How to handle button clicks using the XML onClick within Fragments

Question

Pre-Honeycomb (Android 3), each Activity was registered to handle button clicks via the onClick tag in a Layout\'s XML:

android:onClick=\"m         


        

Answer 1

Though I've spotted some nice answers relying on data binding, I didn't see any going to the full extent with that approach -- in the sense of enabling fragment resolution while allowing for fragment-free layout definitions in XML's.

So assuming data binding is enabled, here's a generic solution I can propose; A bit long but it definitely works (with some caveats):

Step 1: Custom OnClick Implementation

This will run a fragment-aware search through contexts associated with the tapped-on view (e.g. button):

// CustomOnClick.kt

@file:JvmName("CustomOnClick")

package com.example

import android.app.Activity
import android.content.Context
import android.content.ContextWrapper
import android.view.View
import androidx.fragment.app.Fragment
import androidx.fragment.app.FragmentActivity
import java.lang.reflect.Method

fun onClick(view: View, methodName: String) {
    resolveOnClickInvocation(view, methodName)?.invoke(view)
}

private data class OnClickInvocation(val obj: Any, val method: Method) {
    fun invoke(view: View) {
        method.invoke(obj, view)
    }
}

private fun resolveOnClickInvocation(view: View, methodName: String): OnClickInvocation? =
    searchContexts(view) { context ->
        var invocation: OnClickInvocation? = null
        if (context is Activity) {
            val activity = context as? FragmentActivity
                    ?: throw IllegalStateException("A non-FragmentActivity is not supported (looking up an onClick handler of $view)")

            invocation = getTopFragment(activity)?.let { fragment ->
                resolveInvocation(fragment, methodName)
            }?: resolveInvocation(context, methodName)
        }
        invocation
    }

private fun getTopFragment(activity: FragmentActivity): Fragment? {
    val fragments = activity.supportFragmentManager.fragments
    return if (fragments.isEmpty()) null else fragments.last()
}

private fun resolveInvocation(target: Any, methodName: String): OnClickInvocation? =
    try {
        val method = target.javaClass.getMethod(methodName, View::class.java)
        OnClickInvocation(target, method)
    } catch (e: NoSuchMethodException) {
        null
    }

private fun  searchContexts(view: View, matcher: (context: Context) -> T?): T? {
    var context = view.context
    while (context != null && context is ContextWrapper) {
        val result = matcher(context)
        if (result == null) {
            context = context.baseContext
        } else {
            return result
        }
    }
    return null
}

Note: loosely based on the original Android implementation (see https://android.googlesource.com/platform/frameworks/base/+/a175a5b/core/java/android/view/View.java#3025)

Step 2: Declarative application in layout files

Then, in data-binding aware XML's:

Caveats

• Assumes a 'modern' FragmentActivity based implementation
• Can only lookup method of "top-most" (i.e. last) fragment in stack (though that can be fixed, if need be)