I am working on an Android app which has a recycler view. I have items ordered randomly in the recycler view. I want to move the fifth item of the list to the first position. And move the item previously at the first position to second position. Can anybody help me with this please?
You can use Collections.swap()
Swaps the elements at the specified positions in the specified list. (If the specified positions are equal, invoking this method leaves the list unchanged.)
METHOD
public static void swap(List<?> list,
int i,
int j)
Parameters:
list - The list in which to swap elements.
i - the index of one element to be swapped.
j - the index of the other element to be swapped.
SAMPLE CODE
// first swap the item using Collections.swap() method
Collections.swap(yourList, firstPosition, positionToSwap);
// than notify your adapter about change in list using notifyItemMoved() method
YourAdapter.notifyItemMoved(firstPosition, positionToSwap);
The way RecyclerView works is every time a new item comes on screen it calls your custom RecyclerView.Adapter subclass. Adapter has a reference for the dataset you take data for views from and gets passed the ViewHolder (the layout of an item) and index into onBindViewHolder(MyViewHolder holder, int position) to take dataset[position] and put the data into holder.textview.setText(dataset[position]).
As such, to swap places of elements you have 2 options:
rearrange data in the initial dataset. Chances are it's an ArrayList<> of some kind and you do TEMP=dataset[5];ArrayList.remove(TEMP);ArrayList.insert(TEMP,1); ArrayList will shift everything as required.
if it is important to keep dataset intact, you can rewrite your adapter, so that it keeps a map of { position : dataset_index } and populates items according to that map. That should be trivial.
And then you have to refresh the data with adapter.notifyDataSetChaged();
Related
I have a recycler view I want to update one attribute of each item after the initial creation to provide the user with a nicer experience. I would like to do this asynchronously as it takes time to get the data.
How do you iterate over the items in a recycler view and subsequently update. I moved from listview to recycler because it has a the method NotifyItemChanged.
So Ideally I would like to do
void OnRefresh(IList<data> data)
{
Data = data;
NotifyDataSetChanged();
Task.Run(() => UpdateAllAttribute1Fields());
}
void UpdateAllAttribute1Fields()
{
foreach(var myItem in myRecyclerView.Items)
{
UpdateAttribute1(myItem));
}
}
But I do not understand how to access myItems. On windows (sorry), in a listview this would be listview.items I think.
I could save the views OnBindViewHolder but that will be a bit more work.
Thanks all.
You dont need to iterate over all items in recyclerview to update, Just set the RecyclerView to point to your custom data set. And asynchronously update the data set whenever you want. You can then notify the recyclerview that the data in the dataset has changed with the following methods of its adapter (which will automatically update the recyclerview content)
adapterObj.notifyItemChanged(pos) //for one object
adapterObj.notifyDataSetChanged() //for the entire dataset
adapterObj.notifyItemRangeChanged(start, end) //for a range
What is the purpose of the associated data with the Android ListView footer? There is a method to add the footer view to a ListView
void addFooterView (View v, Object data, boolean isSelectable)
And the documentation on this parameter says:
Data to associate with this view
However, the documentation did not describe what it actually does. So, what effect does the data parameter have?
The Object data returned from Adapter's getItem method.
By documentation:
Add a fixed view to appear at the top of the list. If this method is called more than once, the views will appear in the order they were added. Views added using this call can take focus if they want.
Note: When first introduced, this method could only be called before setting the adapter with setAdapter(ListAdapter). Starting with KITKAT, this method may be called at any time. If the ListView's adapter does not extend HeaderViewListAdapter, it will be wrapped with a supporting instance of WrapperListAdapter.
Parameters:
v View: The view to add.
data Object: Data to associate with this view
isSelectable boolean: whether the item is selectable
Then
From the Adapters documentation
getItem Added in API level 1
Object getItem (int position)
Get the data item associated with the specified position in the data set.
Parameters
position int: Position of the item whose data we want within the adapter's data set.
Returns
Object The data at the specified position.
For instance, you have an adapter and in onBindViewHolder method you set OnClickListener to some views (and do some actions there depending on view position). You should assign final to position param of method onBindViewHolder so it could be accessible from onClick().
After changing dataset (remove or add item in list) you call onItemInserted or onItemRemoved and this really adds/removes a view in the recyclerview, BUT it does not refresh other viewitems so when you click on a neighbor viewitem it will open a screen or show data with invalid index. To avoid this I basically call notifyDatasetChanged to call onBind to all visible views and remove/add some views.
So how to refresh other views when you call notifyItemInserted/removed or how to work with these methots appropriately?
Assigning the position to a variable in onBindViewHolder will lead to an inconsistent state if items in the dataset are inserted or deleted without calling notifyDataSetChanged.
To use onItemInserted or onItemRemoved the data in the viewholder should remain consistent since it will not be redrawn and onClick would use this invalid position assigned before an item was added or removed.
For this and other use cases the RecyclerView.ViewHolder provides methods to access its position and id:
Use getAdapterPosition() or getItemId() to get valid positions and ids.
Also have a look on the other methods available in RecyclerView.ViewHolder.
So, the way I fix the problem I had is by changing the position into viewHolder.getAdapterPosition()
Cheers!
I advise you to add notifyItemRangeChanged after insert or remove list inside adapter. This work for my project.
Example in remove item :
public void removeItem (int pos) {
simpanList.remove(pos);
notifyItemRemoved(pos);
notifyItemRangeChanged(pos, simpanList.size());//add here, this can refresh position cmiiw
}
For future readers, this is what I do when inserting/removing in recyclerview
For example, my model class is CarsModel
In my adapter
ArrayList<CarsModel> carsModel;
In onBindViewHolder
CardModel model = carsModel.get(position);
When removing data in list using button in holder:
int position = holder.getAdapterPosition();
carsModel.remove(position);
notifyItemRemoved(position);
Then when inserting
carsModel.add(0, model);
notifyItemInserted(0);
or insert in last row
carsModel.add(carsModel.size() - 1 , model);
notifyItemInserted(carsModel.size()-1);
I'm having a bit of trouble preserving the scroll position of a list view when changing it's adapter's data.
What I'm currently doing is to create a custom ArrayAdapter (with an overridden getView method) in the onCreate of a ListFragment, and then assign it to its list:
mListAdapter = new CustomListAdapter(getActivity());
mListAdapter.setNotifyOnChange(false);
setListAdapter(mListAdapter);
Then, when I receive new data from a loader that fetches everything periodically, I do this in its onLoadFinished callback:
mListAdapter.clear();
mListAdapter.addAll(data.items);
mListAdapter.notifyDataSetChanged();
The problem is, calling clear() resets the listview's scroll position. Removing that call preserves the position, but it obviously leaves the old items in the list.
What is the proper way to do this?
As you pointed out yourself, the call to 'clear()' causes the position to be reset to the top.
Fiddling with scroll-position, etc. is a bit of a hack to get this working.
If your CustomListAdapter subclasses from ArrayAdapter, this could be the issue:
The call to clear(), calls 'notifyDataSetChanged()'. You can prevent this:
mListAdapter.setNotifyOnChange(false); // Prevents 'clear()' from clearing/resetting the listview
mListAdapter.clear();
mListAdapter.addAll(data.items);
// note that a call to notifyDataSetChanged() implicitly sets the setNotifyOnChange back to 'true'!
// That's why the call 'setNotifyOnChange(false) should be called first every time (see call before 'clear()').
mListAdapter.notifyDataSetChanged();
I haven't tried this myself, but try it :)
Check out: Maintain/Save/Restore scroll position when returning to a ListView
Use this to save the position in the ListView before you call .clear(), .addAll(), and . notifyDataSetChanged().
int index = mList.getFirstVisiblePosition();
View v = mList.getChildAt(0);
int top = (v == null) ? 0 : v.getTop();
After updating the ListView adapter, the Listview's items will be changed and then set the new position:
mList.setSelectionFromTop(index, top);
Basically you can save you position and scroll back to it, save the ListView state or the entire application state.
Other helpful links:
Save Position:
How to save and restore ListView position in Android
Save State:
Android ListView y position
Regards,
Please let me know if this helps!
There is one more use-case I came across recently (Android 8.1) - caused by bug in Android code. If I use mouse-wheel to scroll list view - consecutive adapter.notifyDataSetChanged() resets scroll position to zero. Use this workaround until bug gets fixed in Android
listView.onTouchModeChanged(true); // workaround
adapter.notifyDataSetChanged();
More details is here: https://issuetracker.google.com/u/1/issues/130103876
In your Expandable/List Adapter, put this method
public void refresh(List<MyDataClass> dataList) {
mDataList.clear();
mDataList.addAll(events);
notifyDataSetChanged();
}
And from your activity, where you want to update the list, put this code
if (mDataListView.getAdapter() == null) {
MyDataAdapter myDataAdapter = new MyDataAdapter(mContext, dataList);
mDataListView.setAdapter(myDataAdapter);
} else {
((MyDataAdapter)mDataListView.getAdapter()).refresh(dataList);
}
In case of Expandable List View, you will use
mDataListView.getExpandableListAdapter() instead of
mDataListView.getAdapter()
I want to iterate a list of items into a ListView. This code below is not enough to iterate all the items into the list because of the weird behaviour of getChildCount() function which only returns the visible item count.
for (int i = 0; i < list.getChildCount(); i++) {
item = (View)list.getChildAt(i);
product = (Product)item.getTag();
// make some visual changes if product.id == someProductId
}
My screen displays 7 results and when there are more than 7 items into the list, it's not possible to access to the 8th item or so.. Only visible items..
Should I use ListIterator instead?
Thanks.
You need to customize your list adapter's getView() method, and put your check inside it to check if the current item's id matches:
Product product = items.get(position);
if(product.id == someProductId) {
//make visual changes
} else {
//reset visual changes to default to account for recycled views
}
Since typically only the visible items only exist at a specific time, getView is called whenever more need to be seen. They're created at that time, typically recycling views from the now-invisible items in the list (hence why you want to reset the changes if the criteria does NOT match).
So #kcoppock solved your first problem it seems you got another problem. How to update the view item? The android SMS application shows one way:
create your own list view item like this:
public class MyListItem extends RelativeLayout {
...
}
in your list view item layout file:
< MyListItem android:layout_width=.....>
...
</ MyListItem >
and in your code, when your view item can be seen, register MyListItem as a data changed listener(the data is up to you). I mean, when your data changed, then you can update the item directly.
Check out the SMS application source code to read more.
The number of views in the ListView(7) is different from the number of items in the adapter(which is more than 7) .Try to use a BaseAdapter.