Take snapshot of the underlying View or AccessibilityNodeInfo tree.

This improves reliability.
Add JavaDoc on order of getChildren.

Change-Id: Iec4a4b693ef29eea1e067d538bab0078699e3d50

16 files changed, 466 insertions, 392 deletions

diff --git a/src/com/google/android/droiddriver/UiElement.java b/src/com/google/android/droiddriver/UiElement.java
index a0d2ae3..1ae6254 100644
--- a/src/com/google/android/droiddriver/UiElement.java
+++ b/src/com/google/android/droiddriver/UiElement.java
@@ -196,6 +196,13 @@ public interface UiElement {
    * does not include off-screen children, but may include invisible on-screen
    * children.</li>
    * </ul>
+   * <p>
+   * Another discrepancy between {@link InstrumentationDriver}
+   * {@link UiAutomationDriver} is the order of children. The Accessibility API
+   * returns children in the order of layout (see
+   * {@link android.view.ViewGroup#addChildrenForAccessibility}, which is added
+   * in API16).
+   * </p>
    */
   List<? extends UiElement> getChildren(Predicate<? super UiElement> predicate);
 
diff --git a/src/com/google/android/droiddriver/base/BaseUiElement.java b/src/com/google/android/droiddriver/base/BaseUiElement.java
index dce0ee5..6a2f88c 100644
--- a/src/com/google/android/droiddriver/base/BaseUiElement.java
+++ b/src/com/google/android/droiddriver/base/BaseUiElement.java
@@ -16,6 +16,8 @@
 
 package com.google.android.droiddriver.base;
 
+import android.graphics.Rect;
+
 import com.google.android.droiddriver.UiElement;
 import com.google.android.droiddriver.actions.Action;
 import com.google.android.droiddriver.actions.ClickAction;
@@ -31,12 +33,14 @@ import com.google.common.base.Objects;
 import com.google.common.base.Objects.ToStringHelper;
 import com.google.common.base.Predicate;
 import com.google.common.base.Predicates;
-import com.google.common.collect.Lists;
+import com.google.common.collect.Collections2;
+import com.google.common.collect.ImmutableList;
 
 import org.w3c.dom.Element;
 
 import java.lang.ref.WeakReference;
 import java.util.List;
+import java.util.Map;
 import java.util.concurrent.Callable;
 import java.util.concurrent.FutureTask;
 
@@ -46,9 +50,90 @@ import java.util.concurrent.FutureTask;
 public abstract class BaseUiElement implements UiElement {
   private WeakReference<Element> domNode;
 
+  @SuppressWarnings("unchecked")
   @Override
   public <T> T get(Attribute attribute) {
-    return attribute.getValue(this);
+    return (T) getAttributes().get(attribute);
+  }
+
+  @Override
+  public String getText() {
+    return get(Attribute.TEXT);
+  }
+
+  @Override
+  public String getContentDescription() {
+    return get(Attribute.CONTENT_DESC);
+  }
+
+  @Override
+  public String getClassName() {
+    return get(Attribute.CLASS);
+  }
+
+  @Override
+  public String getResourceId() {
+    return get(Attribute.RESOURCE_ID);
+  }
+
+  @Override
+  public String getPackageName() {
+    return get(Attribute.PACKAGE);
+  }
+
+  @Override
+  public boolean isCheckable() {
+    return (Boolean) get(Attribute.CHECKABLE);
+  }
+
+  @Override
+  public boolean isChecked() {
+    return (Boolean) get(Attribute.CHECKED);
+  }
+
+  @Override
+  public boolean isClickable() {
+    return (Boolean) get(Attribute.CLICKABLE);
+  }
+
+  @Override
+  public boolean isEnabled() {
+    return (Boolean) get(Attribute.ENABLED);
+  }
+
+  @Override
+  public boolean isFocusable() {
+    return (Boolean) get(Attribute.FOCUSABLE);
+  }
+
+  @Override
+  public boolean isFocused() {
+    return (Boolean) get(Attribute.FOCUSED);
+  }
+
+  @Override
+  public boolean isScrollable() {
+    return (Boolean) get(Attribute.SCROLLABLE);
+  }
+
+  @Override
+  public boolean isLongClickable() {
+    return (Boolean) get(Attribute.LONG_CLICKABLE);
+  }
+
+  @Override
+  public boolean isPassword() {
+    return (Boolean) get(Attribute.PASSWORD);
+  }
+
+  @Override
+  public boolean isSelected() {
+    return (Boolean) get(Attribute.SELECTED);
+  }
+
+  @Override
+  public Rect getBounds() {
+    return get(Attribute.BOUNDS);
   }
 
   @Override
@@ -122,9 +207,9 @@ public abstract class BaseUiElement implements UiElement {
     perform(SwipeAction.toScroll(direction));
   }
 
-  protected abstract int getChildCount();
+  protected abstract Map<Attribute, Object> getAttributes();
 
-  protected abstract BaseUiElement getChild(int index);
+  protected abstract List<? extends BaseUiElement> getChildren();
 
   protected abstract InputInjector getInjector();
 
@@ -135,21 +220,16 @@ public abstract class BaseUiElement implements UiElement {
   }
 
   @Override
-  public List<BaseUiElement> getChildren(Predicate<? super UiElement> predicate) {
-    if (predicate == null) {
-      predicate = Predicates.notNull();
-    } else {
-      predicate = Predicates.and(Predicates.notNull(), predicate);
+  public List<? extends BaseUiElement> getChildren(Predicate<? super UiElement> predicate) {
+    List<? extends BaseUiElement> children = getChildren();
+    if (children == null) {
+      return ImmutableList.of();
     }
-
-    List<BaseUiElement> list = Lists.newArrayList();
-    for (int i = 0; i < getChildCount(); i++) {
-      BaseUiElement child = getChild(i);
-      if (predicate.apply(child)) {
-        list.add(child);
-      }
+    if (predicate == null || predicate.equals(Predicates.alwaysTrue())) {
+      return children;
     }
-    return list;
+
+    return ImmutableList.copyOf(Collections2.filter(children, predicate));
   }
 
   @Override
diff --git a/src/com/google/android/droiddriver/finders/Attribute.java b/src/com/google/android/droiddriver/finders/Attribute.java
index 2f8bbf6..1aeb86c 100644
--- a/src/com/google/android/droiddriver/finders/Attribute.java
+++ b/src/com/google/android/droiddriver/finders/Attribute.java
@@ -16,124 +16,23 @@
 
 package com.google.android.droiddriver.finders;
 
-import android.graphics.Rect;
-
-import com.google.android.droiddriver.UiElement;
-
 public enum Attribute {
-  CHECKABLE("checkable") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Boolean getValue(UiElement element) {
-      return element.isCheckable();
-    }
-  },
-  CHECKED("checked") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Boolean getValue(UiElement element) {
-      return element.isChecked();
-    }
-  },
-  CLASS("class") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public String getValue(UiElement element) {
-      return element.getClassName();
-    }
-  },
-  CLICKABLE("clickable") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Boolean getValue(UiElement element) {
-      return element.isClickable();
-    }
-  },
-  CONTENT_DESC("content-desc") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public String getValue(UiElement element) {
-      return element.getContentDescription();
-    }
-  },
-  ENABLED("enabled") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Boolean getValue(UiElement element) {
-      return element.isEnabled();
-    }
-  },
-  FOCUSABLE("focusable") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Boolean getValue(UiElement element) {
-      return element.isFocusable();
-    }
-  },
-  FOCUSED("focused") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Boolean getValue(UiElement element) {
-      return element.isFocused();
-    }
-  },
-  LONG_CLICKABLE("long-clickable") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Boolean getValue(UiElement element) {
-      return element.isLongClickable();
-    }
-  },
-  PACKAGE("package") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public String getValue(UiElement element) {
-      return element.getPackageName();
-    }
-  },
-  PASSWORD("password") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Boolean getValue(UiElement element) {
-      return element.isPassword();
-    }
-  },
-  RESOURCE_ID("resource-id") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public String getValue(UiElement element) {
-      return element.getResourceId();
-    }
-  },
-  SCROLLABLE("scrollable") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Boolean getValue(UiElement element) {
-      return element.isScrollable();
-    }
-  },
-  SELECTED("selected") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Boolean getValue(UiElement element) {
-      return element.isSelected();
-    }
-  },
-  TEXT("text") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public String getValue(UiElement element) {
-      return element.getText();
-    }
-  },
-  BOUNDS("bounds") {
-    @SuppressWarnings("unchecked")
-    @Override
-    public Rect getValue(UiElement element) {
-      // TODO: clip by boundsInParent?
-      return element.getBounds();
-    }
-  };
+  CHECKABLE("checkable"),
+  CHECKED("checked"),
+  CLASS("class"),
+  CLICKABLE("clickable"),
+  CONTENT_DESC("content-desc"),
+  ENABLED("enabled"),
+  FOCUSABLE("focusable"),
+  FOCUSED("focused"),
+  LONG_CLICKABLE("long-clickable"),
+  PACKAGE("package"),
+  PASSWORD("password"),
+  RESOURCE_ID("resource-id"),
+  SCROLLABLE("scrollable"),
+  SELECTED("selected"),
+  TEXT("text"),
+  BOUNDS("bounds");
 
   private final String name;
 
@@ -145,8 +44,6 @@ public enum Attribute {
     return name;
   }
 
-  public abstract <T> T getValue(UiElement element);
-
   @Override
   public String toString() {
     return name;
diff --git a/src/com/google/android/droiddriver/finders/By.java b/src/com/google/android/droiddriver/finders/By.java
index b4e1a74..0f1592b 100644
--- a/src/com/google/android/droiddriver/finders/By.java
+++ b/src/com/google/android/droiddriver/finders/By.java
@@ -32,6 +32,18 @@ import java.util.List;
  * Convenience methods to create commonly used finders.
  */
 public class By {
+  private static final MatchFinder ANY = new MatchFinder(null) {
+    @Override
+    public String toString() {
+      return "any";
+    }
+  };
+
+  /** Matches any UiElement. */
+  public static MatchFinder any() {
+    return ANY;
+  }
+
   /** Matches by {@link Object#equals}. */
   public static final MatchStrategy<Object> OBJECT_EQUALS = new MatchStrategy<Object>() {
     @Override
diff --git a/src/com/google/android/droiddriver/finders/ByAttribute.java b/src/com/google/android/droiddriver/finders/ByAttribute.java
index 3356533..a797a6d 100644
--- a/src/com/google/android/droiddriver/finders/ByAttribute.java
+++ b/src/com/google/android/droiddriver/finders/ByAttribute.java
@@ -33,7 +33,7 @@ public class ByAttribute<T> extends MatchFinder {
     super(new Predicate<UiElement>() {
       @Override
       public boolean apply(UiElement element) {
-        T value = attribute.getValue(element);
+        T value = element.get(attribute);
         return strategy.match(expected, value);
       }
     });
diff --git a/src/com/google/android/droiddriver/finders/ByXPath.java b/src/com/google/android/droiddriver/finders/ByXPath.java
index 50b1d3e..dbe4363 100644
--- a/src/com/google/android/droiddriver/finders/ByXPath.java
+++ b/src/com/google/android/droiddriver/finders/ByXPath.java
@@ -136,7 +136,10 @@ public class ByXPath implements Finder {
     setAttribute(element, Attribute.SELECTED, uiElement.isSelected());
     element.setAttribute(Attribute.BOUNDS.getName(), uiElement.getBounds().toShortString());
 
-    // TODO: visitor pattern
+    // TODO: Make VISIBLE optional so that the DOM dump contains all children.
+    // This is especially useful for InstrumentationDriver which sees more than
+    // uiautomatorviewer does. For now users can temporarily change VISIBLE to
+    // null, rebuild test apk and run to get a more comprehensive dump.
     for (BaseUiElement child : uiElement.getChildren(UiElement.VISIBLE)) {
       element.appendChild(child.getDomNode());
     }
diff --git a/src/com/google/android/droiddriver/instrumentation/InstrumentationContext.java b/src/com/google/android/droiddriver/instrumentation/InstrumentationContext.java
index 3868082..27fa975 100644
--- a/src/com/google/android/droiddriver/instrumentation/InstrumentationContext.java
+++ b/src/com/google/android/droiddriver/instrumentation/InstrumentationContext.java
@@ -68,10 +68,10 @@ class InstrumentationContext implements DroidDriverContext {
     return injector;
   }
 
-  public ViewElement getUiElement(View view) {
+  public ViewElement getUiElement(View view, ViewElement parent) {
     ViewElement element = map.get(view);
     if (element == null) {
-      element = new ViewElement(this, view);
+      element = new ViewElement(this, view, parent);
       map.put(view, element);
     }
     return element;
diff --git a/src/com/google/android/droiddriver/instrumentation/InstrumentationDriver.java b/src/com/google/android/droiddriver/instrumentation/InstrumentationDriver.java
index 8686098..dbe9789 100644
--- a/src/com/google/android/droiddriver/instrumentation/InstrumentationDriver.java
+++ b/src/com/google/android/droiddriver/instrumentation/InstrumentationDriver.java
@@ -46,7 +46,7 @@ public class InstrumentationDriver extends BaseDroidDriver {
 
   @Override
   protected ViewElement getNewRootElement() {
-    return context.getUiElement(findRootView());
+    return context.getUiElement(findRootView(), null /* parent */);
   }
 
   @Override
@@ -58,6 +58,7 @@ public class InstrumentationDriver extends BaseDroidDriver {
     Activity runningActivity = getRunningActivity();
     View[] views = RootFinder.getRootViews();
     if (views.length > 1) {
+      Logs.log(Log.VERBOSE, "views.length=" + views.length);
       for (View view : views) {
         if (view.hasWindowFocus()) {
           return view;
diff --git a/src/com/google/android/droiddriver/instrumentation/RootFinder.java b/src/com/google/android/droiddriver/instrumentation/RootFinder.java
index 2cdbd51..eec06c0 100644
--- a/src/com/google/android/droiddriver/instrumentation/RootFinder.java
+++ b/src/com/google/android/droiddriver/instrumentation/RootFinder.java
@@ -17,11 +17,9 @@
 package com.google.android.droiddriver.instrumentation;
 
 import android.os.Build;
-import android.util.Log;
 import android.view.View;
 
 import com.google.android.droiddriver.exceptions.DroidDriverException;
-import com.google.android.droiddriver.util.Logs;
 
 import java.lang.reflect.Field;
 import java.lang.reflect.InvocationTargetException;
@@ -78,7 +76,6 @@ public class RootFinder {
 
     try {
       views = (View[]) viewsField.get(windowManagerObj);
-      Logs.log(Log.DEBUG, "View size:" + views.length);
       return views;
     } catch (RuntimeException re) {
       throw new DroidDriverException(String.format("Reflective access to %s on %s failed.",
diff --git a/src/com/google/android/droiddriver/instrumentation/ViewElement.java b/src/com/google/android/droiddriver/instrumentation/ViewElement.java
index 5106975..c4aecc1 100644
--- a/src/com/google/android/droiddriver/instrumentation/ViewElement.java
+++ b/src/com/google/android/droiddriver/instrumentation/ViewElement.java
@@ -20,32 +20,164 @@ import static com.google.android.droiddriver.util.TextUtils.charSequenceToString
 
 import android.content.res.Resources;
 import android.graphics.Rect;
-import android.util.Log;
 import android.view.View;
 import android.view.ViewGroup;
-import android.view.ViewParent;
 import android.view.accessibility.AccessibilityNodeInfo;
 import android.widget.Checkable;
 import android.widget.TextView;
 
 import com.google.android.droiddriver.actions.InputInjector;
 import com.google.android.droiddriver.base.BaseUiElement;
-import com.google.android.droiddriver.util.Logs;
+import com.google.android.droiddriver.exceptions.DroidDriverException;
+import com.google.android.droiddriver.finders.Attribute;
+import com.google.common.base.Function;
 import com.google.common.base.Preconditions;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 
+import java.util.List;
 import java.util.Map;
 
 /**
- * A UiElement that is backed by a View.
+ * A UiElement that is backed by a View. A snapshot of all attributes is taken
+ * at construction. The attributes of a {@code ViewElement} instance are
+ * immutable. If the underlying view is updated, a new {@code ViewElement}
+ * instance will be created in
+ * {@link com.google.android.droiddriver.DroidDriver#refreshUiElementTree}.
  */
-// TODO: always accessing view on the UI thread even when only get access is
-// needed -- the field may be in the middle of updating.
 public class ViewElement extends BaseUiElement {
-  private static final Map<String, String> CLASS_NAME_OVERRIDES = Maps.newHashMap();
+  private static class SnapshotViewAttributesRunnable implements Runnable {
+    private final View view;
+    final Map<Attribute, Object> attribs = Maps.newEnumMap(Attribute.class);
+    boolean visible;
+    Rect visibleBounds;
+    List<View> childViews;
+    Throwable exception;
 
-  private final InstrumentationContext context;
-  private final View view;
+    private SnapshotViewAttributesRunnable(View view) {
+      this.view = view;
+    }
+
+    @Override
+    public void run() {
+      try {
+        put(Attribute.PACKAGE, view.getContext().getPackageName());
+        put(Attribute.CLASS, getClassName());
+        put(Attribute.TEXT, getText());
+        put(Attribute.CONTENT_DESC, charSequenceToString(view.getContentDescription()));
+        put(Attribute.RESOURCE_ID, getResourceId());
+        put(Attribute.CHECKABLE, view instanceof Checkable);
+        put(Attribute.CHECKED, isChecked());
+        put(Attribute.CLICKABLE, view.isClickable());
+        put(Attribute.ENABLED, view.isEnabled());
+        put(Attribute.FOCUSABLE, view.isFocusable());
+        put(Attribute.FOCUSED, view.isFocused());
+        put(Attribute.LONG_CLICKABLE, view.isLongClickable());
+        put(Attribute.PASSWORD, isPassword());
+        put(Attribute.SCROLLABLE, isScrollable());
+        put(Attribute.SELECTED, view.isSelected());
+        put(Attribute.BOUNDS, getBounds());
+
+        // Order matters as setVisible() depends on setVisibleBounds().
+        this.visibleBounds = getVisibleBounds();
+        // isShown() checks the visibility flag of this view and ancestors; it
+        // needs to have the VISIBLE flag as well as non-empty bounds to be
+        // visible.
+        this.visible = view.isShown() && !visibleBounds.isEmpty();
+        setChildViews();
+      } catch (Throwable e) {
+        exception = e;
+      }
+    }
+
+    private void put(Attribute key, Object value) {
+      if (value != null) {
+        attribs.put(key, value);
+      }
+    }
+
+    private String getText() {
+      if (!(view instanceof TextView)) {
+        return null;
+      }
+      return charSequenceToString(((TextView) view).getText());
+    }
+
+    private String getClassName() {
+      String className = view.getClass().getName();
+      return CLASS_NAME_OVERRIDES.containsKey(className) ? CLASS_NAME_OVERRIDES.get(className)
+          : className;
+    }
+
+    private String getResourceId() {
+      if (view.getId() != View.NO_ID && view.getResources() != null) {
+        try {
+          return charSequenceToString(view.getResources().getResourceName(view.getId()));
+        } catch (Resources.NotFoundException nfe) {
+          /* ignore */
+        }
+      }
+      return null;
+    }
+
+    private boolean isChecked() {
+      return view instanceof Checkable && ((Checkable) view).isChecked();
+    }
+
+    private boolean isScrollable() {
+      // TODO: find a meaningful implementation
+      return true;
+    }
+
+    private boolean isPassword() {
+      // TODO: find a meaningful implementation
+      return false;
+    }
+
+    private Rect getBounds() {
+      Rect rect = new Rect();
+      int[] xy = new int[2];
+      view.getLocationOnScreen(xy);
+      rect.set(xy[0], xy[1], xy[0] + view.getWidth(), xy[1] + view.getHeight());
+      return rect;
+    }
+
+    private Rect getVisibleBounds() {
+      Rect visibleBounds = new Rect();
+      if (!view.isShown() || !view.getGlobalVisibleRect(visibleBounds)) {
+        visibleBounds.setEmpty();
+      }
+      int[] xyScreen = new int[2];
+      view.getLocationOnScreen(xyScreen);
+      int[] xyWindow = new int[2];
+      view.getLocationInWindow(xyWindow);
+      int windowLeft = xyScreen[0] - xyWindow[0];
+      int windowTop = xyScreen[1] - xyWindow[1];
+
+      // Bounds are relative to root view; adjust to screen coordinates.
+      visibleBounds.offset(windowLeft, windowTop);
+      return visibleBounds;
+    }
+
+    private void setChildViews() {
+      if (!(view instanceof ViewGroup)) {
+        return;
+      }
+      ViewGroup group = (ViewGroup) view;
+      int childCount = group.getChildCount();
+      childViews = Lists.newArrayListWithExpectedSize(childCount);
+      for (int i = 0; i < childCount; i++) {
+        View child = group.getChildAt(i);
+        if (child != null) {
+          childViews.add(child);
+        }
+      }
+    }
+  }
+
+  private static final Map<String, String> CLASS_NAME_OVERRIDES = Maps.newHashMap();
 
   /**
    * Typically users find the class name to use in tests using SDK tool
@@ -66,161 +198,62 @@ public class ViewElement extends BaseUiElement {
     CLASS_NAME_OVERRIDES.put(actualClassName, overridingClassName);
   }
 
-  public ViewElement(InstrumentationContext context, View view) {
-    this.context = Preconditions.checkNotNull(context);
-    this.view = Preconditions.checkNotNull(view);
-  }
-
-  @Override
-  public String getText() {
-    if (!(view instanceof TextView)) {
-      return null;
-    }
-    return charSequenceToString(((TextView) view).getText());
-  }
-
-  @Override
-  public String getContentDescription() {
-    return charSequenceToString(view.getContentDescription());
-  }
-
-  @Override
-  public String getClassName() {
-    String className = view.getClass().getName();
-    return CLASS_NAME_OVERRIDES.containsKey(className) ? CLASS_NAME_OVERRIDES.get(className)
-        : className;
-  }
+  private final InstrumentationContext context;
+  private final Map<Attribute, Object> attributes;
+  private final boolean visible;
+  private final Rect visibleBounds;
+  private final ViewElement parent;
+  private final List<ViewElement> children;
 
-  @Override
-  public String getResourceId() {
-    if (view.getId() != View.NO_ID && view.getResources() != null) {
-      try {
-        return charSequenceToString(view.getResources().getResourceName(view.getId()));
-      } catch (Resources.NotFoundException nfe) {
-        /* ignore */
-      }
+  public ViewElement(final InstrumentationContext context, View view, ViewElement parent) {
+    this.context = Preconditions.checkNotNull(context);
+    Preconditions.checkNotNull(view);
+    this.parent = parent;
+    SnapshotViewAttributesRunnable attributesSnapshot = new SnapshotViewAttributesRunnable(view);
+    context.getInstrumentation().runOnMainSync(attributesSnapshot);
+    if (attributesSnapshot.exception != null) {
+      throw new DroidDriverException(attributesSnapshot.exception);
     }
-    return null;
-  }
 
-  @Override
-  public String getPackageName() {
-    return view.getContext().getPackageName();
+    attributes = ImmutableMap.copyOf(attributesSnapshot.attribs);
+    this.visibleBounds = attributesSnapshot.visibleBounds;
+    this.visible = attributesSnapshot.visible;
+    this.children =
+        attributesSnapshot.childViews == null ? null : ImmutableList.copyOf(Lists.transform(
+            attributesSnapshot.childViews, new Function<View, ViewElement>() {
+              public ViewElement apply(View input) {
+                return context.getUiElement(input, ViewElement.this);
+              }
+            }));
   }
 
   @Override
-  public InputInjector getInjector() {
-    return context.getInjector();
+  public Rect getVisibleBounds() {
+    return visibleBounds;
   }
 
   @Override
   public boolean isVisible() {
-    // isShown() checks the visibility flag of this view and ancestors; it needs
-    // to have the VISIBLE flag as well as non-empty bounds to be visible.
-    return view.isShown() && !getVisibleBounds().isEmpty();
-  }
-
-  @Override
-  public boolean isCheckable() {
-    return view instanceof Checkable;
-  }
-
-  @Override
-  public boolean isChecked() {
-    if (!isCheckable()) {
-      return false;
-    }
-    return ((Checkable) view).isChecked();
-  }
-
-  @Override
-  public boolean isClickable() {
-    return view.isClickable();
-  }
-
-  @Override
-  public boolean isEnabled() {
-    return view.isEnabled();
-  }
-
-  @Override
-  public boolean isFocusable() {
-    return view.isFocusable();
-  }
-
-  @Override
-  public boolean isFocused() {
-    return view.isFocused();
-  }
-
-  @Override
-  public boolean isScrollable() {
-    // TODO: find a meaningful implementation
-    return true;
-  }
-
-  @Override
-  public boolean isLongClickable() {
-    return view.isLongClickable();
+    return visible;
   }
 
   @Override
-  public boolean isPassword() {
-    // TODO: find a meaningful implementation
-    return false;
-  }
-
-  @Override
-  public boolean isSelected() {
-    return view.isSelected();
-  }
-
-  @Override
-  public Rect getBounds() {
-    Rect rect = new Rect();
-    int[] xy = new int[2];
-    view.getLocationOnScreen(xy);
-    rect.set(xy[0], xy[1], xy[0] + view.getWidth(), xy[1] + view.getHeight());
-    return rect;
-  }
-
-  @Override
-  public Rect getVisibleBounds() {
-    Rect visibleBounds = new Rect();
-    if (!view.getGlobalVisibleRect(visibleBounds)) {
-      Logs.log(Log.VERBOSE, "View is invisible: " + toString());
-      visibleBounds.setEmpty();
-    }
-    int[] xy = new int[2];
-    view.getLocationOnScreen(xy);
-    // Bounds are relative to root view; adjust to screen coordinates.
-    visibleBounds.offsetTo(xy[0], xy[1]);
-    return visibleBounds;
+  public ViewElement getParent() {
+    return parent;
   }
 
   @Override
-  protected int getChildCount() {
-    if (!(view instanceof ViewGroup)) {
-      return 0;
-    }
-    return ((ViewGroup) view).getChildCount();
+  protected List<ViewElement> getChildren() {
+    return children;
   }
 
   @Override
-  protected ViewElement getChild(int index) {
-    if (!(view instanceof ViewGroup)) {
-      return null;
-    }
-    View child = ((ViewGroup) view).getChildAt(index);
-    return child == null ? null : context.getUiElement(child);
+  protected Map<Attribute, Object> getAttributes() {
+    return attributes;
   }
 
   @Override
-  public ViewElement getParent() {
-    ViewParent parent = view.getParent();
-    if (!(parent instanceof View)) {
-      return null;
-    }
-    return context.getUiElement((View) parent);
+  protected InputInjector getInjector() {
+    return context.getInjector();
   }
 }
diff --git a/src/com/google/android/droiddriver/scroll/AbstractSentinelStrategy.java b/src/com/google/android/droiddriver/scroll/AbstractSentinelStrategy.java
index 4a1c373..2c339ad 100644
--- a/src/com/google/android/droiddriver/scroll/AbstractSentinelStrategy.java
+++ b/src/com/google/android/droiddriver/scroll/AbstractSentinelStrategy.java
@@ -15,6 +15,8 @@
  */
 package com.google.android.droiddriver.scroll;
 
+import android.util.Log;
+
 import com.google.android.droiddriver.DroidDriver;
 import com.google.android.droiddriver.UiElement;
 import com.google.android.droiddriver.exceptions.ElementNotFoundException;
@@ -23,6 +25,7 @@ import com.google.android.droiddriver.finders.Finder;
 import com.google.android.droiddriver.scroll.Direction.DirectionConverter;
 import com.google.android.droiddriver.scroll.Direction.LogicalDirection;
 import com.google.android.droiddriver.scroll.Direction.PhysicalDirection;
+import com.google.android.droiddriver.util.Logs;
 import com.google.common.base.Predicate;
 import com.google.common.base.Predicates;
 
@@ -105,6 +108,24 @@ public abstract class AbstractSentinelStrategy implements SentinelStrategy {
   };
 
   /**
+   * Returns the second last child as the sentinel. Useful when the activity
+   * always shows the last child as an anchor (for example a footer).
+   * <p>
+   * Sometimes uiautomatorviewer may not show the anchor as the last child, due
+   * to the reordering by layout described in {@link UiElement#getChildren}.
+   * This is not a problem with UiAutomationDriver because it sees the same as
+   * uiautomatorviewer does, but could be a problem with InstrumentationDriver.
+   * </p>
+   */
+  public static final GetStrategy SECOND_LAST_CHILD_GETTER = new GetStrategy(
+      Predicates.alwaysTrue(), "SECOND_LAST_CHILD") {
+    @Override
+    protected UiElement getSentinel(List<? extends UiElement> children) {
+      return children.size() < 2 ? null : children.get(children.size() - 2);
+    }
+  };
+
+  /**
    * Returns the second child as the sentinel. Useful when the activity shows a
    * fixed first child.
    */
@@ -130,6 +151,7 @@ public abstract class AbstractSentinelStrategy implements SentinelStrategy {
       if (sentinel == null) {
         throw new ElementNotFoundException(this);
       }
+      Logs.log(Log.INFO, "Found match: " + sentinel);
       return sentinel;
     }
 
@@ -156,6 +178,7 @@ public abstract class AbstractSentinelStrategy implements SentinelStrategy {
 
   protected UiElement getSentinel(DroidDriver driver, Finder containerFinder,
       PhysicalDirection direction) {
+    Logs.call(this, "getSentinel", driver, containerFinder, direction);
     Finder sentinelFinder;
     LogicalDirection logicalDirection = directionConverter.toLogicalDirection(direction);
     if (logicalDirection == LogicalDirection.BACKWARD) {
diff --git a/src/com/google/android/droiddriver/scroll/DynamicSentinelStrategy.java b/src/com/google/android/droiddriver/scroll/DynamicSentinelStrategy.java
index 488b231..3ae0e50 100644
--- a/src/com/google/android/droiddriver/scroll/DynamicSentinelStrategy.java
+++ b/src/com/google/android/droiddriver/scroll/DynamicSentinelStrategy.java
@@ -20,6 +20,7 @@ import android.util.Log;
 import com.google.android.droiddriver.DroidDriver;
 import com.google.android.droiddriver.UiElement;
 import com.google.android.droiddriver.exceptions.ElementNotFoundException;
+import com.google.android.droiddriver.finders.By;
 import com.google.android.droiddriver.finders.Finder;
 import com.google.android.droiddriver.scroll.Direction.DirectionConverter;
 import com.google.android.droiddriver.scroll.Direction.PhysicalDirection;
@@ -150,6 +151,28 @@ public class DynamicSentinelStrategy extends AbstractSentinelStrategy {
     }
   }
 
+  /**
+   * Determines whether the sentinel is updated by checking the resource-id of a
+   * descendant element of the sentinel (often itself). This is useful when the
+   * children of the container are heterogeneous -- they don't have a common
+   * pattern to get a unique string.
+   */
+  public static class ResourceIdUpdated extends SingleStringUpdated {
+    /**
+     * Uses the resource-id of the sentinel itself.
+     */
+    public static final ResourceIdUpdated SELF = new ResourceIdUpdated(By.any());
+
+    public ResourceIdUpdated(Finder uniqueStringFinder) {
+      super(uniqueStringFinder);
+    }
+
+    @Override
+    protected String getUniqueString(UiElement uniqueStringElement) {
+      return uniqueStringElement.getResourceId();
+    }
+  }
+
   private final IsUpdatedStrategy isUpdatedStrategy;
 
   /**
diff --git a/src/com/google/android/droiddriver/scroll/SentinelScroller.java b/src/com/google/android/droiddriver/scroll/SentinelScroller.java
index dd0b17e..273d6fa 100644
--- a/src/com/google/android/droiddriver/scroll/SentinelScroller.java
+++ b/src/com/google/android/droiddriver/scroll/SentinelScroller.java
@@ -41,8 +41,13 @@ import com.google.android.droiddriver.util.Logs;
  * {@link SentinelStrategy} is used to determine whether more scrolling is
  * possible.
  * <p>
- * This algorithm is needed unless the DroidDriver implementation supports
- * directly jumping to the item.
+ * This implementation may not work well with InstrumentationDriver if
+ * {@link UiElement#getChildren} returns children in an order different from the
+ * Accessibility API. In this case you may try
+ * {@link AbstractSentinelStrategy#SECOND_LAST_CHILD_GETTER}.
+ * </p>
+ * TODO: A {@link Scroller} that directly jumps to the item if an
+ * InstrumentationDriver is used.
  */
 public class SentinelScroller implements Scroller {
 
diff --git a/src/com/google/android/droiddriver/uiautomation/UiAutomationContext.java b/src/com/google/android/droiddriver/uiautomation/UiAutomationContext.java
index 6e56dfb..0b56d33 100644
--- a/src/com/google/android/droiddriver/uiautomation/UiAutomationContext.java
+++ b/src/com/google/android/droiddriver/uiautomation/UiAutomationContext.java
@@ -62,10 +62,10 @@ class UiAutomationContext implements DroidDriverContext {
     return injector;
   }
 
-  public UiAutomationElement getUiElement(AccessibilityNodeInfo node) {
+  public UiAutomationElement getUiElement(AccessibilityNodeInfo node, UiAutomationElement parent) {
     UiAutomationElement element = map.get(node);
     if (element == null) {
-      element = new UiAutomationElement(this, node);
+      element = new UiAutomationElement(this, node, parent);
       map.put(node, element);
     }
     return element;
diff --git a/src/com/google/android/droiddriver/uiautomation/UiAutomationDriver.java b/src/com/google/android/droiddriver/uiautomation/UiAutomationDriver.java
index 49b04ac..0e5ea78 100644
--- a/src/com/google/android/droiddriver/uiautomation/UiAutomationDriver.java
+++ b/src/com/google/android/droiddriver/uiautomation/UiAutomationDriver.java
@@ -53,7 +53,7 @@ public class UiAutomationDriver extends BaseDroidDriver {
 
   @Override
   protected UiAutomationElement getNewRootElement() {
-    return context.getUiElement(getRootNode());
+    return context.getUiElement(getRootNode(), null /* parent */);
   }
 
   @Override
diff --git a/src/com/google/android/droiddriver/uiautomation/UiAutomationElement.java b/src/com/google/android/droiddriver/uiautomation/UiAutomationElement.java
index cc3832f..7c4b5e8 100644
--- a/src/com/google/android/droiddriver/uiautomation/UiAutomationElement.java
+++ b/src/com/google/android/droiddriver/uiautomation/UiAutomationElement.java
@@ -26,14 +26,26 @@ import android.view.accessibility.AccessibilityNodeInfo;
 
 import com.google.android.droiddriver.actions.InputInjector;
 import com.google.android.droiddriver.base.BaseUiElement;
+import com.google.android.droiddriver.finders.Attribute;
 import com.google.android.droiddriver.util.Logs;
 import com.google.common.base.Preconditions;
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import com.google.common.collect.Lists;
+import com.google.common.collect.Maps;
 
+import java.util.List;
+import java.util.Map;
 import java.util.concurrent.FutureTask;
 import java.util.concurrent.TimeoutException;
 
 /**
- * A UiElement that is backed by the UiAutomation object.
+ * A UiElement that is backed by an {@link AccessibilityNodeInfo}. A snapshot of
+ * all attributes is taken at construction. The attributes of a
+ * {@code UiAutomationElement} instance are immutable. If the underlying
+ * {@link AccessibilityNodeInfo} is updated, a new {@code UiAutomationElement}
+ * instance will be created in
+ * {@link com.google.android.droiddriver.DroidDriver#refreshUiElementTree}.
  */
 public class UiAutomationElement extends BaseUiElement {
   private static final AccessibilityEventFilter ANY_EVENT_FILTER = new AccessibilityEventFilter() {
@@ -44,108 +56,76 @@ public class UiAutomationElement extends BaseUiElement {
   };
 
   private final UiAutomationContext context;
-  private final AccessibilityNodeInfo node;
-
-  public UiAutomationElement(UiAutomationContext context, AccessibilityNodeInfo node) {
+  private final Map<Attribute, Object> attributes;
+  private final boolean visible;
+  private final Rect visibleBounds;
+  private final UiAutomationElement parent;
+  private final List<UiAutomationElement> children;
+
+  public UiAutomationElement(UiAutomationContext context, AccessibilityNodeInfo node,
+      UiAutomationElement parent) {
     this.context = Preconditions.checkNotNull(context);
-    this.node = Preconditions.checkNotNull(node);
-  }
-
-  @Override
-  public String getText() {
-    return charSequenceToString(node.getText());
-  }
-
-  @Override
-  public String getContentDescription() {
-    return charSequenceToString(node.getContentDescription());
-  }
-
-  @Override
-  public String getClassName() {
-    return charSequenceToString(node.getClassName());
-  }
-
-  @Override
-  public String getResourceId() {
-    return charSequenceToString(node.getViewIdResourceName());
-  }
-
-  @Override
-  public String getPackageName() {
-    return charSequenceToString(node.getPackageName());
-  }
-
-  @Override
-  public InputInjector getInjector() {
-    return context.getInjector();
-  }
-
-  @Override
-  public boolean isVisible() {
-    return node.isVisibleToUser();
-  }
-
-  @Override
-  public boolean isCheckable() {
-    return node.isCheckable();
-  }
-
-  @Override
-  public boolean isChecked() {
-    return node.isChecked();
-  }
-
-  @Override
-  public boolean isClickable() {
-    return node.isClickable();
-  }
-
-  @Override
-  public boolean isEnabled() {
-    return node.isEnabled();
-  }
-
-  @Override
-  public boolean isFocusable() {
-    return node.isFocusable();
-  }
-
-  @Override
-  public boolean isFocused() {
-    return node.isFocused();
-  }
-
-  @Override
-  public boolean isScrollable() {
-    return node.isScrollable();
-  }
-
-  @Override
-  public boolean isLongClickable() {
-    return node.isLongClickable();
-  }
-
-  @Override
-  public boolean isPassword() {
-    return node.isPassword();
+    Preconditions.checkNotNull(node);
+    this.parent = parent;
+
+    Map<Attribute, Object> attribs = Maps.newEnumMap(Attribute.class);
+    put(attribs, Attribute.PACKAGE, charSequenceToString(node.getPackageName()));
+    put(attribs, Attribute.CLASS, charSequenceToString(node.getClassName()));
+    put(attribs, Attribute.TEXT, charSequenceToString(node.getText()));
+    put(attribs, Attribute.CONTENT_DESC, charSequenceToString(node.getContentDescription()));
+    put(attribs, Attribute.RESOURCE_ID, charSequenceToString(node.getViewIdResourceName()));
+    put(attribs, Attribute.CHECKABLE, node.isCheckable());
+    put(attribs, Attribute.CHECKED, node.isChecked());
+    put(attribs, Attribute.CLICKABLE, node.isClickable());
+    put(attribs, Attribute.ENABLED, node.isEnabled());
+    put(attribs, Attribute.FOCUSABLE, node.isFocusable());
+    put(attribs, Attribute.FOCUSED, node.isFocused());
+    put(attribs, Attribute.LONG_CLICKABLE, node.isLongClickable());
+    put(attribs, Attribute.PASSWORD, node.isPassword());
+    put(attribs, Attribute.SCROLLABLE, node.isScrollable());
+    put(attribs, Attribute.SELECTED, node.isSelected());
+    put(attribs, Attribute.BOUNDS, getBounds(node));
+    attributes = ImmutableMap.copyOf(attribs);
+
+    // Order matters as getVisibleBounds depends on visible
+    visible = node.isVisibleToUser();
+    visibleBounds = getVisibleBounds(node);
+    List<UiAutomationElement> mutableChildren = buildChildren(context, node);
+    this.children = mutableChildren == null ? null : ImmutableList.copyOf(mutableChildren);
+  }
+
+  private void put(Map<Attribute, Object> attribs, Attribute key, Object value) {
+    if (value != null) {
+      attribs.put(key, value);
+    }
   }
 
-  @Override
-  public boolean isSelected() {
-    return node.isSelected();
+  private List<UiAutomationElement> buildChildren(UiAutomationContext context,
+      AccessibilityNodeInfo node) {
+    List<UiAutomationElement> children;
+    int childCount = node.getChildCount();
+    if (childCount == 0) {
+      children = null;
+    } else {
+      children = Lists.newArrayListWithExpectedSize(childCount);
+      for (int i = 0; i < childCount; i++) {
+        AccessibilityNodeInfo child = node.getChild(i);
+        if (child != null) {
+          children.add(context.getUiElement(child, this));
+        }
+      }
+    }
+    return children;
   }
 
-  @Override
-  public Rect getBounds() {
+  private Rect getBounds(AccessibilityNodeInfo node) {
     Rect rect = new Rect();
     node.getBoundsInScreen(rect);
     return rect;
   }
 
-  @Override
-  public Rect getVisibleBounds() {
-    if (!isVisible()) {
+  private Rect getVisibleBounds(AccessibilityNodeInfo node) {
+    if (!visible) {
       Logs.log(Log.DEBUG, "Node is invisible: " + node);
       return new Rect();
     }
@@ -161,20 +141,33 @@ public class UiAutomationElement extends BaseUiElement {
   }
 
   @Override
-  protected int getChildCount() {
-    return node.getChildCount();
+  public Rect getVisibleBounds() {
+    return visibleBounds;
   }
 
   @Override
-  protected UiAutomationElement getChild(int index) {
-    AccessibilityNodeInfo child = node.getChild(index);
-    return child == null ? null : context.getUiElement(child);
+  public boolean isVisible() {
+    return visible;
   }
 
   @Override
   public UiAutomationElement getParent() {
-    AccessibilityNodeInfo parent = node.getParent();
-    return parent == null ? null : context.getUiElement(parent);
+    return parent;
+  }
+
+  @Override
+  protected List<UiAutomationElement> getChildren() {
+    return children;
+  }
+
+  @Override
+  protected Map<Attribute, Object> getAttributes() {
+    return attributes;
+  }
+
+  @Override
+  protected InputInjector getInjector() {
+    return context.getInjector();
   }
 
   @Override