MainActivity: rework layout to avoid lag

ScrollView + RecyclerView lead to very slow loading times for the
recycler, like a few seconds on the biggest Medusae pages. Turns out it
binds every ViewHolder instantly, losing all the recycling behavior!
Following some guidelines on StackOverflow fixed this, because official
docs could not.

app/build.gradle

@@ -25,6 +25,7 @@ android {
         release {
             minifyEnabled false
             proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
+            signingConfig signingConfigs.debug
         }
     }
     compileOptions {
@@ -42,6 +43,7 @@ dependencies {
     implementation 'com.google.android.material:material:1.4.0'
     implementation 'androidx.constraintlayout:constraintlayout:2.1.2'
     implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:2.4.0"
+    implementation 'androidx.legacy:legacy-support-v4:1.0.0'
     testImplementation 'junit:junit:4.13.2'
     androidTestImplementation 'androidx.test.ext:junit:1.1.3'
     androidTestImplementation 'androidx.test.espresso:espresso-core:3.4.0'

app/src/main/java/dev/lowrespalmtree/comet/ContentRecycler.kt

@@ -1,5 +1,6 @@
 package dev.lowrespalmtree.comet
 
+import android.annotation.SuppressLint
 import android.text.SpannableString
 import android.text.style.UnderlineSpan
 import android.util.Log
@@ -12,6 +13,8 @@ import dev.lowrespalmtree.comet.databinding.*
 class ContentAdapter(private var content: List<Line>, private val listener: ContentAdapterListen) :
     RecyclerView.Adapter<ContentAdapter.ContentViewHolder>() {
 
+    private var lastLineCount = 0
+
     interface ContentAdapterListen {
         fun onLinkClick(url: String)
     }
@@ -35,7 +38,6 @@ class ContentAdapter(private var content: List<Line>, private val listener: Cont
         }
 
     override fun onCreateViewHolder(parent: ViewGroup, viewType: Int): ContentViewHolder {
-        Log.d(TAG, "onCreateViewHolder: type $viewType")
         return LayoutInflater.from(parent.context).let {
             when (viewType) {
                 TYPE_EMPTY -> ContentViewHolder.Empty(GemtextEmptyBinding.inflate(it))
@@ -54,7 +56,6 @@ class ContentAdapter(private var content: List<Line>, private val listener: Cont
     }
 
     override fun onBindViewHolder(holder: ContentViewHolder, position: Int) {
-        Log.d(TAG, "onBindViewHolder: position $position")
         val line = content[position]
         when (holder) {
             is ContentViewHolder.Paragraph -> holder.binding.textView.text =
@@ -75,9 +76,28 @@ class ContentAdapter(private var content: List<Line>, private val listener: Cont
 
     override fun getItemCount(): Int = content.size
 
-    fun setContent(content: List<Line>) {
-        this.content = content
-        notifyDataSetChanged()
+    /**
+     * Replace the content rendered by the recycler.
+     *
+     * The new content list may or may not be the same object as the previous one, we don't
+     * assume anything. The assumptions this function do however are:
+     * - If the new content is empty, we are about to load new content, so clear the recycler.
+     * - If it's longer than before, we received new streamed content, so *append* data.
+     * - If it's shorter or the same size than before, we do not notify anything and let the caller
+     *   manage the changes itself.
+     */
+    @SuppressLint("NotifyDataSetChanged")
+    fun setContent(newContent: List<Line>) {
+        content = newContent.toList()  // Shallow copy to avoid concurrent update issues.
+        if (content.isEmpty()) {
+            Log.d(TAG, "setContent: empty content")
+            notifyDataSetChanged()
+        } else if (content.size > lastLineCount) {
+            val numAdded = content.size - lastLineCount
+            Log.d(TAG, "setContent: added $numAdded items")
+            notifyItemRangeInserted(lastLineCount, numAdded)
+        }
+        lastLineCount = content.size
     }
 
     sealed class ContentViewHolder(view: View) : RecyclerView.ViewHolder(view) {

app/src/main/java/dev/lowrespalmtree/comet/Gemtext.kt

@@ -41,6 +41,7 @@ fun parseData(
                 channel.send(line)
             }
         }
+        channel.close()
     }
     return channel
 }

app/src/main/java/dev/lowrespalmtree/comet/MainActivity.kt

@@ -2,6 +2,7 @@ package dev.lowrespalmtree.comet
 
 import android.annotation.SuppressLint
 import android.app.Activity
+import android.content.ActivityNotFoundException
 import android.content.Intent
 import android.content.Intent.ACTION_VIEW
 import android.net.Uri
@@ -18,6 +19,7 @@ import androidx.lifecycle.viewModelScope
 import androidx.recyclerview.widget.LinearLayoutManager
 import dev.lowrespalmtree.comet.databinding.ActivityMainBinding
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.Job
 import kotlinx.coroutines.launch
 import java.net.UnknownHostException
 import java.nio.charset.Charset
@@ -27,6 +29,8 @@ class MainActivity : AppCompatActivity(), ContentAdapter.ContentAdapterListen {
     private lateinit var pageViewModel: PageViewModel
     private lateinit var adapter: ContentAdapter
 
+    private val currentUrl get() = binding.addressBar.text
+
     @SuppressLint("SetTextI18n")
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
@@ -50,8 +54,11 @@ class MainActivity : AppCompatActivity(), ContentAdapter.ContentAdapterListen {
             }
         }
 
-        pageViewModel.linesLiveData.observe(this, { adapter.setContent(it) })
-        pageViewModel.alertLiveData.observe(this, { alert(it) })
+        binding.contentSwipeLayout.setOnRefreshListener { openUrl(currentUrl.toString()) }
+
+        pageViewModel.state.observe(this, { updateState(it) })
+        pageViewModel.lines.observe(this, { updateLines(it) })
+        pageViewModel.alert.observe(this, { alert(it) })
     }
 
     override fun onLinkClick(url: String) {
@@ -63,29 +70,68 @@ class MainActivity : AppCompatActivity(), ContentAdapter.ContentAdapterListen {
         var uri = Uri.parse(url)
         if (!uri.isAbsolute) {
             uri = if (!base.isNullOrEmpty()) joinUrls(base, url) else toGeminiUri(uri)
-            Log.d(TAG, "openUrl: '$url' - '$base' - '$uri'")
-            Log.d(TAG, "openUrl: ${uri.authority} - ${uri.path} - ${uri.query}")
-            binding.addressBar.setText(uri.toString())
         }
 
         when (uri.scheme) {
-            "gemini" -> pageViewModel.sendGeminiRequest(uri)
-            else -> startActivity(Intent(ACTION_VIEW, uri))
+            "gemini" -> {
+                binding.addressBar.setText(uri.toString())
+                pageViewModel.sendGeminiRequest(uri)
+            }
+            else -> openUnknownScheme(uri)
         }
     }
 
-    private fun alert(message: String) {
-        AlertDialog.Builder(this)
-            .setTitle(R.string.alert_title)
+    private fun updateState(state: PageViewModel.State) {
+        Log.d(TAG, "updateState: $state")
+        when (state) {
+            PageViewModel.State.IDLE -> {
+                binding.contentProgressBar.hide()
+                binding.contentSwipeLayout.isRefreshing = false
+            }
+            PageViewModel.State.CONNECTING -> {
+                binding.contentProgressBar.show()
+            }
+            PageViewModel.State.RECEIVING -> {
+                binding.contentSwipeLayout.isRefreshing = false
+            }
+        }
+    }
+
+    private fun updateLines(lines: List<Line>) {
+        Log.d(TAG, "updateLines: ${lines.size} lines")
+        adapter.setContent(lines)
+    }
+
+    private fun alert(message: String, title: String? = null) {
+        val builder = AlertDialog.Builder(this)
+        if (title != null)
+            builder.setTitle(title)
+        else
+            builder.setTitle(title ?: R.string.alert_title)
+        builder
             .setMessage(message)
             .create()
             .show()
     }
 
+    private fun openUnknownScheme(uri: Uri) {
+        try {
+            startActivity(Intent(ACTION_VIEW, uri))
+        } catch (e: ActivityNotFoundException) {
+            alert("Can't open this URL.")
+        }
+    }
+
     class PageViewModel : ViewModel() {
-        private var lines = ArrayList<Line>()
-        val linesLiveData: MutableLiveData<List<Line>> by lazy { MutableLiveData<List<Line>>() }
-        val alertLiveData: MutableLiveData<String> by lazy { MutableLiveData<String>() }
+        private var requestJob: Job? = null
+        val state: MutableLiveData<State> by lazy { MutableLiveData<State>(State.IDLE) }
+        private var linesList = ArrayList<Line>()
+        val lines: MutableLiveData<List<Line>> by lazy { MutableLiveData<List<Line>>() }
+        val alert: MutableLiveData<String> by lazy { MutableLiveData<String>() }
+
+        enum class State {
+            IDLE, CONNECTING, RECEIVING
+        }
 
         /**
          * Perform a request against this URI.
@@ -93,42 +139,57 @@ class MainActivity : AppCompatActivity(), ContentAdapter.ContentAdapterListen {
          * The URI must be valid, absolute and with a gemini scheme.
          */
         fun sendGeminiRequest(uri: Uri) {
-            Log.d(TAG, "sendRequest: $uri")
-            viewModelScope.launch(Dispatchers.IO) {
+            Log.d(TAG, "sendRequest: URI \"$uri\"")
+            state.postValue(State.CONNECTING)
+            requestJob?.apply { if (isActive) cancel() }
+            requestJob = viewModelScope.launch(Dispatchers.IO) {
                 val response = try {
                     val request = Request(uri)
                     val socket = request.connect()
                     val channel = request.proceed(socket, this)
                     Response.from(channel, viewModelScope)
                 } catch (e: UnknownHostException) {
-                    alertLiveData.postValue("Unknown host \"${uri.authority}\".")
+                    signalError("Unknown host \"${uri.authority}\".")
                     return@launch
                 } catch (e: Exception) {
                     Log.e(TAG, "sendGeminiRequest coroutine: ${e.stackTraceToString()}")
-                    alertLiveData.postValue("Oops! Whatever we tried to do failed!")
+                    signalError("Oops! Whatever we tried to do failed!")
                     return@launch
                 }
                 if (response == null) {
-                    alertLiveData.postValue("Can't parse server response.")
+                    signalError("Can't parse server response.")
                     return@launch
                 }
 
                 Log.i(TAG, "sendRequest: got ${response.code} with meta \"${response.meta}\"")
                 when (response.code) {
                     Response.Code.SUCCESS -> handleRequestSuccess(response)
-                    else -> alertLiveData.postValue("Can't handle code ${response.code}.")
+                    else -> signalError("Can't handle code ${response.code}.")
                 }
             }
+        }
 
+        private fun signalError(message: String) {
+            alert.postValue(message)
+            state.postValue(State.IDLE)
         }
 
         private suspend fun handleRequestSuccess(response: Response) {
-            lines.clear()
+            state.postValue(State.RECEIVING)
+            linesList.clear()
+            lines.postValue(linesList)
             val charset = Charset.defaultCharset()
+            var lastUpdate = System.currentTimeMillis()
             for (line in parseData(response.data, charset, viewModelScope)) {
-                lines.add(line)
-                linesLiveData.postValue(lines)
+                linesList.add(line)
+                val time = System.currentTimeMillis()
+                if (time - lastUpdate >= 100) {  // Throttle to 100ms the recycler view updates…
+                    lines.postValue(linesList)
+                    lastUpdate = time
+                }
             }
+            lines.postValue(linesList)
+            state.postValue(State.IDLE)
         }
     }
 

app/src/main/java/dev/lowrespalmtree/comet/Request.kt

@@ -50,6 +50,7 @@ class Request(private val uri: Uri) {
                 }
             }
             Log.d(TAG, "proceed coroutine: reading completed")
+            channel.close()
         }
         return channel
     }

app/src/main/java/dev/lowrespalmtree/comet/Response.kt

@@ -70,6 +70,7 @@ class Response(val code: Code, val meta: String, val data: Channel<ByteArray>) {
                     }
                     // Forward all incoming data to the Response channel.
                     channel.consumeEach { responseChannel.send(it) }
+                    responseChannel.close()
                 }
                 // Return the response here; this stops consuming the channel from this for-loop so
                 // that the coroutine above can take care of it.

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

@@ -24,28 +24,48 @@
                 android:layout_width="match_parent"
                 android:layout_height="match_parent"
                 android:layout_margin="4dp"
-                android:ems="10"
                 android:hint="@string/url"
                 android:imeOptions="actionDone|actionGo"
                 android:importantForAutofill="no"
                 android:inputType="textUri"
-                android:text="" />
+                android:text=""
+                tools:ignore="TextContrastCheck" />
 
         </com.google.android.material.appbar.CollapsingToolbarLayout>
 
     </com.google.android.material.appbar.AppBarLayout>
 
-    <androidx.core.widget.NestedScrollView
-        android:id="@+id/scroll_view"
+    <androidx.swiperefreshlayout.widget.SwipeRefreshLayout
+        android:id="@+id/content_swipe_layout"
         android:layout_width="match_parent"
         android:layout_height="wrap_content"
-        app:layout_behavior="com.google.android.material.appbar.AppBarLayout$ScrollingViewBehavior">
+        app:layout_behavior="@string/appbar_scrolling_view_behavior">
 
-        <androidx.recyclerview.widget.RecyclerView
-            android:id="@+id/content_recycler"
+        <RelativeLayout
+            android:id="@+id/content_inner_layout"
             android:layout_width="match_parent"
             android:layout_height="match_parent"
-            android:orientation="vertical" />
-    </androidx.core.widget.NestedScrollView>
+            app:layout_behavior="@string/appbar_scrolling_view_behavior">
+
+            <androidx.recyclerview.widget.RecyclerView
+                android:id="@+id/content_recycler"
+                android:layout_width="match_parent"
+                android:layout_height="wrap_content"
+                android:orientation="vertical"
+                app:layout_constraintTop_toTopOf="parent" />
+
+            <androidx.core.widget.ContentLoadingProgressBar
+                android:id="@+id/content_progress_bar"
+                style="?android:attr/progressBarStyleHorizontal"
+                android:layout_width="match_parent"
+                android:layout_height="16dp"
+                android:layout_alignParentTop="true"
+                android:layout_marginTop="-8dp"
+                android:indeterminate="true"
+                android:visibility="gone" />
+
+        </RelativeLayout>
+
+    </androidx.swiperefreshlayout.widget.SwipeRefreshLayout>
 
 </androidx.coordinatorlayout.widget.CoordinatorLayout>

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

@@ -2,5 +2,5 @@
 <Space xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="match_parent"
     android:layout_height="8dp"
-    android:background="@color/black">
-</Space>
+    android:background="@color/black"
+    android:orientation="vertical"></Space>