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RecyclerView Parts and Functionality

  • Artikkeli

RecyclerView handles some tasks internally (such as the scrolling and recycling of views), but it is essentially a manager that coordinates helper classes to display a collection. RecyclerView delegates tasks to the following helper classes:

  • Adapter – Inflates item layouts (instantiates the contents of a layout file) and binds data to views that are displayed within a RecyclerView. The adapter also reports item-click events.

  • LayoutManager – Measures and positions item views within a RecyclerView and manages the policy for view recycling.

  • ViewHolder – Looks up and stores view references. The view holder also helps with detecting item-view clicks.

  • ItemDecoration – Allows an app to add special drawing and layout offsets to specific views for drawing dividers between items, highlights, and visual grouping boundaries.

  • ItemAnimator – Defines the animations that take place during item actions or as changes are made to the adapter.

The relationship between the RecyclerView, LayoutManager, and Adapter classes is depicted in the following diagram:

Diagram of RecyclerView containing LayoutManager, using Adapter to access Data Set

As this figure illustrates, the LayoutManager can be thought of as the intermediary between the Adapter and the RecyclerView. The LayoutManager makes calls into Adapter methods on behalf of the RecyclerView. For example, the LayoutManager calls an Adapter method when it is time to create a new view for a particular item position in the RecyclerView. The Adapter inflates the layout for that item and creates a ViewHolder instance (not shown) to cache references to the views at that position. When the LayoutManager calls the Adapter to bind a particular item to the data set, the Adapter locates the data for that item, retrieves it from the data set, and copies it to the associated item view.

When using RecyclerView in your app, creating derived types of the following classes is required:

  • RecyclerView.Adapter – Provides a binding from your app's data set (which is specific to your app) to item views that are displayed within the RecyclerView. The adapter knows how to associate each item-view position in the RecyclerView to a specific location in the data source. In addition, the adapter handles the layout of the contents within each individual item view and creates the view holder for each view. The adapter also reports item-click events that are detected by the item view.

  • RecyclerView.ViewHolder – Caches references to the views in your item layout file so that resource lookups are not repeated unnecessarily. The view holder also arranges for item-click events to be forwarded to the adapter when a user taps the view-holder's associated item view.

  • RecyclerView.LayoutManager – Positions items within the RecyclerView. You can use one of several predefined layout managers or you can implement your own custom layout manager. RecyclerView delegates the layout policy to the layout manager, so you can plug in a different layout manager without having to make significant changes to your app.

Also, you can optionally extend the following classes to change the look and feel of RecyclerView in your app:

  • RecyclerView.ItemDecoration
  • RecyclerView.ItemAnimator

If you do not extend ItemDecoration and ItemAnimator, RecyclerView uses default implementations. This guide does not explain how to create custom ItemDecoration and ItemAnimator classes; for more information about these classes, see RecyclerView.ItemDecoration and RecyclerView.ItemAnimator.

How View Recycling Works

RecyclerView does not allocate an item view for every item in your data source. Instead, it allocates only the number of item views that fit on the screen and it reuses those item layouts as the user scrolls. When the view first scrolls out of sight, it goes through the recycling process illustrated in the following figure:

Diagram illustrating the six steps of view recycling

  1. When a view scrolls out of sight and is no longer displayed, it becomes a scrap view.

  2. The scrap view is placed in a pool and becomes a recycle view. This pool is a cache of views that display the same type of data.

  3. When a new item is to be displayed, a view is taken from the recycle pool for reuse. Because this view must be re-bound by the adapter before being displayed, it is called a dirty view.

  4. The dirty view is recycled: the adapter locates the data for the next item to be displayed and copies this data to the views for this item. References for these views are retrieved from the view holder associated with the recycled view.

  5. The recycled view is added to the list of items in the RecyclerView that are about to go on-screen.

  6. The recycled view goes on-screen as the user scrolls the RecyclerView to the next item in the list. Meanwhile, another view scrolls out of sight and is recycled according to the above steps.

In addition to item-view reuse, RecyclerView also uses another efficiency optimization: view holders. A view holder is a simple class that caches view references. Each time the adapter inflates an item-layout file, it also creates a corresponding view holder. The view holder uses FindViewById to get references to the views inside the inflated item-layout file. These references are used to load new data into the views every time the layout is recycled to show new data.

The Layout Manager

The layout manager is responsible for positioning items in the RecyclerView display; it determines the presentation type (a list or a grid), the orientation (whether items are displayed vertically or horizontally), and which direction items should be displayed (in normal order or in reverse order). The layout manager is also responsible for calculating the size and position of each item in the RecycleView display.

The layout manager has an additional purpose: it determines the policy for when to recycle item views that are no longer visible to the user. Because the layout manager is aware of which views are visible (and which are not), it is in the best position to decide when a view can be recycled. To recycle a view, the layout manager typically makes calls to the adapter to replace the contents of a recycled view with different data, as described previously in How View Recycling Works.

You can extend RecyclerView.LayoutManager to create your own layout manager, or you can use a predefined layout manager. RecyclerView provides the following predefined layout managers:

  • LinearLayoutManager – Arranges items in a column that can be scrolled vertically, or in a row that can be scrolled horizontally.

  • GridLayoutManager – Displays items in a grid.

  • StaggeredGridLayoutManager – Displays items in a staggered grid, where some items have different heights and widths.

To specify the layout manager, instantiate your chosen layout manager and pass it to the SetLayoutManager method. Note that you must specify the layout manager – RecyclerView does not select a predefined layout manager by default.

For more information about the layout manager, see the RecyclerView.LayoutManager class reference.

The View Holder

The view holder is a class that you define for caching view references. The adapter uses these view references to bind each view to its content. Every item in the RecyclerView has an associated view holder instance that caches the view references for that item. To create a view holder, use the following steps to define a class to hold the exact set of views per item:

  1. Subclass RecyclerView.ViewHolder.
  2. Implement a constructor that looks up and stores the view references.
  3. Implement properties that the adapter can use to access these references.

A detailed example of a ViewHolder implementation is presented in A Basic RecyclerView Example. For more information about RecyclerView.ViewHolder, see the RecyclerView.ViewHolder class reference.

The Adapter

Most of the "heavy-lifting" of the RecyclerView integration code takes place in the adapter. RecyclerView requires that you provide an adapter derived from RecyclerView.Adapter to access your data source and populate each item with content from the data source. Because the data source is app-specific, you must implement adapter functionality that understands how to access your data. The adapter extracts information from the data source and loads it into each item in the RecyclerView collection.

The following drawing illustrates how the adapter maps content in a data source through view holders to individual views within each row item in the RecyclerView:

Diagram illustrating Adapter connecting Data Source to ViewHolders

The adapter loads each RecyclerView row with data for a particular row item. For row position P, for example, the adapter locates the associated data at position P within the data source and copies this data to the row item at position P in the RecyclerView collection. In the above drawing, for example, the adapter uses the view holder to lookup the references for the ImageView and TextView at that position so it doesn't have to repeatedly call FindViewById for those views as the user scrolls through the collection and reuses views.

When you implement an adapter, you must override the following RecyclerView.Adapter methods:

  • OnCreateViewHolder – Instantiates the item layout file and view holder.

  • OnBindViewHolder – Loads the data at the specified position into the views whose references are stored in the given view holder.

  • ItemCount – Returns the number of items in the data source.

The layout manager calls these methods while it is positioning items within the RecyclerView.

Notifying RecyclerView of Data Changes

RecyclerView does not automatically update its display when the contents of its data source changes; the adapter must notify RecyclerView when there is a change in the data set. The data set can change in many ways; for example, the contents within an item can change or the overall structure of the data may be altered. RecyclerView.Adapter provides a number of methods that you can call so that RecyclerView responds to data changes in the most efficient manner:

  • NotifyItemChanged – Signals that the item at the specified position has changed.

  • NotifyItemRangeChanged – Signals that the items in the specified range of positions have changed.

  • NotifyItemInserted – Signals that the item in the specified position has been newly inserted.

  • NotifyItemRangeInserted – Signals that the items in the specified range of positions have been newly inserted.

  • NotifyItemRemoved – Signals that the item in the specified position has been removed.

  • NotifyItemRangeRemoved – Signals that the items in the specified range of positions have been removed.

  • NotifyDataSetChanged – Signals that the data set has changed (forces a full update).

If you know exactly how your data set has changed, you can call the appropriate methods above to refresh RecyclerView in the most efficient manner. If you do not know exactly how your data set has changed, you can call NotifyDataSetChanged, which is far less efficient because RecyclerView must refresh all the views that are visible to the user. For more information about these methods, see RecyclerView.Adapter.

In the next topic, A Basic RecyclerView Example, an example app is implemented to demonstrate real code examples of the parts and functionality outlined above.