如何正确使用 kotlin 委托创建捆绑包

Question

Trying to create some shortcut to simplify legacy code. Here how fragments are instantiated:

  CustomFragmentManager.showFragment(
                            Car::class.java.name,
                            Car.createBundle(
                                    Color.Red
                                    "modelaName",
                                    4,
                            ),
                            context
                    )

The showFragment is the legacy code whose signature I can't change: showFragment(String className, Bundle args, Context context)

Inside the Car fragment:

    private var color: ColorEnum by argument()
    private var modelName: String? by argumentNullable()
    private var cylinders: Int by argument()

companion object {

    @JvmStatic
    fun newInstance(
             color: ColorEnum,
             modelName: String? = null,
             cylinders: Int = -1,
    ) = Car().apply {

         this.color: color,
         this.modelName: modelName,
         this.cylinders: cylinders,

    }

    @JvmStatic
    fun createBundle(
            color: ColorEnum,
            modelName: String? = null,
            cylinders: Int = -1,
    ) = Bundle().apply {
        putSerializable(ARG_1, color)
        putString(ARG_2, modelName)
        putInt(ARG_3, cylinders)

}

I took inspiration from this nice article ( https://proandroiddev.com/kotlin-delegates-in-android-1ab0a715762d ) to create the delegates and get rid of the ARGS stuff in the newIntance. Is there a way to simplify also the createBundle and definitely remove all these ARGS?

Answer 1

To use the names of the properties as keys like that delegate does, you can create an extension function that uses a property to call through to the other extension function Bundle.put() that the article you linked describes.

fun Bundle.putPropertyByName(property: KProperty0<*>) = put(property.name, property.get())

KProperty0 means you don't separately have to pass an instance of the class that has the property to be able to get() its value. When you pass member properties of your Fragment to this function, the Fragment instance will be bound to the property object.

Call it by passing properties this way:

Bundle().apply {
    putPropertyByName(::color)
    putPropertyByName(::modelName)
    putPropertyByName(::cylinders)
}

Or make a helper function to simplify:

fun bundleOfProperties(vararg properties: KProperty0<*>): Bundle = Bundle(properties.size).apply {
    properties.forEach(::putPropertyByName)
}

//...

fun createBundle(
    color: ColorEnum,
    modelName: String? = null,
    cylinders: Int = -1,
) = bundleOfProperties(::color, ::modelName, ::cylinders)

Also, I think that function Bundle.get() from the article needs to have a case added at the top for null values or it will reach the else clause on null values.

fun <T> Bundle.put(key: String, value: T) {
    when (value) {
        is null -> putString(key, null)
        is Boolean -> putBoolean(key, value)
        is String -> putString(key, value)
        is Int -> putInt(key, value)
//...