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Correct way to implement onMeasure() and onLayout() in custom AdapterView

I know that I should measure children in onMeasure() and layout them in onLayout(). The question is in what of these methods should I add/recycle views so I could measure all children together with an eye on how they are mutually positioned (i.e. grid, list or whatever)?
My first approach was to add/recycle views in onLayout() but from that point I can't measure my children because they aren't added to AdapterView yet and getChildCount() returns 0 in onMeasure(). And I can't measure AdapterView itself without children being already layouted because it really depends upon their mutual positions, right?
I'm really confused with android layouting process in AdapterView when childrens are added/removed dynamically.

I can't post a comment because I'm a new user, but can you describe WHAT you're trying to do, as opposed to HOW you're trying to do it? Frequently, you will find that this is an issue of design as opposed to coding. Especially if you're coming from a different platform (example, iOS). From experience, I found that measuring and manual layouts in Android is mostly unnecessary if you design your layout properly in light of your business need.
EDIT:
As I mentioned this can be solved using some design decisions. I will use your Nodes/List example (hoping that this is your actual use case, but the solution can be expanded for a more general problem).
So if we think about your Header as a comment in a forum, and the List as replies to your comment, we can make the following assumption:
One list is enough, not two. Each item in the list can either be a header (comment) or a list item (reply). Each reply is a comment, but not all comments are replies.
For item n, I know if it's a comment or a reply (i.e. is it a header or an item in your list).
For item n, I have a boolean member isVisible (default false; View.GONE).
Now, you can use the following components:
One extended adapter class
Two Layout XMLs: One for your Comment, one for your reply. You can have unlimited comments and each comment can have unlimited replies. Both those satisfy your requirements.
Your fragment or activity container class that implements OnItemClickListener to show/hide your list.
So let's look at some code, shall we?
First, your XML files:
Comment row (your header)
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/overall"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:animateLayoutChanges="true">
<TextView
android:id="#+id/comment_row_label"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
</RelativeLayout>
Now your reply row (an element in your list)
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="#+id/overall"
android:layout_width="match_parent"
android:layout_height="wrap_content"> <!-- this is important -->
<TextView
android:id="#+id/reply_row_label"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:visibility="gone"/> <!-- important -->
</RelativeLayout>
Ok, now your adapter class
public class CommentsListAdapter extends BaseAdapter implements OnClickListener
{
public static String TAG = "CommentsListAdapter";
private final int NORMAL_COMMENT_TYPE = 0;
private final int REPLY_COMMENT_TYPE = 1;
private Context context = null;
private List<Comment> commentEntries = null;
private LayoutInflater inflater = null;
//All replies are comments, but not all comments are replies. The commentsList includes all your data. (Remember that the refresh method allows you to add items to the list at runtime.
public CommentsListAdapter(Context context, List<Comment> commentsList)
{
super();
this.context = context;
this.inflater = (LayoutInflater)context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
this.commentEntries = commentsList;
}
//For our first XML layout file
public static class CommentViewHolder
{
public RelativeLayout overall;
public TextView label;
}
//For our second XML
public static class ReplyViewHolder
{
public RelativeView replyOverall;
public TextView replyLabel;
}
#Override
public int getViewTypeCount()
{
return 2; //Important. We have two views, Comment and reply.
}
//Change the following method to determine if the current item is a header or a list item.
#Override
public int getItemViewType(int position)
{
int type = -1;
if(commentEntries.get(position).getParentKey() == null)
type = NORMAL_COMMENT_TYPE;
else if(commentEntries.get(position).getParentKey() == 0L)
type = NORMAL_COMMENT_TYPE;
else
type = REPLY_COMMENT_TYPE;
return type;
}
#Override
public int getCount()
{
return this.commentEntries.size(); //all data
}
#Override
public Object getItem(int position)
{
return this.commentEntries.get(position);
}
#Override
public long getItemId(int position)
{
return this.commentEntries.indexOf(this.commentEntries.get(position));
}
#Override
public View getView(int position, View convertView, ViewGroup parent)
{
CommentViewHolder holder = null;
ReplyViewHolder replyHolder = null;
int type = getItemViewType(position);
if(convertView == null)
{
if(type == NORMAL_COMMENT_TYPE)
{
convertView = inflater.inflate(R.layout.row_comment_entry, null);
holder = new CommentViewHolder();
holder.label =(TextView)convertView.findViewById(R.id.comment_row_label);
convertView.setTag(holder);
}
else if(type == REPLY_COMMENT_TYPE)
{
convertView = inflater.inflate(R.layout.row_comment_reply_entry, null);
replyHolder = new ReplyViewHolder();
replyHolder.replyLable = (TextView)convertView.findViewById(R.id.reply_row_label);
convertView.setTag(replyHolder);
}
}
else
{
if(type == NORMAL_COMMENT_TYPE)
{
holder = (CommentViewHolder)convertView.getTag();
}
else if(type == REPLY_COMMENT_TYPE)
{
replyHolder = (ReplyViewHolder)convertView.getTag();
}
}
//Now, set the values of your labels
if(type == NORMAL_COMMENT_TYPE)
{
holder.label.setTag((Integer)position); //Important for onClick handling
//your data model object
Comment entry = (Comment)getItem(position);
holder.label.setText(entry.getLabel());
}
else if(type == REPLY_COMMENT_TYPE)
{
replyHolder = (ReplyViewHolder)convertView.getTag(); //if you want to implement onClick for list items.
//Or another data model if you decide to use multiple Lists
Comment entry = (Comment)getItem(position);
replyHolder.replyLabel.setText(entry.getLabel()));
//This is the key
if(entry.getVisible() == true)
replyHolder.replyLabel.setVisibility(View.VISIBLE);
else
replyHolder.replyLabel.setVisibility(View.GONE);
}
return convertView;
}
//You can use this method to add items to your list. Remember that if you are using two data models, then you will have to send the correct model list here and create another refresh method for the other list.
public void refresh(List<Comment> commentsList)
{
try
{
this.commentEntries = commentsList;
notifyDataSetChanged();
}
catch(Exception e)
{
e.printStackTrace();
Log.d(TAG, "::Error refreshing comments list.");
}
}
//Utility method to show/hide your list items
public void changeVisibility(int position)
{
if(this.commentEntries == null || this.commentEntries.size() == 0)
return;
Comment parent = (Comment)getItem(position);
for(Comment entry : this.commentEntries)
{
if(entry.getParent().isEqual(parent))
entry.setVisible(!entry.getVisible()); //if it's shown, hide it. Show it otherwise.
}
notifyDataSetChanged(); //redraw
}
}
Ok great, now we have a list of headers with hidden children (remember, we set the default visibility of children to 'gone'). Not what we wanted, so let's fix that.
Your container class (fragment or activity) you will have the following XML definition
<!-- the #null divider means transparent -->
<ListView
android:id="#+id/comments_entries_list"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:divider="#null"
android:dividerHeight="5dp" />
And your onCreateView will implement OnItemClickListener and have the following
private ListView commentsListView = null;
private List<Comment>comments = null;
private static CommentsListAdapter adapter = null;
....
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
...
//comments list can be null here, and you can use adapter.refresh(data) to set the data
adapter = new CommentsListAdapter(getActivity(), comments);
this.commentsListView.setAdapter(adapter);
this.commentsListView.setOnClickListener(this); //to show your list
}
Now to show your list when you click a header
#Override
public void onItemClick(AdapterView<?> parent, View view, int position,
long id)
{
adapter.changeVisibility(position);
}
Now, if an item is clicked and that item has a parent (i.e. list item), it will be shown/hidden according to its current state.
Some comments about the code:
I wrote this on WordPad as I don't have a dev environment handy. Sorry for any compilation errors.
This code can be optimized: If you have a very large data set, this code would be slow since you're redrawing the entire list on every call to changeVisibility(). You can maintain two lists (one for headers, one for list items) and in changeVisibility you can query over the list items only).
I re-enforce that idea that some design decisions would make your life a lot easier. For example, if your list items were actually just a list of labels, then you can have one custom XML file (for your header) and a ListView view within it that you can set to View.GONE. This will make all other views pretend that it's not even there and your layout will work properly.
Hope this helps.

Add a different element to ArrayAdapter/ListView

I have a ListView that's being populated by an ArrayAdapter:
someListView.setAdapter(adapter);
Each element in the adapter is inflated using the same layout.xml. Now I want to add an element of a different type (inflated using a different layout file) to the beginning of the ListView.
What I want to achieve is, to have a special element on top of all other elements in the list view, but also scrolls with the list (exits the screen from top if the user scrolls down).
I've tried to add the new element to the array but it's a different type so that won't work.
I've tried to insert a dummy element to the array at position 0, and modify the adapter's getView() so that if (position == 0) return myUniqueView, but that screwed up the entire list view somehow: items not showing, stuff jumping all over the place, huge gaps between elements, etc.
I start to think the best practice of achieving what I want, is not through editing the array adapter. But I don't know how to do it properly.

You don't need anything special to do what you ask. Android already provides that behavior built in to every ListView. Just call:
mListView.addHeaderView(viewToAdd);
That's it.
ListView Headers API
Tutorial

Do't know exactly but it might usefull
https://github.com/chrisjenx/ParallaxScrollView

In your adapter add a check on the position
private static final int LAYOUT_CONFIG_HEADER = 0;
private static final int LAYOUT_CONFIG_ITEMS = 1;
int layoutType;
#Override
public int getItemViewType(int position) {
if (position== 0){
layoutType = LAYOUT_CONFIG_HEADER;
} else {
layoutType = LAYOUT_CONFIG_ITEMS;
}
return layoutType;
}
#Override
public View getView(int position, View convertView, ViewGroup parent) {
View row = convertView;
LayoutInflater inflater = null;
int layoutType = getItemViewType(position);
if (row == null) {
if (layoutType == LAYOUT_CONFIG_HEADER) {
//inflate layout header
}
} else {
//inflate layout of others rows
}
}

How can I get a view by id from a layout in Android?

This is the only way I can find but it seems hacky:
public View getViewByIdFromLayout(int id) {
for (int i = 0; i < layout.getChildCount(); i++) {
View v = layout.getChildAt(i);
if (v.getId() == id)
return v;
}
return null;
}
Is there a better way to do this? These views were created programmatically, not via xml, and the views' ids will be unique in a given layout but may be reused in other layouts.

You can still use findViewById() as long as you've given it an ID. Just make sure to call findViewById() on the layout for which the ID is unique.

You should change
if (v.getId() == id)
return v;
to
if (v.getId().equals(id))
return v;

Equivalent of CSS class selectors for Android views?

Do Android views have something equivalent to CSS class selectors? Something like R.id but usable for multiple views? I would like to hide some group of views independent of their position in the layout tree.

I think that you will need to iterate through all of the views in your layout, looking for the android:id you want. You can then use View setVisibility() to change the visibility. You could also use the View setTag() / getTag() instead of android:id to mark the views that you want to handle. E.g., the following code uses a general purpose method to traverse the layout:
// Get the top view in the layout.
final View root = getWindow().getDecorView().findViewById(android.R.id.content);
// Create a "view handler" that will hide a given view.
final ViewHandler setViewGone = new ViewHandler() {
public void process(View v) {
// Log.d("ViewHandler.process", v.getClass().toString());
v.setVisibility(View.GONE);
}
};
// Hide any view in the layout whose Id equals R.id.textView1.
findViewsById(root, R.id.textView1, setViewGone);
/**
* Simple "view handler" interface that we can pass into a Java method.
*/
public interface ViewHandler {
public void process(View v);
}
/**
* Recursively descends the layout hierarchy starting at the specified view. The viewHandler's
* process() method is invoked on any view that matches the specified Id.
*/
public static void findViewsById(View v, int id, ViewHandler viewHandler) {
if (v.getId() == id) {
viewHandler.process(v);
}
if (v instanceof ViewGroup) {
final ViewGroup vg = (ViewGroup) v;
for (int i = 0; i < vg.getChildCount(); i++) {
findViewsById(vg.getChildAt(i), id, viewHandler);
}
}
}

You can set same tag for all such views and then you can get all the views having that tag with a simple function like this:
private static ArrayList<View> getViewsByTag(ViewGroup root, String tag){
ArrayList<View> views = new ArrayList<View>();
final int childCount = root.getChildCount();
for (int i = 0; i < childCount; i++) {
final View child = root.getChildAt(i);
if (child instanceof ViewGroup) {
views.addAll(getViewsByTag((ViewGroup) child, tag));
}
final Object tagObj = child.getTag();
if (tagObj != null && tagObj.equals(tag)) {
views.add(child);
}
}
return views;
}
As explained in Shlomi Schwartz answer. Obviously this is not as useful as css classes are. But this might be a little useful as compared to writing code to iterate your views again and again.

How does findViewById initialise a view

I just wrote an answer for someone confused by findViewById and I realised that I have a gap in my understanding. This question is for knowledge and curiosity only.
Consider this:
button = (Button)findViewById(R.id.button);
findViewById returns an instance of View, which is then cast to the target class. All good so far.
To setup the view, findViewById constructs an AttributeSet from the parameters in the associated XML declaration which it passes to the constructor of View.
We then cast the View instance to Button.
How does the AttributeSet get passed in turn to the Button constructor?
[EDIT]
So I was the confused one :). The whole point is that when the layout is inflated, the view hierarchy already contains an instance of the view descendant class. findViewById simply returns a reference to it. Obvious when you think about it - doh..

findViewById does nothing. It just looks through view hierarchy and returns reference to a view with requested viewId. View is already created and exists. If you do not call findViewById for some view nothing changes.
Views are inflated by LayoutInflator. When you call setContentView xml layout is parsed and view hierarchy is created.
attributes passed to Button's constructor by LayoutInflater. check LayoutInflator source code.

I don't think findViewById() constructs or instantiates a View. It will search in View hierarchy of already inflated layout, for a View with matching id.This method works differently for a View and for a ViewGroup.
from Android Source code:
View.findViewById() returns the same View object if this view has the given id or null, it calls:
protected View findViewTraversal(int id) {
if (id == mID) {
return this;
}
return null;
}
ViewGroup.findViewById() iterates through child views and calls same method on these Views, it calls:
protected View findViewTraversal(int id) {
if (id == mID) {
return this;
}
final View[] where = mChildren;
final int len = mChildrenCount;
for (int i = 0; i < len; i++) {
View v = where[i];
if ((v.mPrivateFlags & IS_ROOT_NAMESPACE) == 0) {
v = v.findViewById(id);
if (v != null) {
return v;
}
}
}
return null;
}