How to define a delegate property which need to be initialized delayed in Kotlin? - kotlin

Question

I need to define a property isBackgroundWindow by delegate class PreferenceTool, but the following code will cause NullPointerException error.
I know that private val prefs: SharedPreferences by lazy { } in PreferenceTool<T> is lazy, so the object this is not initialized when system invoke PreferenceTool(this, getString(R.string.IsBackgroundName) , false), it will cause null error.
I hope to use the code private lateinit var isBackgroundWindow: Boolean by PreferenceTool(this, getString(R.string.IsBackgroundName) , false), but
'lateinit' modifier is not allowed on delegated properties .
How can I do?
Main
class UIHome : AppCompatActivity() {
//I think the object this is not initialized, it will cause null error.
private var isBackgroundWindow: Boolean by PreferenceTool(this, getString(R.string.IsBackgroundName) , false)
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.layout_home)
isBackgroundWindow=false
}
}
Delegate Class
class PreferenceTool<T>(private val context: Context, private val name: String, private val default: T) {
private val prefs: SharedPreferences by lazy {
context.defaultSharedPreferences
}
operator fun getValue(thisRef: Any?, property: KProperty<*>): T = findPreference(name, default)
operator fun setValue(thisRef: Any?, property: KProperty<*>, value: T) {
putPreference(name, value)
}
#Suppress("UNCHECKED_CAST")
private fun findPreference(name: String, default: T): T = with(prefs) {
val res: Any = when (default) {
is Long -> getLong(name, default)
is String -> getString(name, default)
is Int -> getInt(name, default)
is Boolean -> getBoolean(name, default)
is Float -> getFloat(name, default)
else -> throw IllegalArgumentException("This type can be saved into Preferences")
}
res as T
}
#SuppressLint("CommitPrefEdits")
private fun putPreference(name: String, value: T) = with(prefs.edit()) {
when (value) {
is Long -> putLong(name, value)
is String -> putString(name, value)
is Int -> putInt(name, value)
is Boolean -> putBoolean(name, value)
is Float -> putFloat(name, value)
else -> throw IllegalArgumentException("This type can't be saved into Preferences")
}.apply()
}
}

Answer 1

You can lateinit the variable isBackground like this way .Your UIHome class should be
class UIHome : AppCompatActivity() {
var isBackground: Boolean by Delegates.notNull<Boolean>()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_uihome)
isBackground = isBackgroundWindow()
}
fun isBackgroundWindow(): Boolean {
val isBackgroundWindow: Boolean by PreferenceTool(
this, getString(R.string.IsBackgroundName), false
)
return isBackgroundWindow
}
}
This is avoid null pointer exception
isBackground?.let {
// your code
}