BaseAdapter在扩展时如何工作？

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()，比getView()会从中得到，当我们返回getView()_return_type还有谁在那里get_the_return_value_of getView()。

我完全与这些方法混淆。

Answer 1

下面的帖子是根据我的理解。所以，如果你想改善它，而不是批评某些特定的点，那么随便吧。

private ArrayList<HashMap<String, String>> mProjectsList = new ArrayList<HashMap<String, String>>();（您可以使用实际上是由数据的指针或数组，你想使用适配器将其绑定）

公众诠释getCount将（） - >给你目前的元素总数，在适配器（如数组的大小）

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

公共对象的getItem（INT位置） - >讲述被点击这里简单地回到我并指定位置的方式哪个项目。它实际上返回您点击与它的所有属性和这就是为什么我们刚刚回到这里，我们为了告诉BASEADAPTER类，这种观点是cliked

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

众长getItemId点击的位置从整体biwe （int position）会给你主要的ID，你想要返回时，你会在一些列表项上点击。当你真的点击列表视图的某个项时，它会返回长格式的主键和int格式的位置。

实际上返回主键的getItemId（）方法。

我们通常在数据库中指定主键为“_id”，所以当我们使用简单适配器而不是扩展baseadapter类时，它会自动以长格式返回_id字段作为主ID。 但是，我们必须在这里BaseAdapter手动指定我们想返回

@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 
    } 

公共查看getView（INT位置，查看convertView，一个ViewGroup父） actaully创建，你绑定您的自定义布局视图

@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; 
    } 

您只需将此适配器绑定到您的控件，因为我们正在覆盖这里的方法 所有内容都将自动处理。