I'm trying to implement a custom ArrayAdapter for a ListView.
The ArrayList I want to adapt have some positions I want to ignore. (for example when myobject.getValue()>20) how can I indicate that object shouldn't appear in the ListView?
Returning null in the method getView result in a nullPointerException
public View getView(int position, View convertView, ViewGroup parent)
Thanks in advance
Filter out your list before you create the adapter. Go through the original list, checking values, and only add the ones you need to a second list. Then use the second list for your adapter.
Related
I have to show some HTML with images from the internet in a ListView. The images are handled in a custom Html.ImageGetter. The problem is, I need the TextView available to download the images to inside. This leads to having to incorporate the HTML parsing inside the ArrayAdapter's getView method. But that method gets called very often by the Android system as it recycles and redraws elements.
How can I stop this recycling from happening? I assume I have to use a different flow. This is my current getView() method:
#Override
public View getView(int position, View convertView, ViewGroup parent) {
TextView view = new TextView(context);
CharSequence element = getItem(position);
CharSequence html = Html.fromHtml(element.toString(), new MessageImageGetter(context, view), null);
view.setText(html);
view.setPadding(12, 4, 12, 4);
return view;
}
Maybe a list-like behavior is possible without the use of ListView and ArrayAdapter, that doesn't recycle and redraw often.
How can I stop this recycling from happening?
You don't. You create a richer model object. Rather than having what appears to be a ListAdapter<String>, have a ListAdapter<Thing>, where:
Thing holds your existing string value (element)
Thing caches the Html.fromHtml() result
Your getView() gets the html CharSequence from the Thing and therefore can take advantage of caching
Feel free to substitute a more applicable noun than Thing, of course... :-)
I'am trying to use a custom listView in my application and i have some questions about its working principles.When i implemented a custom adapter,how its methods(especially the getView method)work without calling them from any other class ?
When you set the adapter to a view (e.g. ListView or GridView), that view at some point wants to have some items to show. So it calls getView in the adapter:
getView(int position, View convertView, ViewGroup parent).
The position is the position of the item in the list/grid/etc. The convertView is a recycled view that can be already inflated by a previous getView() call, or null when it's not inflated yet (see this answer about convertView and view recycling.
The parent is used to inflated the view, so the correct layout parameters can be computed in relation to the parent view.
Note that inflation is slow. That is why the convertView mechanism exists, to recycle views so the number of inflations is minimized. Next to that, finding views (findViewById()) is also relatively slow. To improve on that, check out the ViewHolder pattern, which keeps references to views in memory so they don't have to be searched for each time.
I think this link can help you. getview is a callback function which will be called automatically when you will display your listview on Activity. When you display your listview then you overrides getview and inflates your row from XML or dynamically creates your row. That row you return as a view which displays in your listview.
How does the getView() method work when creating your own custom adapter?
For each row getview will be called once. You create your layouts and return them as view. Those respective views displays in your lisview rows.
You are calling the custom adapter class from you activity class.Your customised adapter class extends a BaseAdapter which is an abstract class.The methods of an abstract will be used by the extended class(the methods like getView(), getItemId(), getItem(), and getCount()).These methods should not need a seperate call from your Class since you are calling the customised adapter classs .
I had a problem with the list view having both parent list and the child list of the list activity(implemented through database query). I wish to show them differing their properties by changing the text style (parent list items are in bold, child list items are in normal style).
I wish to differ with Parentid(Null, NotNull)in their text style(bold, normal) for parent and child items respectively. Please help me with the code/links. Thanks a lot in advance.
Maybe an ExpandableListAdapter would be a way to go? (You can make the parent elements non-expandable if you want.)
If you want to stick with the simple list, you can override getView(int position, View convertView, ViewGroup parent).
And maybe also override bindView(View view, Context context, Cursor cursor).
I have used the option of overriding bindView() to add a tag to a view and examine this later on. Maybe this would also work for you...
I'm thoroughly confused about the life cycle of list view. More specifically, what does the list adapter do exactly? Does it just provide data to the given view? And when/where does the getView() function gets called? And what purpose does this getView() function provide? From just looking at the code, it looks like getView() is "assigning" data to the view to be displayed. I'd like to be able to use list views without having to memorize, do this and then this in order for it to work. I'd much rather understand it so I can use it properly. Someone please help me understand all of this.
Also, if someone can explain to me.. what's the difference between BaseAdapter and ArrayAdapter? and any other kind of adapters that comes with Android.
What I have understood is your adapter constructor instantiated by activity and then on activity launch the getView() method is called. the {#param position, view, viewGroup}
position: it refers to the position of the view as given by adapter. Please Note it is different from the position in {OnItemClick(AdapterView adapter, View v, int position,long id)} here position is the list item position. The {position} in {getView()} changes after particular object in the list are displayed again for eg. when you scroll.
view: the view here is the view you want to be presented through getView(). It can be a particular XML layout for each row. So this states clearly that getView is called to plot every row. this view needs to be valid one or another layout (LinearLayout by default) will be selected to maintain uniqueness.
viewgroup: as you might know and as name says will be the container of your #param:view
any other point is appreciated.
getView() fills in the data into the item's view with the given index. The view which is given as a parameter may be a pre-inflated view. If it is not, you have to infalte it yourself.
An ArrayAdapter simply calls setText on the given view with the result of toString() of the object with the respective index from the array. If you override it, you can do more complex stuff, like assigning a picture or filling in more TextViews.
I recommend the following tutorial: http://www.softwarepassion.com/android-series-custom-listview-items-and-adapters/
Hi list adaper provides view for listview.
when user scrolls listview at that time getview is called.
getview is used to populate your view with data hence the name adapter.
The Adapter does all the "rember to do this" for you. If you change a list view's backing data structure through the adapter's methods (e.g. "add()") it will fire all the datachanged and update events you'll need for the list view to show the new state of the data.
I want to be able to access my ArrayAdapter's view:
public View getView(int position, View convertView, ViewGroup parent)
I'm not sure how I can access this:
View myView = myArrayAdapter.getView(myPosition, myView, ?);
I am not sure how I can get a ViewGroup parent?
Well as far as I understand you don't suppose to Adapter in general serves as a source to instance of AdapterView so getView contract is follows :
in params : inPosition,convertView,viewParent
result : View witch will be shown at position inPosition , you can use convertView to bind data to if it's not null, returned view will be attached to viewParent.
So if you want to acquire view that will be shown by adapter view at specific position, why don't just call
AdapterView<?> adapterView = getAdapterView();
View myView = adapterView.getChildAt(position);
If you want to change way of showing view at specific position you should use
Adapter myAdapter = getAdapter();
//change data inside adapter
myAdapter.notifyDataSetChanged();
Update:
Try adding a field to the data in the data set that holds the background resource/color to be used when returning the view, and in the getView do
holder.background = dataSet.get(position).getBackground()
If you don't know what I'm talking about with this holder thing, check How to load the Listview "smoothly" in android
Old:
I'd say the ListView containing the adapter is the parent ViewGroup.
Anyway, why don't you use the ListView containing the ArrayAdapter? The ListView is the container of the Views, rather than the adapter, that only holds the data collection and creates and returns Views on demand when the ListView asks for them (f.e, when scrolling). Even if you were able to fetch a view for the position X, I'd say that won't work, as the convertView you pass as a parameter is not the one the ListView is holding and showing.
Take into account that in a ListView there are at most 7-8 Views inflated at any given moment (I don't know the exact number), and what the ListView does is fill them in by calling the adapter's getview.
This said, to update a single row (a single view) you'll need to update the adapter's data collection. Then, speculating, try to get that View from the ListView and invalidate it, but I doubt this'll work.
I think these concepts are right, if they are wrong I'd be grateful if anyone corrects me.
Why do you want to update only one view at a time?