Javascript 如何从React Native中的三个不同选项有条件地进行渲染？

我想有条件地在React Native中呈现，但有三种不同的选项：加载屏幕、用户没有消息或显示所有用户消息。我有一个名为conversations的变量，它是从我的数据库中提取的，但是在提取完成之前它是空的，所以我想显示一个加载指示器。获取完成后，如果用户没有当前聊天记录，我想显示一条消息；如果用户没有现有聊天记录，我想显示一个包含所有用户聊天记录的表。我的想法是这样的：

return (
  <View>
    {conversations === null && <LoadingIndicator />} //The loading indicator will be present only when conversartions is null
    {conversations !== null && conversations.length === 0 ? ( // The conversations.length will be evaluated only when conversations is different to null
      <Text>You have no messages.</Text>
    ) : (
      <ConversationTable conversations={conversations} />
    )}
  </View>
)

render() { 
   ...
   let view = null;
   if (conversation == null) {
           view = <ActivityIndicator />
   } else {
       if (conversation.length == 0) 
           view = <Text>You have no messages.</Text>
       else
           view = <ConversationTable conversations={conversations} />
   }       

   ...
   return (
       <View>
           {view}
       </View>
   )
}

回来 {conversations？conversations.length==0？ 你没有留言。 : : }

---

有更好的方法吗？

您可以这样做：

return (
  <View>
    {conversations === null && <LoadingIndicator />} //The loading indicator will be present only when conversartions is null
    {conversations !== null && conversations.length === 0 ? ( // The conversations.length will be evaluated only when conversations is different to null
      <Text>You have no messages.</Text>
    ) : (
      <ConversationTable conversations={conversations} />
    )}
  </View>
)

render() { 
   ...
   let view = null;
   if (conversation == null) {
           view = <ActivityIndicator />
   } else {
       if (conversation.length == 0) 
           view = <Text>You have no messages.</Text>
       else
           view = <ConversationTable conversations={conversations} />
   }       

   ...
   return (
       <View>
           {view}
       </View>
   )
}

---

您可以这样做：

return (
  <View>
    {conversations === null && <LoadingIndicator />} //The loading indicator will be present only when conversartions is null
    {conversations !== null && conversations.length === 0 ? ( // The conversations.length will be evaluated only when conversations is different to null
      <Text>You have no messages.</Text>
    ) : (
      <ConversationTable conversations={conversations} />
    )}
  </View>
)

render() { 
   ...
   let view = null;
   if (conversation == null) {
           view = <ActivityIndicator />
   } else {
       if (conversation.length == 0) 
           view = <Text>You have no messages.</Text>
       else
           view = <ConversationTable conversations={conversations} />
   }       

   ...
   return (
       <View>
           {view}
       </View>
   )
}

你可以这样走

{conversations ? (
            {conversations.length===0? (
                <Text>You have no messages.</Text>
            ):(
                <ConversationTable conversations={conversations} />
            )}
        ) :(
            <ActivityIndicator />
        )}

你可以这样走

{conversations ? (
            {conversations.length===0? (
                <Text>You have no messages.</Text>
            ):(
                <ConversationTable conversations={conversations} />
            )}
        ) :(
            <ActivityIndicator />
        )}

---

在我看来，如果在JSX中使用分支，可能会给跟踪代码带来一些混乱和混乱

所以我的建议是把你的if分支放在报税表之外，比如：

return (
  <View>
    {conversations === null && <LoadingIndicator />} //The loading indicator will be present only when conversartions is null
    {conversations !== null && conversations.length === 0 ? ( // The conversations.length will be evaluated only when conversations is different to null
      <Text>You have no messages.</Text>
    ) : (
      <ConversationTable conversations={conversations} />
    )}
  </View>
)

render() { 
   ...
   let view = null;
   if (conversation == null) {
           view = <ActivityIndicator />
   } else {
       if (conversation.length == 0) 
           view = <Text>You have no messages.</Text>
       else
           view = <ConversationTable conversations={conversations} />
   }       

   ...
   return (
       <View>
           {view}
       </View>
   )
}

---

在我看来，如果在JSX中使用分支，可能会给跟踪代码带来一些混乱和混乱

所以我的建议是把你的if分支放在报税表之外，比如：

return (
  <View>
    {conversations === null && <LoadingIndicator />} //The loading indicator will be present only when conversartions is null
    {conversations !== null && conversations.length === 0 ? ( // The conversations.length will be evaluated only when conversations is different to null
      <Text>You have no messages.</Text>
    ) : (
      <ConversationTable conversations={conversations} />
    )}
  </View>
)

render() { 
   ...
   let view = null;
   if (conversation == null) {
           view = <ActivityIndicator />
   } else {
       if (conversation.length == 0) 
           view = <Text>You have no messages.</Text>
       else
           view = <ConversationTable conversations={conversations} />
   }       

   ...
   return (
       <View>
           {view}
       </View>
   )
}

---

在JSX之外执行分支逻辑，但仍然在render方法而不是内联方法中，这可能更清楚。例如：如果conversations&&conversations.length返回；，在JSX之外执行分支逻辑，但仍然在render方法而不是内联方法中，这可能更清楚。例如：如果conversations&&conversations.length返回；，以此类推。这实际上不起作用，因为如果对话为空，则对话！==null&&conversations.length==0将计算为false，这将导致呈现ConversationTable。这实际上不起作用，因为如果conversations为null，那么conversations！==null&&conversations.length==0将计算为false，这将导致呈现ConversationTable。此代码存在语法错误，能否像那样嵌套JSX表达式？此代码存在语法错误，能否像那样嵌套JSX表达式？