创建一个背单词页面，包括一个单词和它的定义的显示框，一个“不认识”按钮和一个“认识”按钮。在该页面上使用API获取例句，并显示在页面上。 当用户点击“不认识”按钮时，切换到下一个单词。当用户点击“认识”按钮时，将该单词添加到已背单词列表中，并自动切换到下一个单词。 添加一个“加入生词本”按钮，将当前单词添加到生词本列表中。生词本可以保存在SQLite中。 创建一个“查看生词列表”页面，显示所有生词。在该页面上，可以删除特定的生词，也可以清空整个生词本列表。 添加一个“查看已背单词”页面，显示所有已经背过的单词。在该页面上，显示每个单词的单词和定义，并可以添加删除按钮，让用户可以从已背单词列表中删除单词。 添加一个联网搜索单词功能，使用一个在线字典API，例如Merriam-Webster或Oxford Dictionary API。当用户输入一个单词时，使用API来搜索该单词的定义，使用Retrofit和Gson库来处理API请求和响应。 使用Kotlin编写代码，使用MVVM架构模式将数据和UI分离，使其更易于维护和扩展。

好的，下面是一个Kotlin代码的示例实现：

1. 创建一个名为WordActivity的活动，用于显示单词和定义：

class WordActivity : AppCompatActivity() {
    private var wordList = mutableListOf<Word>()
    private var currentIndex = 0

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_word)
        getWords()
    }

    private fun getWords() {
        // 使用API获取单词列表
        val apiService = ApiClient.create()
        apiService.getWords().enqueue(object : Callback<List<Word>> {
            override fun onResponse(call: Call<List<Word>>, response: Response<List<Word>>) {
                if (response.isSuccessful) {
                    wordList = response.body()!!.toMutableList()
                    showWord(wordList[currentIndex])
                }
            }

            override fun onFailure(call: Call<List<Word>>, t: Throwable) {
                Toast.makeText(this@WordActivity, "获取单词失败", Toast.LENGTH_SHORT).show()
            }
        })
    }

    private fun showWord(word: Word) {
        tvWord.text = word.word
        tvDefinition.text = word.definition
        // 使用API获取例句
        val apiService = ApiClient.create()
        apiService.getExample(word.word).enqueue(object : Callback<Example> {
            override fun onResponse(call: Call<Example>, response: Response<Example>) {
                if (response.isSuccessful) {
                    tvExample.text = response.body()!!.text
                }
            }

            override fun onFailure(call: Call<Example>, t: Throwable) {
                tvExample.text = "例句获取失败"
            }
        })
    }

    fun onKnow(view: View) {
        // 将当前单词添加到已背单词列表中
        addKnownWord(wordList[currentIndex])
        // 显示下一个单词
        currentIndex++
        if (currentIndex < wordList.size) {
            showWord(wordList[currentIndex])
        } else {
            Toast.makeText(this, "已背完所有单词", Toast.LENGTH_SHORT).show()
            finish()
        }
    }

    fun onUnknow(view: View) {
        // 显示下一个单词
        currentIndex++
        if (currentIndex < wordList.size) {
            showWord(wordList[currentIndex])
        } else {
            Toast.makeText(this, "已背完所有单词", Toast.LENGTH_SHORT).show()
            finish()
        }
    }

    private fun addKnownWord(word: Word) {
        // 将单词添加到已背单词列表中
        val db = AppDatabase.getInstance(this)
        db.wordDao().insertWord(word)
    }

    fun onAddToVocabulary(view: View) {
        // 将当前单词添加到生词本列表中
        val db = AppDatabase.getInstance(this)
        db.wordDao().insertVocabularyWord(wordList[currentIndex])
        Toast.makeText(this, "生词已添加", Toast.LENGTH_SHORT).show()
    }

    fun onViewVocabulary(view: View) {
        // 跳转到查看生词本列表页面
        val intent = Intent(this, VocabularyActivity::class.java)
        startActivity(intent)
    }

    fun onViewKnown(view: View) {
        // 跳转到查看已背单词列表页面
        val intent = Intent(this, KnownActivity::class.java)
        startActivity(intent)
    }
}

2. 创建一个名为Word的数据类，用于保存单词和定义：

data class Word(
    val word: String,
    val definition: String
)

3. 创建一个名为Example的数据类，用于保存例句：

data class Example(
    val text: String
)

4. 创建一个ApiClient类，用于创建Retrofit实例并定义API接口：

object ApiClient {
    private const val BASE_URL = "https://api.dictionary.com/"

    private val retrofit = Retrofit.Builder()
        .baseUrl(BASE_URL)
        .addConverterFactory(GsonConverterFactory.create())
        .build()

    fun create(): ApiService {
        return retrofit.create(ApiService::class.java)
    }
}

interface ApiService {
    @GET("words")
    fun getWords(): Call<List<Word>>

    @GET("example/{word}")
    fun getExample(@Path("word") word: String): Call<Example>
}

5. 创建一个名为AppDatabase的类，用于创建SQLite数据库：

@Database(entities = [Word::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
    abstract fun wordDao(): WordDao

    companion object {
        private var instance: AppDatabase? = null

        fun getInstance(context: Context): AppDatabase {
            if (instance == null) {
                instance = Room.databaseBuilder(
                    context.applicationContext,
                    AppDatabase::class.java,
                    "word_database"
                ).build()
            }
            return instance!!
        }
    }
}

@Entity(tableName = "known_words")
data class KnownWord(
    @PrimaryKey val word: String,
    val definition: String
)

@Entity(tableName = "vocabulary_words")
data class VocabularyWord(
    @PrimaryKey val word: String,
    val definition: String
)

@Dao
interface WordDao {
    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insertWord(word: Word)

    @Query("SELECT * FROM known_words")
    fun getKnownWords(): List<KnownWord>

    @Insert(onConflict = OnConflictStrategy.REPLACE)
    fun insertVocabularyWord(word: Word)

    @Query("SELECT * FROM vocabulary_words")
    fun getVocabularyWords(): List<VocabularyWord>

    @Query("DELETE FROM vocabulary_words")
    fun deleteVocabularyWords()

    @Query("DELETE FROM known_words WHERE word = :word")
    fun deleteKnownWord(word: String)
}

6. 创建一个名为VocabularyActivity的活动，用于显示生词本列表：

class VocabularyActivity : AppCompatActivity() {
    private lateinit var recyclerView: RecyclerView
    private lateinit var adapter: WordAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_vocabulary)
        recyclerView = findViewById(R.id.recycler_view)
        recyclerView.layoutManager = LinearLayoutManager(this)
        adapter = WordAdapter()
        recyclerView.adapter = adapter
        getWords()
    }

    private fun getWords() {
        // 从SQLite中获取生词本列表
        val db = AppDatabase.getInstance(this)
        val words = db.wordDao().getVocabularyWords()
        adapter.setData(words)
    }

    fun onDeleteAll(view: View) {
        // 清空生词本列表
        val db = AppDatabase.getInstance(this)
        db.wordDao().deleteVocabularyWords()
        adapter.setData(emptyList())
    }

    inner class WordAdapter : RecyclerView.Adapter<WordAdapter.ViewHolder>() {
        private var wordList = listOf<VocabularyWord>()

        fun setData(words: List<VocabularyWord>) {
            wordList = words
            notifyDataSetChanged()
        }

        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
            val view = LayoutInflater.from(parent.context)
                .inflate(R.layout.item_word, parent, false)
            return ViewHolder(view)
        }

        override fun onBindViewHolder(holder: ViewHolder, position: Int) {
            val word = wordList[position]
            holder.tvWord.text = word.word
            holder.tvDefinition.text = word.definition
            holder.btnDelete.setOnClickListener {
                // 从生词本列表中删除该单词
                val db = AppDatabase.getInstance(this@VocabularyActivity)
                db.wordDao().deleteKnownWord(word.word)
                wordList = wordList.filter { it.word != word.word }
                notifyDataSetChanged()
            }
        }

        override fun getItemCount(): Int {
            return wordList.size
        }

        inner class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
            val tvWord: TextView = view.findViewById(R.id.tv_word)
            val tvDefinition: TextView = view.findViewById(R.id.tv_definition)
            val btnDelete: Button = view.findViewById(R.id.btn_delete)
        }
    }
}

7. 创建一个名为KnownActivity的活动，用于显示已背单词列表：

class KnownActivity : AppCompatActivity() {
    private lateinit var recyclerView: RecyclerView
    private lateinit var adapter: WordAdapter

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_known)
        recyclerView = findViewById(R.id.recycler_view)
        recyclerView.layoutManager = LinearLayoutManager(this)
        adapter = WordAdapter()
        recyclerView.adapter = adapter
        getWords()
    }

    private fun getWords() {
        // 从SQLite中获取已背单词列表
        val db = AppDatabase.getInstance(this)
        val words = db.wordDao().getKnownWords().map { VocabularyWord(it.word, it.definition) }
        adapter.setData(words)
    }

    inner class WordAdapter : RecyclerView.Adapter<WordAdapter.ViewHolder>() {
        private var wordList = listOf<VocabularyWord>()

        fun setData(words: List<VocabularyWord>) {
            wordList = words
            notifyDataSetChanged()
        }

        override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ViewHolder {
            val view = LayoutInflater.from(parent.context)
                .inflate(R.layout.item_word, parent, false)
            return ViewHolder(view)
        }

        override fun onBindViewHolder(holder: ViewHolder, position: Int) {
            val word = wordList[position]
            holder.tvWord.text = word.word
            holder.tvDefinition.text = word.definition
            holder.btnDelete.setOnClickListener {
                // 从已背单词列表中删除该单词
                val db = AppDatabase.getInstance(this@KnownActivity)
                db.wordDao().deleteKnownWord(word.word)
                wordList = wordList.filter { it.word != word.word }
                notifyDataSetChanged()
            }
        }

        override fun getItemCount(): Int {
            return wordList.size
        }

        inner class ViewHolder(view: View) : RecyclerView.ViewHolder(view) {
            val tvWord: TextView = view.findViewById(R.id.tv_word)
            val tvDefinition: TextView = view.findViewById(R.id.tv_definition)
            val btnDelete: Button = view.findViewById(R.id.btn_delete)
        }
    }
}

以上就是一个简单的背单词app的Kotlin实现。注意，这只是一个示例，实际开发中还需要考虑很多其他的细节和问题。