Add simple and clean empty state behavior.

frontend/android/src/main/java/com/jakewharton/sdksearch/ui/MainActivity.kt

@@ -11,6 +11,8 @@ import android.support.v7.util.DiffUtil
 import android.support.v7.widget.DividerItemDecoration
 import android.support.v7.widget.LinearLayoutManager
 import android.support.v7.widget.RecyclerView
+import android.text.Editable
+import android.text.TextWatcher
 import android.view.KeyEvent.KEYCODE_ENTER
 import android.view.View
 import android.view.View.INVISIBLE
@@ -20,6 +22,7 @@ import android.view.inputmethod.EditorInfo.IME_ACTION_GO
 import android.view.inputmethod.InputMethodManager
 import android.view.inputmethod.InputMethodManager.HIDE_NOT_ALWAYS
 import android.widget.EditText
+import android.widget.TextView
 import androidx.content.systemService
 import com.jakewharton.rxbinding2.support.v7.widget.scrollEvents
 import com.jakewharton.rxbinding2.view.keys
@@ -101,10 +104,45 @@ class MainActivity : Activity() {
     dividerDecoration.setDrawable(getDrawable(R.drawable.list_divider))
     recycler.addItemDecoration(dividerDecoration)
 
+    val emptyStateTextView = findViewById<TextView>(R.id.emptyState)
+
     val queryInput = findViewById<EditText>(R.id.query)
     if (savedInstanceState == null) {
       queryInput.setText(intent.getStringExtra("query") ?: "")
     }
+    queryInput.addTextChangedListener(object : TextWatcher {
+      override fun beforeTextChanged(s: CharSequence?, start: Int, count: Int, after: Int) = Unit
+      override fun onTextChanged(s: CharSequence?, start: Int, before: Int, count: Int) = Unit
+      override fun afterTextChanged(s: Editable) {
+        if (s.isEmpty()) {
+          emptyStateTextView.text = getString(R.string.enter_class_name)
+          emptyStateTextView.animate()
+                  .scaleX(1f)
+                  .scaleY(1f)
+                  .alpha(1f)
+                  .start()
+        } else {
+          emptyStateTextView.animate()
+                  .scaleX(0f)
+                  .scaleY(0f)
+                  .alpha(0f)
+                  .start()
+        }
+      }
+    })
+    adapter.registerAdapterDataObserver(object : RecyclerView.AdapterDataObserver() {
+      override fun onItemRangeRemoved(positionStart: Int, itemCount: Int) {
+        if (queryInput.length() > 0 && adapter.itemCount == 0) {
+          emptyStateTextView.visibility = View.VISIBLE
+          emptyStateTextView.text = getString(R.string.no_results)
+          emptyStateTextView.animate()
+                  .scaleX(1f)
+                  .scaleY(1f)
+                  .alpha(1f)
+                  .start()
+        }
+      }
+    })
 
     val enterKeys = queryInput.keys(Predicate { it.keyCode == KEYCODE_ENTER })
         .filter { it.keyCode == KEYCODE_ENTER }

frontend/android/src/main/res/drawable/ic_search_black_24dp.xml

@@ -0,0 +1,4 @@
+<vector android:height="96dp" android:viewportHeight="24.0"
+    android:viewportWidth="24.0" android:width="96dp" xmlns:android="http://schemas.android.com/apk/res/android">
+    <path android:fillColor="#8A000000" android:pathData="M15.5,14h-0.79l-0.28,-0.27C15.41,12.59 16,11.11 16,9.5 16,5.91 13.09,3 9.5,3S3,5.91 3,9.5 5.91,16 9.5,16c1.61,0 3.09,-0.59 4.23,-1.57l0.27,0.28v0.79l5,4.99L20.49,19l-4.99,-5zM9.5,14C7.01,14 5,11.99 5,9.5S7.01,5 9.5,5 14,7.01 14,9.5 11.99,14 9.5,14z"/>
+</vector>

frontend/android/src/main/res/layout/main.xml

@@ -62,4 +62,16 @@
       app:layout_constraintStart_toStartOf="parent"
       app:layout_constraintTop_toBottomOf="@id/toolbar"
       />
+  <TextView
+      android:id="@+id/emptyState"
+      android:layout_width="wrap_content"
+      android:layout_height="wrap_content"
+      android:gravity="center"
+      android:text="@string/enter_class_name"
+      android:drawableTop="@drawable/ic_search_black_24dp"
+      app:layout_constraintLeft_toLeftOf="parent"
+      app:layout_constraintRight_toRightOf="parent"
+      app:layout_constraintTop_toTopOf="parent"
+      app:layout_constraintBottom_toBottomOf="parent"
+  />
 </android.support.constraint.ConstraintLayout>

frontend/android/src/main/res/values/strings.xml

@@ -7,6 +7,8 @@
     <item quantity="one">Search 1 class…</item>
     <item quantity="other">Search %d classes…</item>
   </plurals>
+  <string name="enter_class_name">Enter a class name</string>
+  <string name="no_results">No results</string>
   <string name="copy">Copy</string>
   <string name="share">Share…</string>
   <string name="copied">%s URL copied to clipboard!</string>