Java中SeparatedListAdapter类的实现

需要引入的包有：

import android.content.Context;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Adapter;
import android.widget.ArrayAdapter;
import android.widget.BaseAdapter;

import java.util.LinkedHashMap;
import java.util.Map;

以下开始类的构建：

public class SeparatedListAdapter extends BaseAdapter {
    public final Map<String, Adapter> sections = new LinkedHashMap<>();
    public final ArrayAdapter<String> headers;
    public final static int TYPE_SECTION_HEADER = 0；

首先，声明一个终态的Map<String, Adapter>型变量sections，键值对的类型实参为String和Adapter，且创建一个新的LinkedHashMap()型的变量，赋给变量sections。和一个ArrayAdapter型的变量headers，且类型实参为String。LinkedHashMap 是 Map 的实现类，Map是一个公有接口，LinkedHashMap等一系列Map是公有类，继承于HashMap，调用Map接口。

public SeparatedListAdapter(Context context) {
        headers = new ArrayAdapter<String>(context, R.layout.list_header);
    }

    public void addSection(String section, Adapter adapter) {
        this.headers.add(section);
        this.sections.put(section, adapter);
    }

接着，将addSection方法中传入的参数分别写入headers和sections中。

其中，headers类型为ArrayAdapter，sections类型为LinkedHashMap，结构分别为：ArrayAdapter(Context, int)和LinkedHashMap(int, float), add(sections)表示将section插入队列headers队尾，put(section, adapter)表示将字符串section置于adapter键位。

public Object getItem(int position) {
        for (Object section : this.sections.keySet()) {
            Adapter adapter = sections.get(section);
            int size = adapter.getCount() + 1;
            // check if position inside this section
            if (position == 0)
                return section;
            if (position < size)
                return adapter.getItem(position - 1);
            // otherwise jump into next section
            position -= size;
        }
        return null;
    }

获取Item方法，在sections.keySet()中遍历keys，（其中keySet()返回全部的keys）将adapter的数量赋给size，如果Item的位置参数为0，则返回第一个，否则，倒叙遍历查找

public int getCount() {
        // total together all sections, plus one for each section header
        int total = 0;
        for (Adapter adapter : this.sections.values())
            total += adapter.getCount() + 1;
        return total;
    }

递归得到sections数量

public int getViewTypeCount() {
        // assume that headers count as one, then total all sections
        int total = 1;
        for (Adapter adapter : this.sections.values())
            total += adapter.getViewTypeCount();
        return total;
    }

返回adapter的types of view 的数量

public int getItemViewType(int position) {
        int type = 1;
        for (Object section : this.sections.keySet()) {
            Adapter adapter = sections.get(section);
            int size = adapter.getCount() + 1;
            // check if position inside this section
            if (position == 0)
                return TYPE_SECTION_HEADER;
            if (position < size)
                return type + adapter.getItemViewType(position - 1);

            // otherwise jump into next section
            position -= size;
            type += adapter.getViewTypeCount();
        }
        return -1;
    }

递归得到position位置的viewtype

public boolean isEnabled(int position) {
        return (getItemViewType(position) != TYPE_SECTION_HEADER);
    }

    public View getView(int position, View convertView, ViewGroup parent) {
        int sectionnum = 0;
        for (Object section : this.sections.keySet()) {
            Adapter adapter = sections.get(section);
            int size = adapter.getCount() + 1;

            // check if position inside this section
            if (position == 0)
                return headers.getView(sectionnum, convertView, parent);
            if (position < size)
                return adapter.getView(position - 1, convertView, parent);

            // otherwise jump into next section
            position -= size;
            sectionnum++;
        }
        return null;
    }

    public long getItemId(int position) {
        return position;
    }