Debounce input from an EditText and relay to a TextView with a timeout.

debounce-broadcast.kt

package me.vishna.kdebounce

import android.support.v7.app.AppCompatActivity
import android.os.Bundle
import android.text.Editable
import android.text.TextWatcher
import android.widget.EditText
import android.widget.TextView
import kotlinx.coroutines.experimental.*
import kotlinx.coroutines.experimental.android.UI
import kotlinx.coroutines.experimental.channels.*
import java.util.concurrent.TimeUnit

class MainActivity : AppCompatActivity() {

    lateinit var inputText: EditText
    lateinit var outputText: TextView
    var broadcastChannel: BroadcastChannel<String>? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        inputText = findViewById(R.id.input_text)
        outputText = findViewById(R.id.output_text)

        launch(CommonPool) {
            broadcastChannel = inputText.textChanged.consumeDebounced(800, TimeUnit.MILLISECONDS) {
                launch(UI) { outputText.text = it }
            }
        }
    }

    override fun onDestroy() {
        broadcastChannel?.close()
        super.onDestroy()
    }
}

public val EditText.textChanged: BroadcastChannel<String>
    get() = EditTextBroadcastChannel(this)

private class EditTextBroadcastChannel(
        val editText: EditText,
        val broadcastChannel: BroadcastChannel<String> = ConflatedBroadcastChannel())
    : BroadcastChannel<String> by broadcastChannel {

    init {
        launch(UI) {
            editText.addTextChangedListener(textWatcher)
        }
    }

    val textWatcher = object : TextWatcher {
        override fun afterTextChanged(editable: Editable?) {
            if (!isClosedForSend) {
                offer(editable.toString())
            }
        }

        override fun beforeTextChanged(sequence: CharSequence?, start: Int, count: Int, after: Int) {}
        override fun onTextChanged(sequence: CharSequence?, start: Int, before: Int, count: Int) {}
    }

    override fun close(cause: Throwable?): Boolean {
        editText.removeTextChangedListener(textWatcher)
        return broadcastChannel.close(cause)
    }
}

/**
 * Subscribes to this [BroadcastChannel] and performs the specified action for received elements omitting those
 * elements that are published too quickly in succession. This is done by dropping elements which are
 * followed up by other elements before a specified timer has expired. If the timer expires and no follow up element was received (yet)
 * the last received element is passed to the specified action.
 */
public suspend fun <E> BroadcastChannel<E>.consumeDebounced(timeout: Long, unit: TimeUnit = TimeUnit.MILLISECONDS, action: suspend (E) -> Unit): BroadcastChannel<E> {
    openSubscription().use { channel ->
        var job: Job? = null
        var last = System.currentTimeMillis()

        for (x in channel) {
            val now = System.currentTimeMillis()
            val diff = now - last
            job?.cancel()
            if (!channel.isClosedForReceive) {
                job = launch {
                    if (diff < timeout) {
                        delay(Math.max(diff, 0), unit)
                    }
                    action(x)
                    last = now
                }
            }
        }
    }
    return this
}