处理
    public Object getItem(int position) {
        // Header (negative positions will throw an ArrayIndexOutOfBoundsException)
        int numHeaders = getHeadersCount();
        if (position &lt; numHeaders) {
            return mHeaderViewInfos.get(position).data;
        }

        // Adapter
        final int adjPosition = position - numHeaders;
        int adapterCount = 0;
        if (mAdapter != null) {
            adapterCount = mAdapter.getCount();
            if (adjPosition &lt; adapterCount) {
                return mAdapter.getItem(adjPosition);
            }
        }

        // Footer (off-limits positions will throw an ArrayIndexOutOfBoundsException)
        return mFooterViewInfos.get(adjPosition - adapterCount).data;
    }

    /// 这可不包括Header和Footer
    public long getItemId(int position) {
        int numHeaders = getHeadersCount();
        if (mAdapter != null && position >= numHeaders) {
            int adjPosition = position - numHeaders;
            int adapterCount = mAdapter.getCount();
            if (adjPosition < adapterCount) {
                return mAdapter.getItemId(adjPosition);
            }
        }
        return -1;
    }

    public boolean hasStableIds() {
        if (mAdapter != null) {
            return mAdapter.hasStableIds();
        }
        return false;
    }

    /// 需要分段处理，其中Header和Footer视图已经事先存储在ListView.FixedViewInfo对象中了
    public View getView(int position, View convertView, ViewGroup parent) {
        // Header (negative positions will throw an ArrayIndexOutOfBoundsException)
        int numHeaders = getHeadersCount();
        if (position &lt; numHeaders) {
            return mHeaderViewInfos.get(position).view;
        }

        // Adapter
        final int adjPosition = position - numHeaders;
        int adapterCount = 0;
        if (mAdapter != null) {
            adapterCount = mAdapter.getCount();
            if (adjPosition &lt; adapterCount) {
                return mAdapter.getView(adjPosition, convertView, parent);
            }
        }

        // Footer (off-limits positions will throw an ArrayIndexOutOfBoundsException)
        return mFooterViewInfos.get(adjPosition - adapterCount).view;
    }

    /// 支持同一个List中不同类型的视图
    public int getItemViewType(int position) {
        int numHeaders = getHeadersCount();
        if (mAdapter != null && position >= numHeaders) {
            int adjPosition = position - numHeaders;
            int adapterCount = mAdapter.getCount();
            if (adjPosition &lt; adapterCount) {
                return mAdapter.getItemViewType(adjPosition);
            }
        }

        return AdapterView.ITEM_VIEW_TYPE_HEADER_OR_FOOTER;
    }

    public int getViewTypeCount() {
        if (mAdapter != null) {
            return mAdapter.getViewTypeCount();
        }
        return 1;
    }

    public void registerDataSetObserver(DataSetObserver observer) {
        if (mAdapter != null) {
            mAdapter.registerDataSetObserver(observer);
        }
    }

    public void unregisterDataSetObserver(DataSetObserver observer) {
        if (mAdapter != null) {
            mAdapter.unregisterDataSetObserver(observer);
        }
    }

    /// 这有点意思，有待发掘
    public Filter getFilter() {
        if (mIsFilterable) {
            return ((Filterable) mAdapter).getFilter();
        }
        return null;
    }
    public ListAdapter getWrappedAdapter() {
        return mAdapter;
    }
}