JsonCommit.kt

import com.google.gson.JsonObject

fun main(args: Array<String>) {

    val json = JsonObject()
    json.commit {
        addProperty("added_property", "This is the added property")
    }
    json.commit {
        remove("added_property")
    }
    json.commit {
        addProperty("name", "The name")
    }
    json.commit {
        addProperty("name", "The NEW name")
    }
}

fun JsonObject.commit(block: JsonObject.() -> Unit) {
    val before = xpathToValue(this).map { it.first + ": " + it.second }.toMutableList()
    block()
    val after = xpathToValue(this).map { it.first + ": " + it.second }.toMutableList()
    (before + after).forEach {
        if (before.contains(it) && after.contains(it)) {
            before.remove(it)
            after.remove(it)
        }
    }
    before.forEach { println("- $it") }
    after.forEach { println("+ $it") }
}

fun xpathToValue(json: JsonObject): List<Pair<String, String>> {
    return json.entrySet().flatMap { nameToObj ->
        if (nameToObj.value.isJsonObject) {
            xpathToValue(nameToObj.value.asJsonObject).map { "${nameToObj.key}/${it.first}" to it.second }
        } else if (nameToObj.value.isJsonArray) {
            val res = nameToObj.value.asJsonArray
                    .mapIndexed { i, value -> i to value }
                    .fold(JsonObject()) { total, next -> total.apply { add("${nameToObj.key}/${next.first}", next.second) } }
            xpathToValue(res)
        } else {
            listOf(nameToObj.key to nameToObj.value.asJsonPrimitive.toString())
        }
    }
}