feat(chat): implement ChatViewModel for state management

app/src/main/java/com/stand/standapp/ui/chat/ChatViewModel.kt

@@ -0,0 +1,98 @@
+package com.stand.standapp.ui.chat
+
+import androidx.lifecycle.ViewModel
+import androidx.lifecycle.viewModelScope
+import com.stand.standapp.ui.chat.model.ChatMessage
+import com.stand.standapp.ui.chat.model.ChatSession
+import com.stand.standapp.ui.chat.repo.AiChatRepository
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.catch
+import kotlinx.coroutines.flow.onCompletion
+import kotlinx.coroutines.launch
+import java.util.UUID
+
+class ChatViewModel : ViewModel() {
+    private val repository = AiChatRepository()
+
+    private val _sessions = MutableStateFlow<List<ChatSession>>(emptyList())
+    val sessions: StateFlow<List<ChatSession>> = _sessions.asStateFlow()
+
+    private val _currentSessionId = MutableStateFlow<String?>(null)
+    val currentSessionId: StateFlow<String?> = _currentSessionId.asStateFlow()
+
+    private val _messages = MutableStateFlow<List<ChatMessage>>(emptyList())
+    val messages: StateFlow<List<ChatMessage>> = _messages.asStateFlow()
+
+    init {
+        createNewSession()
+    }
+
+    fun createNewSession() {
+        val newSession = ChatSession()
+        _sessions.value = listOf(newSession) + _sessions.value
+        _currentSessionId.value = newSession.id
+    }
+
+    fun switchSession(sessionId: String) {
+        _currentSessionId.value = sessionId
+    }
+
+    fun sendMessage(content: String) {
+        val sessionId = _currentSessionId.value ?: return
+        
+        // 1. 如果是新会话，更新标题
+        updateSessionTitleIfFirstMessage(sessionId, content)
+
+        // 2. 添加用户消息
+        val userMsg = ChatMessage(sessionId = sessionId, role = ChatMessage.Role.USER, content = content)
+        
+        // 3. 准备 AI 空消息，处于 streaming 状态
+        val aiMsgId = UUID.randomUUID().toString()
+        val aiMsg = ChatMessage(id = aiMsgId, sessionId = sessionId, role = ChatMessage.Role.AI, content = "", isStreaming = true)
+        
+        _messages.value = _messages.value + userMsg + aiMsg
+
+        // 4. 发起网络请求，流式追加内容
+        viewModelScope.launch {
+            repository.streamChat(content)
+                .catch { e ->
+                    appendAiMessageChunk(aiMsgId, "\n[请求异常: ${e.message}]")
+                    finalizeAiMessage(aiMsgId)
+                }
+                .onCompletion {
+                    finalizeAiMessage(aiMsgId)
+                }
+                .collect { chunk ->
+                    appendAiMessageChunk(aiMsgId, chunk)
+                }
+        }
+    }
+
+    private fun appendAiMessageChunk(messageId: String, chunk: String) {
+        _messages.value = _messages.value.map { msg ->
+            if (msg.id == messageId) {
+                msg.copy(content = msg.content + chunk)
+            } else msg
+        }
+    }
+
+    private fun finalizeAiMessage(messageId: String) {
+        _messages.value = _messages.value.map { msg ->
+            if (msg.id == messageId) {
+                msg.copy(isStreaming = false)
+            } else msg
+        }
+    }
+
+    private fun updateSessionTitleIfFirstMessage(sessionId: String, content: String) {
+        val isFirst = _messages.value.none { it.sessionId == sessionId }
+        if (isFirst) {
+            val title = if (content.length > 10) content.take(10) + "..." else content
+            _sessions.value = _sessions.value.map {
+                if (it.id == sessionId) it.copy(title = title) else it
+            }
+        }
+    }
+}