Android开发Compose remember原理解析

private val items = arrayOf(Color.Red, Color.Gray, Color.Magenta, Color.Blue, Color.Green, Color.Cyan)
@Composable
fun ColorText(name: String) {
    val color = items.random()
    val clicked = mutableStateOf(0)
    Log.d("ct", "ui compose")
    Column {
        Text(
            text = "I'm colored: ${clicked.value}", modifier = Modifier
                .padding(16.dp)
                .background(color)
                .clickable {
                    Log.d("ct", "clicked")
                    clicked.value = clicked.value + 1
                }
        )
    }
}

@Composable
fun ColorText(name: String) {
    val color = remember { items.random() }
    val clicked = remember { mutableStateOf(0) }
    //...
}

@Composable
inline fun <T> remember(calculation: @DisallowComposableCalls () -> T): T =
    currentComposer.cache(false, calculation)

@ComposeCompilerApi
inline fun <T> Composer.cache(invalid: Boolean, block: () -> T): T {
    @Suppress("UNCHECKED_CAST")
    return rememberedValue().let {
        // 无效或Empty值时，走if流程，计算并保存值，否则直接返回
        // remember传入的invalid为false，所以肯定走值判断
        if (invalid || it === Composer.Empty) {
            val value = block()
            updateRememberedValue(value)
            value
        } else it
    } as T
}
// 要么返回Composer.Empty ，要么返回传给updateRememberedValue的值
@ComposeCompilerApi
fun rememberedValue(): Any?
// 更新调用rememberedValue()后的值，且此值在下一次调用rememberedValue()时返回
@ComposeCompilerApi
fun updateRememberedValue(value: Any?)
interface Composer {
    // ....
    companion object {
        /**
         * 用于标记无值的状态
         */
        val Empty = object {
            override fun toString() = "Empty"
        }
    }
}