Add validate extension of TextView

api/current.txt

@@ -648,6 +648,11 @@ package androidx.core.view {
 
 package androidx.core.widget {
 
+  public final class TextViewKt {
+    ctor public TextViewKt();
+    method public static void validate(android.widget.TextView, kotlin.jvm.functions.Function1<? super java.lang.CharSequence,java.lang.String> check);
+  }
+
   public final class ToastKt {
     ctor public ToastKt();
     method public static android.widget.Toast toast(android.content.Context, CharSequence text, int duration = "Toast.LENGTH_SHORT");

src/androidTest/java/androidx/core/widget/TextViewTest.kt

@@ -0,0 +1,49 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * 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
+ *
+ *       http://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 androidx.core.widget
+
+import android.support.test.InstrumentationRegistry
+import android.widget.TextView
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class TextViewTest {
+    private val context = InstrumentationRegistry.getContext()
+    private val textView = TextView(context)
+
+    @Test
+    fun validateTest() {
+        textView.setText("1234")
+        textView.validate {
+            if (it.length > 2) {
+                "length>2"
+            } else {
+                null
+            }
+        }
+        assertEquals(textView.error, "length>2")
+        textView.setText("1")
+        textView.validate {
+            if (it.length > 2) {
+                "length>2"
+            } else {
+                null
+            }
+        }
+        assertEquals(textView.error, null)
+    }
+}

src/main/java/androidx/core/widget/TextView.kt

@@ -0,0 +1,27 @@
+/*
+ * Copyright (C) 2018 The Android Open Source Project
+ *
+ * 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
+ *
+ *       http://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 androidx.core.widget
+
+import android.widget.TextView
+
+/**
+ * check if text of TextView is valid and set error message by the return value of [check] lambda,
+ * if [check] return null means the text is valid
+ */
+fun TextView.validate(check: (CharSequence) -> String?) {
+    error = check(text)
+}