I would like to know the difference between:

var textFieldState = remember {
    mutableStateOf("")
}

and

var textFieldState by remember {
    mutableStateOf("")
}

Is there any advantage over the other?

CodePudding user response：

Is there any advantage over the other?

The first really should be a val and not a var. Otherwise, they are equivalent. Or, to quote the documentation:

There are three ways to declare a MutableState object in a composable:

• val mutableState = remember { mutableStateOf(default) }
• var value by remember { mutableStateOf(default) }
• val (value, setValue) = remember { mutableStateOf(default) }

These declarations are equivalent, and are provided as syntax sugar for different uses of state. You should pick the one that produces the easiest-to-read code in the composable you're writing.

In those three:

• In the first, mutableState holds a MutableState, and you use .value and .value= to manipulate the contents
• In the second, value holds a MutableState, but the by syntax tells the compiler to treat it as a property delegate, so we can pretend that value just holds the underlying data
• In the third, a destructuring declaration gives you getter and setter references to manipulate the content in the underlying MutableState

CodePudding user response：

The by in this context is a kotlin property delegate. Any class that implements the operator fun operator fun getValue(thisRef: Any?, prop: KProperty<*>): T can use this syntax. Using = will eagerly assign the variable (importantly without delegation), the by will delegate to the operator function. The remember in this case is just a shortcut function to creating the Remember delgate that wraps the value you are creating inside the { ... } block.

A typical example is the kotlin Lazy<T> class : val myValue : Int by lazy { 1 }. If used with the by operator you will return the Int value, if used with = it will return Lazy<Int> as you have not used delegation.

It is also worth noting that delgates can be setters as well by using this operator fun : operator fun setValue(thisRef: Any?, prop: KProperty<*>, value: T).