In previous post we talked about the companion object Modifier and how it’s implementing the interface Modifier. Now it is time to go deeper and see Modifier’s internals.
Situation 1 What is Modifier after all?
Answer Well, what if I tell you Modifier is just a binary tree data structure.
Let’s dive into the source code to prove this.
First let’s take a look at the class hierachy, we have 3 pieces: Modifier(interface), Element and CombinedModifier.
Let’s talk about Modifier.
Situation 1 What is the simplest Modifier?
Answer Modifier What is it? a class? an object?
It is a companion object, which implements the Modifier interface.
companion object : Modifier { override fun <R> foldIn(initial: R, operation: (R, Element) -> R): R = initial override fun <R> foldOut(initial: R, operation: (Element, R) -> R): R = initial override fun any(predicate: (Element) -> Boolean): Boolean = false override fun all(predicate: (Element) -> Boolean): Boolean = true override infix fun then(other: Modifier): Modifier = other override fun toString() = "Modifier" } The formal version is Modifier.
This time we are focusing on derivedStateOf and how it is different from remember.
Let’s look at this simple example:
Situation 1 What will happen when user clicks on the Text?
@Composable private fun Situation1() { var name by remember { mutableStateOf("guowei") } val uppercase by remember { derivedStateOf { name.uppercase() } } Text(uppercase, modifier = Modifier.clickable { name = "hello" }) } Answer The text will change from “GUOWEI” to “HELLO”.
Let’s do a bit of recap first. (I highly suggest you go through previous post if not already done so)
Situation 1 Will clicking the button trigger the recompose of UserPage?
data class User(var name: String) class MainActivity : ComponentActivity() { override fun onCreate(savedInstanceState: Bundle?) { super.onCreate(savedInstanceState) val user1 = User("Guowei Lv") val user2 = User("Guowei Lv") var user = user1 setContent { var flag by remember { mutableStateOf(true) } Column { Text(text = "Flag is $flag") UserPage(user = user) Button(onClick = { flag = !
