When all you need to do is receive a file from another application, implementing an entire ContentProvider is overkill. Fortunately, Google has provided a convenience class called FileProvider that takes care of everything except the configuration work.

The first step is to declare FileProvider as a ContentProvider hooked up to a specific authority. Do this by adding a content provider declaration to your Android manifest.

<activity android:name=".MainActivity">
    ...
</activity>
<provider
        android:name="androidx.core.content.FileProvider"
        android:authorities="com.bignerdranch.android.criminalintent.fileprovider"
        android:exported="false"
        android:grantUriPermissions="true">
</provider>

The authority is a location – a place that files will be saved to. The string you choose for android:authorities must be unique across the entire system. To help ensure this, the convention is to prepend the authority string with your package name. (We show the package name com.bignerdranch.android.criminalintent above. If your app’s package name is different, use your package name instead.)

By hooking up FileProvider to your authority, you give other apps a target for their requests. By adding the exported="false" attribute, you keep anyone from using your provider except you or anyone you grant permission to. And by adding the grantUriPermissions attribute, you add the ability to grant other apps permission to write to URIs on this authority when you send them out in an intent. (Keep an eye out for this later.)

Now that you have told Android where your FileProvider is, you also need to tell your FileProvider which files it is exposing. This bit of configuration is done with an extra XML resource file. Right-click your app/res folder in the project tool window and select New → Android resource file. Enter files for the name, and for Resource type select XML. Click OK and Android Studio will add and open the new resource file.

In the text view of your new res/xml/files.xml, add details about the file path (Listing 16.5).

<PreferenceScreen xmlns:android="http://schemas.android.com/apk/res/android">

</PreferenceScreen>
<paths>
    <files-path name="crime_photos" path="."/>
</paths>

This XML file says, “Map the root path of my private storage as crime_photos.” You will not use the crime_photos name – FileProvider uses that internally.

Now, hook up files.xml to your FileProvider by adding a meta-data tag in your AndroidManifest.xml.

<provider
        android:name="androidx.core.content.FileProvider"
        android:authorities="com.bignerdranch.android.criminalintent.fileprovider"
        android:exported="false"
        android:grantUriPermissions="true">
    <meta-data
            android:name="android.support.FILE_PROVIDER_PATHS"
            android:resource="@xml/files"/>
</provider>

Time to give your pictures a place to live on disk locally. First, add a computed property to Crime to get a well-known filename.

@Entity
data class Crime(@PrimaryKey val id: UUID = UUID.randomUUID(),
                 var title: String = "",
                 var date: Date = Date(),
                 var isSolved: Boolean = false,
                 var suspect: String = "") {

    val photoFileName
        get() = "IMG_$id.jpg"
}

photoFileName does not include the path to the folder the photo will be stored in. However, the filename will be unique, since it is based on the Crime’s ID.

Next, find where the photos should live. CrimeRepository is responsible for everything related to persisting data in CriminalIntent, so it is a natural owner for this idea. Add a getPhotoFile(Crime) function to CrimeRepository that provides a complete local file path for Crime’s image.

class CrimeRepository private constructor(context: Context) {
    ...
    private val executor = Executors.newSingleThreadExecutor()
    private val filesDir = context.applicationContext.filesDir

    fun addCrime(crime: Crime) {
        ...
    }

    fun getPhotoFile(crime: Crime): File = File(filesDir, crime.photoFileName)
    ...
}

This code does not create any files on the filesystem. It only returns File objects that point to the right locations. Later on, you will use FileProvider to expose these paths as URIs.

Finally, add a function to CrimeDetailViewModel to expose the file information to CrimeFragment.

class CrimeDetailViewModel : ViewModel() {
    ...
    fun saveCrime(crime: Crime) {
        crimeRepository.updateCrime(crime)
    }

    fun getPhotoFile(crime: Crime): File {
        return crimeRepository.getPhotoFile(crime)
    }
}