Property delegates for Activity extras

[gabrielhuff]

This implements #265. Usage:

class MyActivity : Activity {
  
  private val id by extra<Long?>("id")

  private val timestamp by extra<Long>("timestamp")

  private val name by extra<String>("name") { "Default Name" }
}

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[gabrielhuff]

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[dovahkiin98]

You're missing the default value parameter

[gabrielhuff]

@dovahkiin98 What do you mean? Where?

[dovahkiin98]

@gabrielhuff when you use intent.getStringExtra("extra", "") you provide a default value, it's not always 0 or "" or false, sometimes it's different.
I would suggest this implementation :

inline fun <T> Activity.extra(key: String, defaultValue: T) = lazy {
    when(T::class.java) {
        Int::class.java -> intent.getIntExtra(key, defaultValue) as T
        Float::class.java -> intent.getFloatExtra(key, defaultValue) as T
...
    }
}

val intExtra: Int by extra("intExtra", 2)

[gabrielhuff]

@dovahkiin98 The current implementation allows you to define default values. The only difference is that we are getting them from a function instead of a raw value:

val intExtra by extra("intExtra") { 2 }

Note that you can omit the default value function. If so, the implementation will throw an IllegalArgumentException if no extra was passed.

[JakeWharton]

Delegates are an expensive hammer to use for this. I don't think we've decided whether their use is appropriate for simple things like this.

[gabrielhuff]

IMHO delegates (in this case, specifically lazy instantiation) are perfect for this. Consider a scenario in which you need to access the same extra on different scopes. What would be the most appropriate way to do it?

[JakeWharton]

A lateinit property is an order of magnitude less expensive.

I have no idea what "same extra on different scopes" means.

[gabrielhuff]

By different scopes I mean different methods / contexts. We don't want to read the same extra from the input intent more than once (not because of performance - just because the call might be kinda ugly):

override fun onCreate(state: Bundle?) {
  ...
  val extra = intent.getParcelableExtra("my_key") as MyCustomExtra
}

fun onSomethingElse() {
  val extra = intent.getParcelableExtra("my_key") as MyCustomExtra // Eww
}

But I get what you're saying. Performance-wise, lateinit would be ideal. However, the property would have to be a var and would require an additional line of code somewhere else for instantiation.