获取 Composable 中状态的先前值 - Jetpack Compose

假设我的代码如下所示

@Composable
fun ExampleList() {
    val tickers by exampleViewModel.tickers.observeAsState()
    LazyColumn() {
        items(items = tickers) { ticker ->
            ExampleItem(ticker)
        }
    }
}
@Composable
fun ExampleItem(ticker: Ticker) {
    Text(text= ticker.lastPrice)
}

每次更新股票代码时，是否都可以在ExampleItem Compose 中获取股票代码的先前值？
我想知道 React Native 中是否有类似 componentDidUpdate 的东西

虽然答案在技术上是正确的，但第一个示例渲染了太多次，不幸的是我不理解第二个示例。

所以我回到了React看看那里是如何完成的，并且解释得很好here https://usehooks.com/usePrevious/:

这就是钩子（remember函数如你所愿）看起来像（对于好奇的人来说）：

function usePrevious<T>(value: T): T {
  // The ref object is a generic container whose current property is mutable ...
  // ... and can hold any value, similar to an instance property on a class
  const ref: any = useRef<T>();
  // Store current value in ref
  useEffect(() => {
    ref.current = value;
  }, [value]); // Only re-run if value changes
  // Return previous value (happens before update in useEffect above)
  return ref.current;
}

同样的想法可以以可重用的方式在 compose 中实现（重要的是@Composable设置先前值时不应重新渲染）：

/**
 * Returns a dummy MutableState that does not cause render when setting it
 */
@Composable
fun <T> rememberRef(): MutableState<T?> {
    // for some reason it always recreated the value with vararg keys,
    // leaving out the keys as a parameter for remember for now
    return remember() {
        object: MutableState<T?> {
            override var value: T? = null

            override fun component1(): T? = value

            override fun component2(): (T?) -> Unit = { value = it }
        }
    }
}

和实际的rememberPrevious:

@Composable
fun <T> rememberPrevious(
    current: T,
    shouldUpdate: (prev: T?, curr: T) -> Boolean = { a: T?, b: T -> a != b },
): T? {
    val ref = rememberRef<T>()

    // launched after render, so the current render will have the old value anyway
    SideEffect {
        if (shouldUpdate(ref.value, current)) {
            ref.value = current
        }
    }

    return ref.value
}

key值可以添加到remember功能，但我发现remember在我的情况下不起作用，因为即使没有keys被传入。

Usage:

@Composable
fun SomeComponent() {

    ...
    val prevValue = rememberPrevious(currentValue)
}