activity-dsl.md

Launching Activities using Kotlin DSL

Launching activities in android apps is a common task and different developers use different approaches. Some use the traditional ways of creating Intent bundles and passing them in startActivity() methods along side the Intents. But, with Kotlin DSL and writing a handful of extension methods, this can become a lot more easier and readable than the traditional ways.

Let's look at the typical traditional way of starting another activity

    var intent = Intent(myActivity, TargetActivity::class)
    myActivity.startActivity(intent)

Now, if we pass some arguments in it, this becomes more complex.

    var intent = Intent(myActivity, TargetActivity::class)
    intent.putExtra("myIntVal", 10)
    intent.putExtra("myStrVal", "Hello String")
    intent.putExtra("myBoolVal", false)
    myActivity.startActivity(intent)

But, creating some kotlin extension methods and applying DSL rules, we can make these a lot simpler.

Add these methods in any class of your kotlin class, ideally in any extension class:

/**
 * Extensions for simpler launching of Activities
 */

inline fun <reified T : Any> Activity.launchActivity(
        requestCode: Int = -1,
        options: Bundle? = null,
        noinline init: Intent.() -> Unit = {}) {

    val intent = newIntent<T>(this)
    intent.init()
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {
        startActivityForResult(intent, requestCode, options)
    } else {
        startActivityForResult(intent, requestCode)
    }
}

inline fun <reified T : Any> Context.launchActivity(
        options: Bundle? = null,
        noinline init: Intent.() -> Unit = {}) {

    val intent = newIntent<T>(this)
    intent.init()
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN) {
        startActivity(intent, options)
    } else {
        startActivity(intent)
    }
}

inline fun <reified T : Any> newIntent(context: Context): Intent =
        Intent(context, T::class.java)

Now, after this, we can do same launching activity stuff (of previous examples) like this:

    myActivity.launchActivity<TargetActivity> {  }

Or we can pass arguments like this:

    myActivity.launchActivity<TargetActivity> { 
        putExtra("myIntVal", 10)
        putExtra("myStrVal", "Hello String")
        putExtra("myBoolVal", false)
    }

Here are all the cases which you can do with these extensions:

    // super simple, no arguments for simple Activities but doesn't prevent a crash for this one
    launchActivity<UserDetailActivity>()

    // add the required argument
    launchActivity<UserDetailActivity> {
        putExtra(INTENT_USER_ID, user.id)
    }

    // add custom flags
    launchActivity<UserDetailActivity> {
        putExtra(INTENT_USER_ID, user.id)
        addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP)
    }

    // use options for cool animations
    val options = ActivityOptionsCompat.makeSceneTransitionAnimation(this, avatar, "avatar")
    launchActivity<UserDetailActivity>(options = options) {
        putExtra(INTENT_USER_ID, user.id)
    }

    // requestCode, why not!?
    launchActivity<UserDetailActivity>(requestCode = 1234) {
        putExtra(INTENT_USER_ID, user.id)
    }