<div id="readability-page-1" class="page"><div><p id="ad19">Learn about ConcatAdapter.</p><figure><figcaption>Photo by <a href="https://unsplash.com/@homemademedia?utm_source=medium&amp;utm_medium=referral" rel="noopener ugc nofollow" target="_blank">Homemade Media</a> on <a href="https://unsplash.com/?utm_source=medium&amp;utm_medium=referral" rel="noopener ugc nofollow" target="_blank">Unsplash</a></figcaption></figure><h2 id="3ac9">ConcatAdapter</h2><p id="9eb2">ConcatAdapter is an adapter that allows you to add multiple adapters that appeared in the version RecyclerView : 1.2.0-alpha02. In the past, <code>RecyclerView.Adapter</code> had to provides view type to each by view in order to show each view in a list form. This led to the presence of multiple <code>ViewHolder</code> on one adapter, and multiple logics were combined. ConcatAdapter is implemented to add multiple adapters to a single adapter and to add them to a single RecyclerView to benefit from encapsulation and reuse.</p><figure><figcaption><a href="https://medium.com/androiddevelopers/merge-adapters-sequentially-with-mergeadapter-294d2942127a" rel="noopener">이미지</a></figcaption></figure><h2 id="99f4">Component in ConcatAdapter</h2><h2 id="a4ce">Config</h2><p id="5ea9">You can specify default options by passing the <code>ConcatAdapter.Config</code> object to the constructor of the ConcatAdapter.</p><figure></figure><p id="f3a2">You can control the <code>isolateViewTypes</code> and <code>stableIdMode</code> in the <code>Config</code>, each of which is described below.</p><ul><li id="f6f0"><code>isolateViewTypes</code> : If <code>false</code>, the ConcatAdapter assumes that all adapters assigned to it share a global view type pool so that they reference the same view holder using the same view type. Specifying this option <code>false</code> means that nested adapters can share view holders, but they should not have conflicting view types that return the same view type for different view holders. Defaults to <code>true</code> to separate view types between adapters to avoid using the same view holder.</li><li id="f57e"><code>stableIdMode</code> : There are three <code>StableIdMode</code></li></ul><ol><li id="b349"><code>NO_STABLE_IDS</code> : Default value; ignores the stable id of the adapter added to the <code>concatAdapter</code>. If you add an adapter that uses stable id when in this mode, a warning is issued.</li></ol><p id="2737">2. <code>ISOLATED_STABLE_IDS</code> : In this mode, <code>hasStableIds()</code> of the concatAdapter returns <code>true</code> and requires an adapter with a stable id specified. Because two different adapters may not recognize each other and return the same stable id, <code>ConcatAdapter</code> separates the id pool of each adapter from each other, overwrites the returned stable id, and notifies <code>RecyclerView</code> again. In this mode, the value of <code>RecyclerView.Adapter.getItemId(int)</code> and the value of <code>RecyclerView.ViewHolder.getItemId()</code> may differ. Additionally, if you add an adapter with no stable id specified, an <code>IlllegalArgumentException</code> error occurs.</p><p id="139d">3. <code>SHARED_STABLE_IDS</code> : Similar to <code>ISOLATED_STABLE_IDS</code>, <code>hasStableIds()</code> returns <code>true</code> and requires an adapter with a stable id. However, unlike <code>ISOLATED_STABLE_IDS</code>, it does not override the returned stable id. In this mode, the sub-adapters must be aware of each other and should not return the same id unless items are moved between the adapters. <code>IllegalArgumentException</code> error occurs when adding an adapter with no stable id specified.</p><blockquote><p id="138d">⚠️ If you create a Constructor in ConcatAdapter without handing over the Config, it is designated as <strong>Config.DEFAULT</strong>, isolateViewTypes as <strong>true</strong>, and StableIdMode as <strong>NO_STABLE_IDS</strong></p></blockquote><h2 id="cd5d">NestedAdapterWrapper</h2><figure></figure><p id="4128">Most functions in ConcatAdapter delegate actions to the <code>ConcatAdapterController</code>. And the <code>ConcatAdapterController</code> implements <code>NestedAdapterWrapper.Callback</code>, so first let’s look at <code>NestedAdapterWrapper</code>.</p><p id="78a2"><code>NestedAdapterWrapper</code> is a class that wraps the adapter we added. <code>Callback</code> looks the same as <code>RecyclerView.AdapterDataObserver</code>, but when we add the Adapter through the addAdapter method, the Adapter and <code>ConcatAdapterController</code> that we want to add to the constructor’s parameter in <code>NestedAdapterWrapper</code> are handed over, and <code>NestedAdapterWrapper</code> takes over <code>Callback (ConcatAdapterController)</code> is called by mapping to each method in the <code>AdapterDataObserver</code> that it has as an internal field.</p><figure></figure><p id="93c7">The code above shows that <code>NestedAdapterWrapper</code> has an object called <code>ViewTypeStorage.ViewTypeLookup</code>, <code>StableIdStorage.StableIdLookup</code> as a field. These fields determine the value returned when we look up the id or view type of a particular item in ConcatAdapter.</p><h2 id="3c0a">ViewTypeStorage</h2><p id="40df"><code>ViewTypeStorage</code> is a storage interface that stores and manages view types. It is held as a field by the <code>ConcatAdapterController</code>.</p><figure></figure><h2 id="c218"><strong>getItemViewType</strong></h2><p id="8e2d">When we call the <code>getItemViewType</code> function, the viewtype is eventually returned by <code>NestedAdapterWrapper</code>’s <code>getItemViewType</code>, which is determined by the <code>ViewTypeStorage.ViewTypeLookup</code> interface implementation held in the field.</p><figure></figure><figure></figure><p id="9f08">In addition, <code>ViewTypeStorage</code> is created from the constructor of the <code>ConcatAdapterController</code>, which is divided into two implementations, depending on the value of the <code>isolateViewTypes</code>.</p><figure></figure><p id="8a3b"><code>IsolatedViewTypeStorage</code>, <code>SharedIdRangeViewTypeStorage</code>, is declared inside the <code>WrapperViewTypeLookup</code>, the implementation class of the <code>ViewTypeStorage.ViewTypeLookup</code> interface, which returns the view type by referring to the view type managed by the corresponding Storage.</p><ul><li id="b31d"><code>SharedIdRangeViewTypeStorage</code>: <code>getItemViewType</code> returns the view type of the adapter connected to the <code>NestedAdapterWrapper</code>.</li><li id="4c9a"><code>IsolatedViewTypeStorage</code>: <code>getItemViewType</code> returns if it is already stored, otherwise create a unique viewtype and return it. (Even if N adapters with the same view type are added, different unique view types will be returned for <code>getItemViewType</code></li></ul><h2 id="ef42">StableIdStorage</h2><p id="27e5">StableIdStorage is a storage interface that stores Stableid. Similarly, the <code>ConcatAdapterController</code> holds it as a field.</p><figure></figure><h2 id="c115"><strong>getItemId</strong></h2><p id="905e">Similarly, if the <code>NestedAdapterWrapper</code> holds <code>StableIdLookup</code> as a field and calls <code>getItemId</code>, this field returns <code>id</code>.</p><figure></figure><p id="2bc7">Implementations of <code>StableIdLookup</code> are declared inside three implementations of the <code>StableIdStorage</code> interface, each of which behaves in this way.</p><ul><li id="5782"><code>NoStableIdStorage</code>: unconditionally return <code>RecyclerView.NO_ID</code> (default)</li><li id="5b72"><code>SharedPoolStableIdStorage</code>: Return the adapter’s <code>getItemId</code> value as it is</li><li id="d98b"><code>IsolatedStableIdStorage</code>: If the <code>getItemId</code> value of the adapter received as a parameter is already stored, return the <code>id</code> or create a unique <code>id</code> to return it.</li></ul><h2 id="9b40">Under the hood</h2><h2 id="f5eb">addAdapter</h2><p id="25c5">What will happen internally if we call <code>addAdapter</code> to add one adapter? The ConcatAdapter calls the <code>addAdapter</code> of the <code>ConcatAdapterController</code> and the <code>ConcatAdapterController</code> follows the process below to perform the <code>addAdapter</code>.</p><figure></figure><p id="766a">Controller — <code>addAdapter</code></p><ol><li id="7712"><strong>index inspection</strong>: When we hand over the <code>adapter</code> and <code>index</code> to add them to the <code>addAdapter</code>, we first check whether this <code>index</code> is a value between 0 and the current number of adapters. If you try to add an <code>index</code> value beyond this range, <code>IndexOutOfBoundsException</code> occurs. If you call addAdapter without passing the <code>index</code>, you will add the <code>adapter</code> internally in the last order.</li><li id="7a19"><strong>Stable id check</strong>: Exception occurs when <code>StableIdMode</code> of the current ConcatAdapter is not <code>NO_STABLE_IDS</code> and <code>hasStableId</code> of the <code>adapter</code> you are adding is <code>true</code>. If <code>StableIdMode</code> is <code>NO_STABLE_IDS</code> and the <code>hasStableIds</code> of the <code>adapter</code> you want to add is <code>true</code>, it will log the warning.</li><li id="67cd"><strong>Check whether the adapter has already been added</strong>: If the <code>adapter</code> already added to the ConcatAdapter matches the <code>adapter</code> you want to add, return it without adding the adapter.</li><li id="355b">Once you have completed this process, create a <code>NestedAdapterWrapper</code>, connect the adapter you want to add, and cache it. In addition, notify the RecyclerView connected to the ConcatAdapter through the <code>onAttachedToRecyclerView</code> method, and if the item already exists on the adapter you are adding, call <code>notifyItemRangeInserted</code> to update the list.</li><li id="5c4a">Finally, update the <code><a href="https://developer.android.com/reference/androidx/recyclerview/widget/RecyclerView.Adapter.StateRestorationPolicy" rel="noopener ugc nofollow" target="_blank">StateRestorationPolicy</a></code>.</li></ol><h2 id="60f4">getItemViewType &amp; getItemId</h2><p id="2b64">We can call ConcatAdapter’s <code>getItemViewType</code> to find out the view type of a particular location item. The ConcatAdapter then internally calls the <code>ConcatAdapterController</code>’s <code>getItemViewType</code>.</p><figure></figure><p id="ba3e">You can also call <code>getItemId</code> to find out the id of a particular location item. Similarly, call <code>getItemId</code> of <code>ConcatAdapterController</code>.</p><figure></figure><p id="b0cd">We’ve looked at how <code>getItemViewType</code> and <code>getItemId</code> call from <code>wrapper</code> get values from above, so let’s look at <code>findWrapperAndLocalPosition</code> and <code>releaseWrapperAndLocalPosition</code>.</p><h2 id="a018">findWrapperAndLocalPosition</h2><p id="88bc"><code>findWrapperAndLocalPosition</code> is a function that calculates the exact position using the <code>globalPosition</code> received as a parameter.</p><p id="e555">This uses the <code>WrapperAndLocalPosition</code> class, which is defined inside the <code>ConcatAdapterController</code>.</p><figure></figure><p id="0d3b">The <code>findWrapperAndLocalPosition</code> method returns a <code>WrapperAndLocalPosition</code> object with the correct <code>position</code> and corresponding <code>Wrapper</code> in the following procedure.</p><figure></figure><ol><li id="468b">If the <code>mReusableHolder</code> cached using the <code>mInUse</code> flag value of the <code>mReusableHolder</code> field held by the Controller is not used, create a new <code>WrapperAndLocalPosition</code> object.</li><li id="57cc">Use a recurring statement to subtract the cached <code>List&lt;Wrapper&gt;</code> field from the <code>globalPosition</code> by the number of items in the cached wrapper.</li><li id="30b9">If the number of items in the wrapper is larger than the <code>globalPosition</code>, update the <code>wrapper</code> and <code>position</code> of the created <code>wrapperAndLocalPosition</code> object and return it.</li></ol><h2 id="7937">releaseWrapperAndLocalPosition</h2><p id="043b">If <code>mReusableHolder</code> was used due to an action such as <code>getItemId</code> or <code>getItemViewType</code>, the <code>releaseWrapperAndLocalPosition</code> function is called at the end. This is to allow the <code>wrapper</code> and <code>position</code> to be returned without new assignments if possible.</p><figure></figure><h2 id="15c1">conclusion</h2><p id="5a06">Using ConcatAdapter, which represents different types of data as a single adapter and can encapsulate logic, may be a good idea. However, if you don’t understand and use the <code>Config</code> and internal behavior in detail, you may encounter unintended behavior. 🙂</p><h2 id="17cc">Reference</h2><ul><li id="cfbb"><a href="https://medium.com/androiddevelopers/merge-adapters-sequentially-with-mergeadapter-294d2942127a" rel="noopener">https://medium.com/androiddevelopers/merge-adapters-sequentially-with-mergeadapter-294d2942127a</a></li></ul></div></div>

Stories by hongbeom on Medium

Learn about ConcatAdapter.
ConcatAdapter
ConcatAdapter is an adapter that allows you to add multiple adapters that appeared in the version RecyclerView : 1.2.0-alpha02. In the past, RecyclerView.Adapter had to provides view type to each by view in order to show each view in a list form. This led to the presence of multiple ViewHolder on one adapter, and multiple logics were combined. ConcatAdapter is implemented to add multiple adapters to a single adapter and to add them to a single RecyclerView to benefit from encapsulation and reuse.
Component in ConcatAdapter
Config
You can specify default options by passing the ConcatAdapter.Config object to the constructor of the ConcatAdapter.
https://medium.com/media/b11ec7d60184ad8572bfa4b495b88490/href
You can control the isolateViewTypes and stableIdMode in the Config, each of which is described below.
isolateViewTypes : If false, the ConcatAdapter assumes that all adapters assigned to it share a global view type pool so that they reference the same view holder using the same view type. Specifying this option false means that nested adapters can share view holders, but they should not have conflicting view types that return the same view type for different view holders. Defaults to true to separate view types between adapters to avoid using the same view holder.
stableIdMode : There are three StableIdMode
NO_STABLE_IDS : Default value; ignores the stable id of the adapter added to the concatAdapter. If you add an adapter that uses stable id when in this mode, a warning is issued.
2. ISOLATED_STABLE_IDS : In this mode, hasStableIds() of the concatAdapter returns true and requires an adapter with a stable id specified. Because two different adapters may not recognize each other and return the same stable id, ConcatAdapter separates the id pool of each adapter from each other, overwrites the returned stable id, and notifies RecyclerView again. In this mode, the value of RecyclerView.Adapter.getItemId(int) and the value of RecyclerView.ViewHolder.getItemId() may differ. Additionally, if you add an adapter with no stable id specified, an IlllegalArgumentException error occurs.
3. SHARED_STABLE_IDS : Similar to ISOLATED_STABLE_IDS, hasStableIds() returns true and requires an adapter with a stable id. However, unlike ISOLATED_STABLE_IDS, it does not override the returned stable id. In this mode, the sub-adapters must be aware of each other and should not return the same id unless items are moved between the adapters. IllegalArgumentException error occurs when adding an adapter with no stable id specified.
⚠️ If you create a Constructor in ConcatAdapter without handing over the Config, it is designated as Config.DEFAULT, isolateViewTypes as true, and StableIdMode as NO_STABLE_IDS
NestedAdapterWrapper
https://medium.com/media/e4f11e291aa485cc3d7711218aa09050/href
Most functions in ConcatAdapter delegate actions to the ConcatAdapterController. And the ConcatAdapterController implements NestedAdapterWrapper.Callback, so first let’s look at NestedAdapterWrapper.
NestedAdapterWrapper is a class that wraps the adapter we added. Callback looks the same as RecyclerView.AdapterDataObserver, but when we add the Adapter through the addAdapter method, the Adapter and ConcatAdapterController that we want to add to the constructor’s parameter in NestedAdapterWrapper are handed over, and NestedAdapterWrapper takes over Callback (ConcatAdapterController) is called by mapping to each method in the AdapterDataObserver that it has as an internal field.
https://medium.com/media/2f1e4ec81bd34b7feb69bf1aaac80c8b/href
The code above shows that NestedAdapterWrapper has an object called ViewTypeStorage.ViewTypeLookup, StableIdStorage.StableIdLookup as a field. These fields determine the value returned when we look up the id or view type of a particular item in ConcatAdapter.
ViewTypeStorage
ViewTypeStorage is a storage interface that stores and manages view types. It is held as a field by the ConcatAdapterController.
https://medium.com/media/312a87c58d5061644c378b3732c8cd34/href
getItemViewType
When we call the getItemViewType function, the viewtype is eventually returned by NestedAdapterWrapper’s getItemViewType, which is determined by the ViewTypeStorage.ViewTypeLookup interface implementation held in the field.
https://medium.com/media/9931e3eb8ae86b35c9c16b663b5d6766/hrefhttps://medium.com/media/7c1892e5c202c226d63ba9200de8a3ae/href
In addition, ViewTypeStorage is created from the constructor of the ConcatAdapterController, which is divided into two implementations, depending on the value of the isolateViewTypes.
https://medium.com/media/1e6f7fe1f7152d01b04d68d7f99db13d/href
IsolatedViewTypeStorage, SharedIdRangeViewTypeStorage, is declared inside the WrapperViewTypeLookup, the implementation class of the ViewTypeStorage.ViewTypeLookup interface, which returns the view type by referring to the view type managed by the corresponding Storage.
SharedIdRangeViewTypeStorage: getItemViewType returns the view type of the adapter connected to the NestedAdapterWrapper.
IsolatedViewTypeStorage: getItemViewType returns if it is already stored, otherwise create a unique viewtype and return it. (Even if N adapters with the same view type are added, different unique view types will be returned for getItemViewType
StableIdStorage
StableIdStorage is a storage interface that stores Stableid. Similarly, the ConcatAdapterController holds it as a field.
https://medium.com/media/f17cb9d3486e90ef6a9fb9ac9fc9b811/href
getItemId
Similarly, if the NestedAdapterWrapper holds StableIdLookup as a field and calls getItemId, this field returns id.
https://medium.com/media/efba6ce3cd3b9479fde879d5d4ee11c1/href
Implementations of StableIdLookup are declared inside three implementations of the StableIdStorage interface, each of which behaves in this way.
NoStableIdStorage: unconditionally return RecyclerView.NO_ID (default)
SharedPoolStableIdStorage: Return the adapter’s getItemId value as it is
IsolatedStableIdStorage: If the getItemId value of the adapter received as a parameter is already stored, return the id or create a unique id to return it.
Under the hood
addAdapter
What will happen internally if we call addAdapter to add one adapter? The ConcatAdapter calls the addAdapter of the ConcatAdapterController and the ConcatAdapterController follows the process below to perform the addAdapter.
Controller — addAdapter
index inspection: When we hand over the adapter and index to add them to the addAdapter, we first check whether this index is a value between 0 and the current number of adapters. If you try to add an index value beyond this range, IndexOutOfBoundsException occurs. If you call addAdapter without passing the index, you will add the adapter internally in the last order.
Stable id check: Exception occurs when StableIdMode of the current ConcatAdapter is not NO_STABLE_IDS and hasStableId of the adapter you are adding is true. If StableIdMode is NO_STABLE_IDS and the hasStableIds of the adapter you want to add is true, it will log the warning.
Check whether the adapter has already been added: If the adapter already added to the ConcatAdapter matches the adapter you want to add, return it without adding the adapter.
Once you have completed this process, create a NestedAdapterWrapper, connect the adapter you want to add, and cache it. In addition, notify the RecyclerView connected to the ConcatAdapter through the onAttachedToRecyclerView method, and if the item already exists on the adapter you are adding, call notifyItemRangeInserted to update the list.
Finally, update the StateRestorationPolicy.
getItemViewType & getItemId
We can call ConcatAdapter’s getItemViewType to find out the view type of a particular location item. The ConcatAdapter then internally calls the ConcatAdapterController’s getItemViewType.
https://medium.com/media/bb9dcff068a175f52fc04853821d4ef4/href
You can also call getItemId to find out the id of a particular location item. Similarly, call getItemId of ConcatAdapterController.
https://medium.com/media/1c01fc2035b3198057963170db26d3df/href
We’ve looked at how getItemViewType and getItemId call from wrapper get values from above, so let’s look at findWrapperAndLocalPosition and releaseWrapperAndLocalPosition.
findWrapperAndLocalPosition
findWrapperAndLocalPosition is a function that calculates the exact position using the globalPosition received as a parameter.
This uses the WrapperAndLocalPosition class, which is defined inside the ConcatAdapterController.
https://medium.com/media/74dd3620684de8c8c1c0db8ef2a4ee19/href
The findWrapperAndLocalPosition method returns a WrapperAndLocalPosition object with the correct position and corresponding Wrapper in the following procedure.
https://medium.com/media/4fa0a8f08d9cc12a6d866b56b7e40203/href
If the mReusableHolder cached using the mInUse flag value of the mReusableHolder field held by the Controller is not used, create a new WrapperAndLocalPosition object.
Use a recurring statement to subtract the cached List<Wrapper> field from the globalPosition by the number of items in the cached wrapper.
If the number of items in the wrapper is larger than the globalPosition, update the wrapper and position of the created wrapperAndLocalPosition object and return it.
releaseWrapperAndLocalPosition
If mReusableHolder was used due to an action such as getItemId or getItemViewType, the releaseWrapperAndLocalPosition function is called at the end. This is to allow the wrapper and position to be returned without new assignments if possible.
https://medium.com/media/cbd747915ed6a26a4fc1c7a38be1f200/href
conclusion
Using ConcatAdapter, which represents different types of data as a single adapter and can encapsulate logic, may be a good idea. However, if you don’t understand and use the Config and internal behavior in detail, you may encounter unintended behavior. 🙂
Reference
https://medium.com/androiddevelopers/merge-adapters-sequentially-with-mergeadapter-294d2942127a
ConcatAdapter Deep Dive was originally published in ProAndroidDev on Medium, where people are continuing the conversation by highlighting and responding to this story.

ConcatAdapter Deep Dive

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