add custom locator strategy (#1041)

* add custom locator strategy

* use correct strategy constant

* fix lint issues

* add doc links

src/main/java/io/appium/java_client/AppiumDriver.java

@@ -64,7 +64,7 @@
 */
 @SuppressWarnings("unchecked")
 public class AppiumDriver<T extends WebElement>
-    extends DefaultGenericMobileDriver<T> implements ComparesImages, FindsByImage<T> {
+    extends DefaultGenericMobileDriver<T> implements ComparesImages, FindsByImage<T>, FindsByCustom<T> {
 
     private static final ErrorHandler errorHandler = new ErrorHandler(new ErrorCodesMobile(), true);
     // frequently used command parameters

src/main/java/io/appium/java_client/FindsByCustom.java

@@ -0,0 +1,55 @@
+/*
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * See the NOTICE file distributed with this work for additional
+ * information regarding copyright ownership.
+ * 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 io.appium.java_client;
+
+import org.openqa.selenium.NoSuchElementException;
+import org.openqa.selenium.WebElement;
+
+import java.util.List;
+
+public interface FindsByCustom<T extends WebElement> extends FindsByFluentSelector<T> {
+    /**
+     * Performs the lookup for a single element by sending a selector to a custom element finding
+     * plugin. This type of locator requires the use of the 'customFindModules' capability and a
+     * separately-installed element finding plugin.
+     *
+     * @param selector selector to pass to the custom element finding plugin
+     * @return The first element that matches the given selector
+     * @see <a href="https://appium.io/docs/en/advanced-concepts/element-finding-plugins/">
+     * The documentation on custom element finding plugins and their use</a>
+     * @throws NoSuchElementException when no element is found
+     * @since Appium 1.9.2
+     */
+    default T findElementByCustom(String selector) {
+        return findElement(MobileSelector.CUSTOM.toString(), selector);
+    }
+
+    /**
+     * Performs the lookup for a single element by sending a selector to a custom element finding
+     * plugin. This type of locator requires the use of the 'customFindModules' capability and a
+     * separately-installed element finding plugin.
+     *
+     * @param selector selector to pass to the custom element finding plugin
+     * @return a list of elements that match the given selector or an empty list
+     * @see <a href="https://appium.io/docs/en/advanced-concepts/element-finding-plugins/">
+     * The documentation on custom element finding plugins and their use</a>
+     * @since Appium 1.9.2
+     */
+    default List<T> findElementsByCustom(String selector) {
+        return findElements(MobileSelector.CUSTOM.toString(), selector);
+    }
+}

src/main/java/io/appium/java_client/MobileBy.java

@@ -147,6 +147,18 @@ public static By AndroidViewTag(final String tag) {
     public static By image(final String b64Template) {
         return new ByImage(b64Template);
     }
+
+    /**
+     * This type of locator requires the use of the 'customFindModules' capability and a
+     * separately-installed element finding plugin.
+     *
+     * @param selector selector to pass to the custom element finding plugin
+     * @return an instance of {@link ByCustom}
+     * @since Appium 1.9.2
+     */
+    public static By custom(final String selector) {
+        return new ByCustom(selector);
+    }
 
     public static class ByIosUIAutomation extends MobileBy implements Serializable {
 
@@ -572,6 +584,62 @@ protected ByImage(String b64Template) {
         }
     }
 
+    public static class ByCustom extends MobileBy implements Serializable {
+
+        protected ByCustom(String selector) {
+            super(MobileSelector.CUSTOM, selector);
+        }
+
+        /**
+         * {@inheritDoc}
+         *
+         * @throws WebDriverException when current session doesn't support the given selector or when
+         *      value of the selector is not consistent.
+         * @throws IllegalArgumentException when it is impossible to find something on the given
+         * {@link SearchContext} instance
+         */
+        @SuppressWarnings("unchecked")
+        @Override public List<WebElement> findElements(SearchContext context) {
+            Class<?> contextClass = context.getClass();
+
+            if (FindsByCustom.class.isAssignableFrom(contextClass)) {
+                return FindsByCustom.class.cast(context).findElementsByCustom(getLocatorString());
+            }
+
+            if (FindsByFluentSelector.class.isAssignableFrom(contextClass)) {
+                return super.findElements(context);
+            }
+
+            throw formIllegalArgumentException(contextClass, FindsByCustom.class, FindsByFluentSelector.class);
+        }
+
+        /**
+         * {@inheritDoc}
+         *
+         * @throws WebDriverException when current session doesn't support the given selector or when
+         *      value of the selector is not consistent.
+         * @throws IllegalArgumentException when it is impossible to find something on the given
+         * {@link SearchContext} instance
+         */
+        @Override public WebElement findElement(SearchContext context) {
+            Class<?> contextClass = context.getClass();
+
+            if (FindsByCustom.class.isAssignableFrom(contextClass)) {
+                return FindsByCustom.class.cast(context).findElementByCustom(getLocatorString());
+            }
+
+            if (FindsByFluentSelector.class.isAssignableFrom(contextClass)) {
+                return super.findElement(context);
+            }
+
+            throw formIllegalArgumentException(contextClass, FindsByCustom.class, FindsByFluentSelector.class);
+        }
+
+        @Override public String toString() {
+            return "By.Custom: " + getLocatorString();
+        }
+    }
+
     public static class ByAndroidViewTag extends MobileBy implements Serializable {
 
         public ByAndroidViewTag(String tag) {

src/main/java/io/appium/java_client/MobileSelector.java

@@ -24,7 +24,8 @@ public enum MobileSelector {
     IOS_CLASS_CHAIN("-ios class chain"),
     WINDOWS_UI_AUTOMATION("-windows uiautomation"),
     IMAGE("-image"),
-    ANDROID_VIEWTAG("-android viewtag");
+    ANDROID_VIEWTAG("-android viewtag"),
+    CUSTOM("-custom");
 
     private final String selector;