当我们扩展BaseAdapter时它是如何工作的

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()会从中得到什么，当我们返回getView()_return_type时，还有谁会返回get_the_return_value_of getView()。

这些方法把我搞糊涂了

Answer 1

下面的帖子是根据我的理解。所以，如果你想改进它，而不是批评某个特定的点，请随意。

private ArrayList<HashMap<String, String>> mProjectsList = new ArrayList<HashMap<String, String>>();(您可以使用任何实际包含数据的游标或数组，并且您希望使用适配器将其绑定)

public int getCount() ->给出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) ->告诉哪个项目被点击了，就像我指定位置一样返回这里。它会返回你点击的整个视图和它的所有属性这就是为什么我们在这里返回我们点击位置的视图为了告诉BASEADAPTER类这个视图被点击了

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

public long getItemId(int position)将给出在点击某个列表项时想要返回的主Id。当你点击列表视图中的某项时它会返回两件事primarykey为长格式position为int格式。

从这个getItemId()方法返回主键。

我们通常在数据库中指定主键为"_id"，所以当我们使用简单的适配器而不是扩展baseadapter类时，它会自动以长格式返回_id字段作为主id。但是我们需要在BaseAdapter中手动指定我们想要返回的

@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)实际上创建了绑定自定义布局的视图

@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;
    }

你只需要将这个适配器绑定到你的Control，因为我们在这里重写了方法一切都会自动处理。