[android] What is the main purpose of setTag() getTag() methods of View?

What is the main purpose of such methods as setTag() and getTag() of View type objects?

Am I right in thinking that I can associate any number of objects with a single View?

Unlike IDs, tags are not used to identify views. Tags are essentially an extra piece of information that can be associated with a view. They are most often used as a convenience to store data related to views in the views themselves rather than by putting them in a separate structure.

Reference: http://developer.android.com/reference/android/view/View.html

Setting of TAGs is really useful when you have a ListView and want to recycle/reuse the views. In that way the ListView is becoming very similar to the newer RecyclerView.

@Override
public View getView(int position, View convertView, ViewGroup parent)
  {
ViewHolder holder = null;

if ( convertView == null )
{
    /* There is no view at this position, we create a new one. 
       In this case by inflating an xml layout */
    convertView = mInflater.inflate(R.layout.listview_item, null);  
    holder = new ViewHolder();
    holder.toggleOk = (ToggleButton) convertView.findViewById( R.id.togOk );
    convertView.setTag (holder);
}
else
{
    /* We recycle a View that already exists */
    holder = (ViewHolder) convertView.getTag ();
}

// Once we have a reference to the View we are returning, we set its values.

// Here is where you should set the ToggleButton value for this item!!!

holder.toggleOk.setChecked( mToggles.get( position ) );

return convertView;
}

This is very useful for custom ArrayAdapter using. It is some kind of optimization. There setTag used as reference to object that references on some parts of layout (that displaying in ListView) instead of findViewById.

static class ViewHolder {
    TextView tvPost;
    TextView tvDate;
    ImageView thumb;
}

public View getView(int position, View convertView, ViewGroup parent) {

    if (convertView == null) {
        LayoutInflater inflater = myContext.getLayoutInflater();
        convertView = inflater.inflate(R.layout.postitem, null);

        ViewHolder vh = new ViewHolder();
        vh.tvPost = (TextView)convertView.findViewById(R.id.postTitleLabel);
        vh.tvDate = (TextView)convertView.findViewById(R.id.postDateLabel);
        vh.thumb = (ImageView)convertView.findViewById(R.id.postThumb);
        convertView.setTag(vh);
    }
            ....................
}

I'd like to add few words.

Although using get/setTag(Object) seems to be very useful in the particular case of a ViewHolder pattern, I'd recommend to think twice before using it in other cases. There is almost always another solution with better design.

The main reason is that code like that becomes unsupportable pretty quickly.

• It is non-obvious for other developers what you designed to store as tag in the view. The methods setTag/getTag are not descriptive at all.

• It just stores an Object, which requires to be cast when you want to getTag. You can get unexpected crashes later when you decide to change the type of stored object in the tag.

• Here's a real-life story: We had a pretty big project with a lot of adapters, async operations with views and so on. One developer decided to set/getTag in his part of code, but another one had already set the tag to this view. In the end, someone couldn't find his own tag and was very confused. It cost us several hours to find the bug.

setTag(int key, Object tag) looks much better, cause you can generate unique keys for every tag (using id resources), but there is a significant restriction for Android < 4.0. From Lint docs:

Prior to Android 4.0, the implementation of View.setTag(int, Object) would store the objects in a static map, where the values were strongly referenced. This means that if the object contains any references pointing back to the context, the context (which points to pretty much everything else) will leak. If you pass a view, the view provides a reference to the context that created it. Similarly, view holders typically contain a view, and cursors are sometimes also associated with views.

For web developers, this seems to be the equivalent to data-..

We can use setTag() and getTag() to set and get custom objects as per our requirement. The setTag() method takes an argument of type Object, and getTag() returns an Object.

For example,

Person p = new Person();
p.setName("Ramkailash");
p.setId(2000001);
button1.setTag(p);