What is the main purpose of setTag getTag methods of View

Question

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

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

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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

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Let s say you generate a bunch of views that are similar  You could set an OnClickListener for each view individually   button1 setOnClickListener new OnClickListener        button2 setOnClickListener new OnClickListener               Then you have to create a unique onClick method for each view even if they do the similar things  like   public void onClick View v        doAction 1      1 for button1  2 for button2  etc      This is because onClick has only one parameter  a View  and it has to get other information from instance variables or final local variables in enclosing scopes  What we really want is to get information from the views themselves   Enter getTag setTag   button1 setTag 1   button2 setTag 2     Now we can use the same OnClickListener for every button   listener   new OnClickListener          Override     public void onClick View v            doAction v getTag                It s basically a way for views to have memories

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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

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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  lt  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

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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

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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

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

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