Hard Prerequisites |
IMPORTANT: Please review these prerequisites, they include important information that will help you with this content. |
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The sleep-tracker app has two screens, represented by fragments, as shown in the figure below.
The first screen, shown on the left, has buttons for starting and stopping tracking. The screen shows some of the user’s sleep data. The Clear button permanently deletes all the data that the app has collected for the user. The second screen, shown on the right, is for selecting a sleep-quality rating.
This app is architected to use a UI controller, ViewModel and LiveData, and a Room database to persist sleep data.
The sleep data is displayed in a RecyclerView. In this project, you build the DiffUtil and data-binding portion for the RecyclerView. After this project, your app will look exactly the same, but it will be more efficient and easier to scale and maintain.
You can continue using the SleepTracker app from the previous project.
-Run the app.
To tell RecyclerView that an item in the list has changed and needs to be updated, the current code calls notifyDataSetChanged() in the SleepNightAdapter, as shown below.
var data = listOf<SleepNight>()
set(value) {
field = value
notifyDataSetChanged()
}
However, notifyDataSetChanged()
tells RecyclerView that the entire list is potentially invalid. As a result, RecyclerView rebinds and redraws every item in the list, including items that are not visible on screen. This is a lot of unnecessary work. For large or complex lists, this process could take long enough that the display flickers or stutters as the user scrolls through the list.
To fix this problem, you can tell RecyclerView exactly what has changed. RecyclerView can then update only the views that changed on screen.
RecyclerView has a rich API for updating a single element. You could use notifyItemChanged() to tell RecyclerView that an item has changed, and you could use similar functions for items that are added, removed, or moved. You could do it all manually, but that task would be non-trivial and might involve quite a bit of code.
Fortunately, there’s a better way.
RecyclerView has a class called DiffUtil which is for calculating the differences between two lists. DiffUtil takes an old list and a new list and figures out what’s different. It finds items that were added, removed, or changed. Then it uses an algorithm called a Eugene W. Myers’s difference algorithm to figure out the minimum number of changes to make from the old list to produce the new list.
Once DiffUtil figures out what has changed, RecyclerView can use that information to update only the items that were changed, added, removed, or moved, which is much more efficient than redoing the entire list.
In this task, you upgrade the SleepNightAdapter to use DiffUtil to optimize the RecyclerView for changes to the data.
In order to use the functionality of the DiffUtil class, extend DiffUtil.ItemCallback.
class SleepNightDiffCallback : DiffUtil.ItemCallback<SleepNight>() {
}
This generates stubs inside SleepNightDiffCallback for the two methods, as shown below. DiffUtil uses these two methods to figure out how the list and items have changed.
override fun areItemsTheSame(oldItem: SleepNight, newItem: SleepNight): Boolean {
TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
}
override fun areContentsTheSame(oldItem: SleepNight, newItem: SleepNight): Boolean {
TODO("not implemented") //To change body of created functions use File | Settings | File Templates.
}
areItemsTheSame()
, replace the TODO with code that tests whether the two passed-in SleepNight items, oldItem and newItem, are the same. If the items have the same nightId, they are the same item, so return true. Otherwise, return false. DiffUtil uses this test to help discover if an item was added, removed, or moved.override fun areItemsTheSame(oldItem: SleepNight, newItem: SleepNight): Boolean {
return oldItem.nightId == newItem.nightId
}
areContentsTheSame()
, check whether oldItem and newItem contain the same data; that is, whether they are equal. This equality check will check all the fields, because SleepNight is a data class. Data classes automatically define equals and a few other methods for you. If there are differences between oldItem and newItem, this code tells DiffUtil that the item has been updated.override fun areContentsTheSame(oldItem: SleepNight, newItem: SleepNight): Boolean {
return oldItem == newItem
}
It’s a common pattern to use a RecyclerView to display a list that changes. RecyclerView provides an adapter class, ListAdapter, that helps you build a RecyclerView adapter that’s backed by a list.
ListAdapter keeps track of the list for you and notifies the adapter when the list is updated.
import androidx.recyclerview.widget.ListAdapter
.class SleepNightAdapter : ListAdapter<SleepNight, SleepNightAdapter.ViewHolder>(SleepNightDiffCallback()) {
getItemCount()
, because the ListAdapter implements this method for you.onBindViewHolder()
, change the item variable. Instead of using data to get an item, call the getItem(position) method that the ListAdapter provides.val item = getItem(position)
Your code needs to tell the ListAdapter when a changed list is available. ListAdapter provides a method called submitList() to tell ListAdapter that a new version of the list is available. When this method is called, the ListAdapter diffs the new list against the old one and detects items that were added, removed, moved, or changed. Then the ListAdapter updates the items shown by RecyclerView.
onCreateView()
, in the observer on sleepTrackerViewModel, find the error where the data variable that you’ve deleted is referenced.sleepTrackerViewModel.nights.observe(viewLifecycleOwner, Observer {
it?.let {
adapter.submitList(it)
}
})
In this task, you use the same technique as in previous projects to set up data binding, and you eliminate calls to findViewById().
<data>
<variable
name="sleep"
type="com.example.android.trackmysleepquality.database.SleepNight"/>
</data>
Code to delete:
val view = layoutInflater
.inflate(R.layout.list_item_sleep_night, parent, false)
Where the view variable was, define a new variable called binding that inflates the ListItemSleepNightBinding binding object, as shown below. Make the necessary import of the binding object.
val binding =
ListItemSleepNightBinding.inflate(layoutInflater, parent, false)
return ViewHolder(binding)
Scroll up to the class definition of the ViewHolder to see the change in the signature. You see an error for itemView, because you changed itemView to binding in the from() method.
In the ViewHolder class definition, right-click on one of the occurrences of itemView and select Refactor > Rename. Change the name to binding.
Prefix the constructor parameter binding with val to make it a property.
In the call to the parent class, RecyclerView.ViewHolder, change the parameter from binding to binding.root. You need to pass a View, and binding.root is the root ConstraintLayout in your item layout.
Your finished class declaration should look like the code below.
class ViewHolder private constructor(val binding: ListItemSleepNightBinding) : RecyclerView.ViewHolder(binding.root){
You also see an error for the calls to findViewById(), and you fix this next.
You can now update the sleepLength, quality, and qualityImage properties to use the binding object instead of findViewById().
val sleepLength: TextView = binding.sleepLength
val quality: TextView = binding.qualityString
val qualityImage: ImageView = binding.qualityImage
With the binding object in place, you don’t need to define the sleepLength, quality, and qualityImage properties at all anymore. DataBinding will cache the lookups, so there is no need to declare these properties.
In this task, you upgrade your app to use data binding with binding adapters to set the data in your views.
In a previous project, you used the Transformations class to take LiveData and generate formatted strings to display in text views. However, if you need to bind different types, or complex types, you can provide binding adapters to help data binding use those types. Binding adapters are adapters that take your data and adapt it into something that data binding can use to bind a view, like text or an image.
You are going to implement three binding adapters, one for the quality image, and one for each text field. In summary, to declare a binding adapter, you define a method that takes an item and a view, and annotate it with @BindingAdapter. In the body of the method, you implement the transformation. In Kotlin, you can write a binding adapter as an extension function on the view class that receives the data.
Note that you will have to import a number of classes in the step, and it will not be called out individually.
fun TextView.setSleepDurationFormatted(item: SleepNight) {}
text = convertDurationToFormatted(item.startTimeMilli, item.endTimeMilli, context.resources)
@BindingAdapter("sleepDurationFormatted")
The second adapter sets the sleep quality based on the value in a SleepNight object. Create an extension function called setSleepQualityString() on TextView, and pass in a SleepNight.
In the body, bind the data to the view as you did in ViewHolder.bind(). Call convertNumericQualityToString and set the text.
Annotate the function with @BindingAdapter("sleepQualityString").
@BindingAdapter("sleepQualityString")
fun TextView.setSleepQualityString(item: SleepNight) {
text = convertNumericQualityToString(item.sleepQuality, context.resources)
}
@BindingAdapter("sleepImage")
fun ImageView.setSleepImage(item: SleepNight) {
setImageResource(when (item.sleepQuality) {
0 -> R.drawable.ic_sleep_0
1 -> R.drawable.ic_sleep_1
2 -> R.drawable.ic_sleep_2
3 -> R.drawable.ic_sleep_3
4 -> R.drawable.ic_sleep_4
5 -> R.drawable.ic_sleep_5
else -> R.drawable.ic_sleep_active
})
}
fun bind(item: SleepNight) {
}
binding.sleep = item
Below that line, add binding.executePendingBindings(). This call is an optimization that asks data binding to execute any pending bindings right away. It's always a good idea to call executePendingBindings() when you use binding adapters in a RecyclerView, because it can slightly speed up sizing the views.
binding.executePendingBindings()
This property creates the connection between the view and the binding object, via the adapter. Whenever sleepImage is referenced, the adapter will adapt the data from the SleepNight.
app:sleepImage="@{sleep}"
Do the same for the sleep_length and the quality_string text views. Whenever sleepDurationFormatted or sleepQualityString are referenced, the adapters will adapt the data from the SleepNight.
app:sleepDurationFormatted="@{sleep}"
app:sleepQualityString="@{sleep}"
You’ve displayed the same list for the last few exercises. That’s by design, to show you that the Adapter interface allows you to architect your code in many different ways. The more complex your code, the more important it becomes to architect it well. In production apps, these patterns and others are used with RecyclerView. The patterns all work, and each has its benefits. Which one you choose depends on what you are building.
Congrats! At this point you’re well on your way to mastering RecyclerView on Android.