如何在配置更改时恢复菜单状态

Question

我有 2 个菜单项（过滤按钮和搜索按钮）允许在列表中过滤或搜索。 When a list item is selected, it is given to the view throught the viewModel's LiveData called listItemSelected, when this happen, we move from the ListFragment to the DetailFragment and hide the 2 buttons from菜单，因为它们与此处无关。

我的问题是配置更改，例如屏幕旋转，旧的 activity 被破坏，新的 Activity.onCreate() 函数在 onCreateOptionsMenu() 之前被调用，所以我没有不知道如何正确设置菜单状态。

下面的简化代码示例，现在它只是在 filterMenuItem.isVisible = it == null 上崩溃，因为此时 filterMenuItem 尚未初始化。

class MyActivity : AppCompatActivity() {

    private lateinit var viewModel: MyViewModel
    private lateinit var filterMenuItem: MenuItem
    private lateinit var searchMenuItem: MenuItem

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        viewModel = getViewModel { injector.myViewModel }
        viewModel.listItemSelected.observe(this, Observer {
            filterMenuItem.isVisible = it == null
            searchMenuItem.isVisible = it == null
        })
    }

    override fun onCreateOptionsMenu(menu: Menu?): Boolean {
        menuInflater.inflate(R.menu.main_menu, menu)

        menu!!.apply {
            filterMenuItem = findItem(R.id.main_menu_filter)
            searchMenuItem = findItem(R.id.main_menu_search)
        }

        return true
    }

Answer 1

初始化 MenuItem 后如何观察数据：

class MyActivity : AppCompatActivity() {

    private lateinit var viewModel: MyViewModel
    private lateinit var filterMenuItem: MenuItem
    private lateinit var searchMenuItem: MenuItem

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        viewModel = getViewModel { injector.myViewModel }
    }

    override fun onCreateOptionsMenu(menu: Menu?): Boolean {
        menuInflater.inflate(R.menu.main_menu, menu)

        menu!!.apply {
            filterMenuItem = findItem(R.id.main_menu_filter)
            searchMenuItem = findItem(R.id.main_menu_search)
        }

        // observe after the menu items are initialized
        viewModel.listItemSelected.observe(this, Observer {
            filterMenuItem.isVisible = it == null
            searchMenuItem.isVisible = it == null
        })

        return true
    }
}

如何在配置更改时恢复菜单状态

How to restore the menu state on configuration change

android

android-lifecycle

android-viewmodel