ListView and Adapters

Software Development for Portable Devices BITS Pilani Goa Campus

Sem I 2011-12

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Foundation of :

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Problem??• Performance

• 10,000 entries in a list!– Less than 10% shown but fetched

completely

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Solution??• Populate on Demand– Google image search

• Recycle views to reduce object churn

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Android – ListView

• Its a view capable of displaying scrollable list of items

• It only creates views (widget) if needed

• It recycles views, e.g. if a row is not displayed anymore it will be recycled and only its content will change.

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The FLOW diagram

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Adapters•extends "BaseAdapter“

•"ArrayAdapter" can handle data based on Arrays

•"SimpleCursorAdapter" handle database related

data

•Your own "CustomAdapter"

Android - ListActivity• If Activity is primarily showing a list you should

extend the activity "ListActivity“• It simplifies the handling of a "ListView“• Set Listadapter in the onCreate(), via the

method setListAdapter().• Register the click by onListItemClick(ListView l,

View v, int position, long id)

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onListItemClick

• Parameters– l: The ListView where the click happened– v: The view that was clicked within the ListView– position: The position of the view in the list– id: The row id of the item that was clicked

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ListActivity with ArrayAdapter and Android standard layout

• See the code FirstExample

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ListActivity with own layout

• See the code SecondExample

• Define the layout in xml for each row

• Define our own arrayAdapter and using Id telling adapter which UI element is the text

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ListActivity with flexible layout

• See the code ThirdExample• Override getView().• Performance is not optimized.. Think why?

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Performance Issues

• Creating Java objects for each row is time and memory consumption.

• findViewById() is a expensive operation

• Android recycles rows (views) which are not displayed anymore.

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Optimized Way

• Using existing rows saves memory and CPU consumption .

• convertView - The old view to reuse• avoid doing findViewById() if not necessary • ViewHolder pattern

175% faster then the older procedure

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ViewHolder• The ViewHolder stores a reference to the

required views in a row• ViewHolder is then attached to the row via the

method setTag()• row is recycled we can get the ViewHolder via

getTag() methodmuch faster then the repetitive call of

findViewById()

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Optimized way

• See the code

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Your task

• Your task is to implement the View holder pattern in the Add food class

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Interacting rows

• Your row layout can also contain views which interact with the underlying data model.

• Example: Checkboxes

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Try Out

• LongClickListeners• Single Vs Multiple selection• Header and Footers

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References

• Spinner tutorial : http://developer.android.com/resources/tutorials/views/hello-spinner.html

• ArrayAdapter:http://developer.android.com/resources/tutorials/views/hello-spinner.html

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