使用 kotlin 和挂起函数在协程中返回
在行 return@withContext cachedCategories 中，因为它不能仅返回 cachedCategories。什么是@withContext？
完整代码：
@Singleton 类 FoodMenuRemoteSource @Inject 构造函数(private val foodMenuApi: FoodMenuApi) {
private var cachedCategories: List<FoodItem>? = null
suspend fun getFoodCategories(): List<FoodItem> = withContext(Dispatchers.IO) {
var cachedCategories = cachedCategories
if (cachedCategories == null) {
cachedCategories = foodMenuApi.getFoodCategories().mapCategoriesToItems()
[email protected] = cachedCategories
}
return@withContext cachedCategories
}
IN line return@withContext cachedCategories because it can't just be return cachedCategories only. Whats @withContext ?
Code full:
@Singleton
class FoodMenuRemoteSource @Inject constructor(private val foodMenuApi: FoodMenuApi) {
private var cachedCategories: List<FoodItem>? = null
suspend fun getFoodCategories(): List<FoodItem> = withContext(Dispatchers.IO) {
var cachedCategories = cachedCategories
if (cachedCategories == null) {
cachedCategories = foodMenuApi.getFoodCategories().mapCategoriesToItems()
[email protected] = cachedCategories
}
return@withContext cachedCategories
}
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不允许在 lambda 中使用非本地返回。这就是为什么 @withContext 是必要的。您会看到，那里的代码块实际上不是
getFoodCategories的主体，而是作为withContext的第二个参数的 lambda 函数。此外，lambda 中的最后一个表达式也自动成为它的返回值，因此您实际上可以像这样完全保留return@withContextYou are not allowed to have a non-local return in a lambda. That's why @withContext is necessary. You see, the code block there is in fact not the body of the
getFoodCategoriesbut the lambda function that is the second argument ofwithContext. Also, the last expression in a lambda is automatically also the return value of it, so you can actually leave thereturn@withContextout completely like this