prototype on how ui state/controller could work

app/src/main/java/com/dropbox/android/sample/ui/uistate/UIStateFragment.kt

@@ -0,0 +1,61 @@
+package com.dropbox.android.sample.ui.uistate
+
+import android.os.Bundle
+import android.view.LayoutInflater
+import android.view.View
+import android.view.ViewGroup
+import android.widget.TextView
+import androidx.fragment.app.Fragment
+import androidx.lifecycle.lifecycleScope
+import com.dropbox.android.external.store4.Fetcher
+import com.dropbox.android.external.store4.StoreBuilder
+import com.dropbox.android.external.store4.StoreRequest
+import com.dropbox.android.external.store4.UIController
+import com.dropbox.android.sample.R
+import kotlinx.android.synthetic.main.ui_state.buttonRefresh
+import kotlinx.android.synthetic.main.ui_state.loadingInfo
+import kotlinx.android.synthetic.main.ui_state.streamValue
+import kotlinx.coroutines.delay
+import kotlinx.coroutines.flow.collect
+import kotlinx.coroutines.flow.flow
+
+
+class UIStateFragment : Fragment() {
+    val store = StoreBuilder.from<Int, Int>(
+            fetcher = Fetcher.ofFlow {key:Int ->
+                flow {
+                    var latest = key
+                    while(true) {
+                        delay(2000)
+                        emit(latest)
+                        latest += 2
+
+                    }
+                }
+            }
+    ).build()
+    override fun onCreateView(
+            inflater: LayoutInflater,
+            container: ViewGroup?,
+            savedInstanceState: Bundle?
+    ): View? {
+        return inflater.inflate(R.layout.ui_state, container, false)
+    }
+
+    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
+        super.onViewCreated(view, savedInstanceState)
+        val controller = UIController(
+                store = store,
+                request = StoreRequest.fresh(3)
+        )
+        buttonRefresh.setOnClickListener {
+            controller.refresh()
+        }
+        viewLifecycleOwner.lifecycleScope.launchWhenStarted {
+            controller.state.collect {
+                streamValue.text = "${it.data?.dataOrNull()}"
+                loadingInfo.text = "loading: ${it.isLoading()}"
+            }
+        }
+    }
+}

app/src/main/res/layout/ui_state.xml

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout
+    xmlns:android="http://schemas.android.com/apk/res/android" android:layout_width="match_parent"
+    android:orientation="vertical"
+    android:layout_height="match_parent">
+    <Button
+        android:id="@+id/buttonRefresh"
+        android:text="refresh"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"/>
+    <TextView
+        android:id="@+id/streamValue"
+        android:layout_width="match_parent"
+        android:layout_height="wrap_content"/>
+    <TextView
+        android:id="@+id/loadingInfo"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"/>
+
+</LinearLayout>

app/src/main/res/menu/bottom_navigation_menu.xml

@@ -12,4 +12,8 @@
         android:id="@+id/streamFragment"
         android:icon="@drawable/ic_distribution"
         android:title="@string/bottom_navigation_menu_title_stream" />
+    <item
+        android:id="@+id/uiStateFragment"
+        android:icon="@drawable/ic_distribution"
+        android:title="@string/bottom_navigation_menu_title_uistate" />
 </menu>

app/src/main/res/navigation/main_nav_graph.xml

@@ -2,7 +2,7 @@
 <navigation xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     android:id="@+id/main_nav_graph"
-    app:startDestination="@id/redditFragment">
+    app:startDestination="@id/uiStateFragment">
 
     <fragment
         android:id="@+id/redditFragment"
@@ -16,4 +16,8 @@
         android:id="@+id/streamFragment"
         android:name="com.dropbox.android.sample.ui.stream.StreamFragment"
         android:label="StreamFragment" />
+    <fragment
+        android:id="@+id/uiStateFragment"
+        android:name="com.dropbox.android.sample.ui.uistate.UIStateFragment"
+        android:label="UIStateFragment" />
 </navigation>

app/src/main/res/values/strings.xml

@@ -5,4 +5,5 @@
     <string name="bottom_navigation_menu_title_reddit">Reddit</string>
     <string name="bottom_navigation_menu_title_stream">Stream</string>
     <string name="bottom_navigation_menu_title_room">Room</string>
+    <string name="bottom_navigation_menu_title_uistate">UIState</string>
 </resources>

store/src/main/java/com/dropbox/android/external/store4/UIState.kt

@@ -0,0 +1,71 @@
+package com.dropbox.android.external.store4
+
+import kotlinx.coroutines.channels.ConflatedBroadcastChannel
+import kotlinx.coroutines.flow.asFlow
+import kotlinx.coroutines.flow.collect
+import kotlinx.coroutines.flow.emitAll
+import kotlinx.coroutines.flow.flatMapLatest
+import kotlinx.coroutines.flow.flow
+import kotlinx.coroutines.flow.fold
+import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.mapLatest
+
+data class UIState<T>(
+        val loading: StoreResponse.Loading? = null,
+        val data: StoreResponse.Data<T>? = null,
+        val error: StoreResponse.Error? = null
+) {
+    fun isLoading() = loading != null
+    internal fun combine(response: StoreResponse<T>) : UIState<T> {
+        return when(response) {
+            is StoreResponse.Loading -> {
+                copy(
+                        loading = response,
+                        error = null
+                )
+            }
+            is StoreResponse.Data -> {
+                copy(
+                        loading = null,
+                        data = response,
+                        error = null
+                )
+            }
+            is StoreResponse.Error -> {
+                copy(
+                        error = response,
+                        loading = null
+                )
+            }
+            is StoreResponse.NoNewData -> {
+                copy(
+                        loading = null
+                )
+            }
+        }
+    }
+}
+
+class UIController<Key : Any, Output : Any>(
+        val store: Store<Key, Output>,
+        val request: StoreRequest<Key>
+) {
+    // use mutable state flow instead when we can
+    private val refreshTrigger = ConflatedBroadcastChannel<Unit>().also {
+        it.offer(Unit)
+    }
+
+
+    fun refresh() = refreshTrigger.offer(Unit)
+    val state = flow<UIState<Output>> {
+        var prevState : UIState<Output> = UIState()
+        emitAll(refreshTrigger.asFlow().flatMapLatest {
+            store.stream(request)
+        }.map {
+            prevState.combine(it).also {
+                prevState = it
+            }
+            prevState
+        })
+    }
+}

store/src/main/java/com/dropbox/android/external/store4/impl/FallbackController.kt

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2020 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.dropbox.android.external.store4.impl
+
+/**
+ * Internal helper class to help with fallback logic from Fetcher to local data
+ */
+internal class FallbackController {
+
+}