从 SwiftUI 的 ScrollView 列表中获取 onAppear 行为 [英] Get onAppear behaviour from list in ScrollView in SwiftUI

问题描述

当创建一个 List 视图时，onAppear 会以您期望的方式触发该列表中的元素:只要您滚动到该元素，onAppear触发器.但是，我正在尝试实现这样的水平列表

When creating a List view onAppear triggers for elements in that list the way you would expect: As soon as you scroll to that element the onAppear triggers. However, I'm trying to implement a horizontal list like this

ScrollView(.horizontal) { 
   HStack(spacing: mySpacing) {
      ForEach(items) { item in 
         MyView(item: item)
            .onAppear { \\do something }
      } 
   }
}

使用此方法，onAppear 会立即为所有项目触发，也就是说:立即，但我想要与 List 视图相同的行为.我该怎么做呢?是否有手动触发 onAppear 或控制视图加载的方法?

Using this method the onAppear triggers for all items at once, that is to say: immediately, but I want the same behavior as for a List view. How would I go about doing this? Is there a manual way to trigger onAppear, or control when views load?

为什么我要实现这一点:我制作了一个自定义 Image 视图，该视图仅在出现时才从 URL 加载图像(并同时替换占位符)，这适用于List 视图，但我希望它也适用于我的水平列表".

Why I want to achieve this: I have made a custom Image view that loads an image from an URL only when it appears (and substitutes a placeholder in the mean time), this works fine for a List view, but I'd like it to also work for my horizontal 'list'.

推荐答案

根据 SwiftUI 2.0 (XCode 12 beta 1)，这终于在本地解决了:在 LazyHStack(或任何其他带有 Lazy 前缀的网格或堆栈)中，元素只会在它们出现在屏幕上时初始化(并因此触发 onAppear).

As per SwiftUI 2.0 (XCode 12 beta 1) this is finally natively solved: In a LazyHStack (or any other grid or stack with the Lazy prefix) elements will only initialise (and therefore trigger onAppear) when they appear on screen.

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