How does the getView() method work when creating your own custom adapter?

Question

My questions are:

1. What is exactly the function of the LayoutInflater?
2. Why do all the articles that I've read check if convertview is null or not first? What does it mean when it is null and what does it mean when it isn't?
3. What is the parent parameter that this method accepts?

Answer 1

1: The LayoutInflater takes your layout XML-files and creates different View-objects from its contents.

2: The adapters are built to reuse Views, when a View is scrolled so that is no longer visible, it can be used for one of the new Views appearing. This reused View is the convertView. If this is null it means that there is no recycled View and we have to create a new one, otherwise we should use it to avoid creating a new.

3: The parent is provided so you can inflate your view into that for proper layout parameters.

All these together can be used to effectively create the view that will appear in your list (or other view that takes an adapter):

public View getView(int position, @Nullable View convertView, ViewGroup parent){     if (convertView == null) {         //We must create a View:         convertView = inflater.inflate(R.layout.my_list_item, parent, false);     }     //Here we can do changes to the convertView, such as set a text on a TextView      //or an image on an ImageView.     return convertView; } 

Notice the use of the LayoutInflater, that parent can be used as an argument for it, and how convertView is reused.

Answer 2

getView() method in Adapter is for generating item's view of a ListView, Gallery,...

1. LayoutInflater is used to get the View object which you define in a layout xml (the root object, normally a LinearLayout, FrameLayout, or RelativeLayout)

2. convertView is for recycling. Let's say you have a listview which can only display 10 items at a time, and currently it is displaying item 1 -> item 10. When you scroll down one item, the item 1 will be out of screen, and item 11 will be displayed. To generate View for item 11, the getView() method will be called, and convertView here is the view of item 1 (which is not neccessary anymore). So instead create a new View object for item 11 (which is costly), why not re-use convertView? => we just check convertView is null or not, if null create new view, else re-use convertView.

3. parentView is the ListView or Gallery... which contains the item's view which getView() generates.

Note: you don't call this method directly, just need to implement it to tell the parent view how to generate the item's view.