导航组件的默认空值：参数过多错误答案

【问题标题】：Default null value for navigation Component: Too many arguments error导航组件的默认空值：参数过多错误
【发布时间】：2020-09-03 05:43:00
【问题描述】：

我正在从我的片段中实现一个日期选择器。我想在我的片段上显示选定的日期。

日期选择器对话框片段：

class DatePickerDialogFragment :  DialogFragment() , DatePickerDialog.OnDateSetListener {


    @NotNull
    override fun onCreateDialog(savedInstanceState: Bundle?): Dialog {
        //return super.onCreateDialog(savedInstanceState)
        // Use the current date as the default date in the picker.
        val c: Calendar = Calendar.getInstance()
        val year: Int = c.get(Calendar.YEAR)
        val month: Int = c.get(Calendar.MONTH)
        val day: Int = c.get(Calendar.DAY_OF_MONTH)
        // Create a new instance of DatePickerDialog and return it.
        return DatePickerDialog(requireActivity(), this, year, month, day)
    }


    override fun onDateSet(view: DatePicker?, year: Int, month: Int, dayOfMonth: Int) {
        val selectedDate = arrayOf<String>(year.toString(),month.toString(),dayOfMonth.toString())
        val action = DatePickerDialogFragmentDirections.actionDatePickerDialogFragmentToDatePickerDemo(month.toString(),year.toString(),dayOfMonth.toString())
        //Navigation.findNavController().navigate(action)
        findNavController().navigate(action)
    }
}

DatePickerDemo 类：

class DatePickerDemo : Fragment() ,View.OnClickListener {
    private var mSelectedMonth: String? = null
    private var mSelectedDay: String? = null
    private var mSelectedYear: String? = null
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
    }

    override fun onCreateView(
        inflater: LayoutInflater, container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        return inflater.inflate(R.layout.fragment_date_picker, container, false)
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        btnShowDate.setOnClickListener(this)
        arguments?.let {
            mSelectedMonth = DatePickerDemoArgs.fromBundle(it).month.toString()
            mSelectedDay = DatePickerDemoArgs.fromBundle(it).day.toString()
            mSelectedYear = DatePickerDemoArgs.fromBundle(it).year.toString()
        }
        if(mSelectedMonth != null && mSelectedDay != null && mSelectedYear != null){
            processDatePickerResult(mSelectedYear!!.toInt(),mSelectedMonth!!.toInt(),mSelectedDay!!.toInt())
        }
    }

    override fun onClick(v: View?) {
        findNavController().navigate(R.id.datePickerDialogFragment)
    }
}

片段和对话框片段的导航图如下：

<fragment
        android:id="@+id/datePickerDemo"
        android:name="com.kgandroid.droidcafeuimaster.DatePickerDemo"
        android:label="DatePickerDemo" >
        <action
            android:id="@+id/action_datePickerDemo_to_datePickerDialogFragment"
            app:destination="@id/datePickerDialogFragment" />
        <argument
            android:name="year"
            app:argType="string"
            app:nullable="true"
            android:defaultValue="@null" />
        <argument
            android:name="month"
            app:argType="string"
            app:nullable="true"
            android:defaultValue="@null" />
        <argument
            android:name="day"
            app:argType="string"
            app:nullable="true"
            android:defaultValue="@null" />
    </fragment>
  <dialog
        android:id="@+id/datePickerDialogFragment"
        android:name="com.kgandroid.droidcafeuimaster.DatePickerDialogFragment"
        android:label="fragment_date_picker_dialog"
        tools:layout="@layout/fragment_date_picker_dialog" >
        <action
            android:id="@+id/action_datePickerDialogFragment_to_datePickerDemo"
            app:destination="@id/datePickerDemo" />
    </dialog>

问题：

很明显，当片段启动时，我不需要 DatePikerDemo 片段中的参数。当我从 DatePickerDialogFragment 返回并将其显示为 toast 时，我需要它。因此，如您所见，我在参数中添加了 null 值： app:nullable="true" android:defaultValue="@null" 在导航图中。但问题是，编译器显示错误：

public open fun actionDatePickerDialogFragmentToDatePickerDemo 的参数过多

如果没有默认值，程序正在编译但由于错误而崩溃：

 java.lang.IllegalArgumentException: Required argument "year" is missing and does not have an android:defaultValue

这很明显。那么有什么解决办法呢？

注意到在从fragment到dilogs之间通过导航库进行通信时会出现这种问题。

【问题讨论】：

标签： android android-fragments android-jetpack android-jetpack-navigation

【解决方案1】：

如果参数类型支持空值，您可以声明一个默认值使用 android:defaultValue="@null"

<argument
            android:name="customer"
            app:argType="com.test.CustomerEntity"
            app:nullable="true"
            android:defaultValue="@null"/>

在出现的添加参数链接窗口中，输入参数名称，参数类型，参数是否可以为空（app:nullable="true"），如果需要，还有一个默认值。

https://developer.android.com/guide/navigation/navigation-pass-data

【讨论】：