android - 当我们扩展它时，BaseAdapter 是如何工作的？

Question

当我们扩展 Baseadapter 时，这些方法是如何工作的。

public int getCount() 
public Object getItem(int position) 
public long getItemId(int position) 
public View getView(int position, View convertView, ViewGroup parent) 

因为如果我们有 some_return_type_from_getCount() ，那么 getView() 将从中得到什么以及当我们返回 getView()_return_type 谁还有 get_the_return_value_of getView() 。

我对这些方法一头雾水。

Answer 1

下面的帖子是根据我的理解写的。所以如果你想改进它而不是批评某些特定点，请随意。

private ArrayList<HashMap<String, String>> mProjectsList = new ArrayList<HashMap<String, String>>(); (你可以使用任何实际包含数据的游标或数组，并且你想使用适配器绑定(bind)它)

public int getCount() -> 为您提供适配器中存在的总元素(如数组的大小)

@Override
    public int getCount() {
        // TODO Auto-generated method stub
        return mProjectsList.size();
    }

public Object getItem(int position) -> 告诉点击了哪个项目，只需按照我指定位置的方式返回这里即可。它实际上会返回您单击的整个 biwe 及其所有属性，这就是为什么我们只在此处返回我们单击的位置的 View ，以便告诉 BASEADAPTER 类该 View 已被点击

@Override
    public Object getItem(int position) {
        // TODO Auto-generated method stub
        return mProjectsList.get(position);
    }

public long getItemId(int position) 将提供您在点击某些列表项时要返回的主要 ID。当您实际单击 ListView 的某些项目时，它会返回长格式的主键和 int 格式的位置这两个东西..

实际上从这个 getItemId() 方法返回主键。

我们通常在数据库中将主键指定为“_id”，因此当我们使用简单的适配器而不是扩展 baseadapter 类时，它会自动返回 _id 字段作为长格式的主 ID。但是我们必须在BaseAdapter这里手动指定我们要返回的内容

@Override
    public long getItemId(int position) {
        // TODO Auto-generated method stub
        return Long.parseLong(mProjectsList.get(position).get("ID")) ;
// retuning my Primary id from the arraylist by
    }

public View getView(int position, View convertView, ViewGroup parent) 实际创建您绑定(bind)自定义布局的 View

@Override
    public View getView(int position, View convertView, ViewGroup parent) {
        // TODO Auto-generated method stub


        //**position**  index of the item whose view we want.
        //**convertView**   the old view to reuse, if possible. Note: You should 
//check that this view is non-null and of an appropriate type before using. If it is 
//not possible to convert this view to display the correct data, this method can create a 
//new view.
  //                        (Lets see if you have total 100 elements in listview , but 
//currently only 10 are visible in screen. So it will create only 10 items at a time as 
//only those are visible.
                //and when you will scroll that listView it will use these same 10 
//Views(elemnts in rows to display new data, instead of creating 10 more new views, which 
//obviously will be efficeient)
                //While alternatively if we dont extend base adapter (but rather 
//use simple binding using simpleadapter) it will then generates whole list of 100 Views 
//in one short whic will be time consuimng/ memory consuming depending uopn the amount of 
//data to be bind   



        //**parent**    the parent that this view will eventually be attached to


        View rowView = convertView; 
        if (rowView == null) { //always required to be checked as mentioned in google docs 
                               // this line checks for if initially row View is null then we have to create the Row View. Once it will be created then it will always Surpass this check and we will keep on reusing this rowView (thats what actually we are looking for) 
          LayoutInflater inflater = mActivitycontext.getLayoutInflater();  // creating instance of layout inflater to inflate our custom layout
          rowView = inflater.inflate(R.layout.projectslist_row, null); //assigend our custom row layout to convertview whic is to be reused
          ViewHolder viewHolder = new ViewHolder();     // ViewHolder is a custom class in which we are storing are UI elaments of Row  
          viewHolder.mprojectslistRowView = (TextView) rowView.findViewById(R.id.projectslist_row); //assigned the id of actual textView thats on our custom layout to the instance of TextView in our static class       
          rowView.setTag(viewHolder);
        }

        ViewHolder holder = (ViewHolder) rowView.getTag();
        String projectName = mProjectsList.get(position).get("ProjectName"); // here i am fetching data from my HashMap ArrayList
        holder.mprojectslistRowView.setText(projectName);       // here i am just assigning what to show in my text view 

        return rowView;
    }

    I created this as inner class 
    static class ViewHolder 
    {
        // create instances for all UI elemnts of your Row layout (here i am showing only text data as row of my list view so only instance of TextView has been created)
        // It is a statci class hence will keep this instance alive all the time and thats Why we will be able to  reuse it again and again.
        TextView mprojectslistRowView;
    }

您只需将此适配器绑定(bind)到您的控件，因为我们正在覆盖此处的方法一切都会自动处理。