[iOS] Upstream //ios/chrome/browser/memory

ios/chrome/browser/memory/memory_debugger.mm

Index: ios/chrome/browser/memory/memory_debugger.mm
diff --git a/ios/chrome/browser/memory/memory_debugger.mm b/ios/chrome/browser/memory/memory_debugger.mm
new file mode 100644
index 0000000000000000000000000000000000000000..e069b850b1a670af0d6702e854618cad743715cd
--- /dev/null
+++ b/ios/chrome/browser/memory/memory_debugger.mm
@@ -0,0 +1,601 @@
+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import "ios/chrome/browser/memory/memory_debugger.h"
+
+#include "base/ios/ios_util.h"
+#import "base/mac/scoped_nsobject.h"
+#import "base/memory/scoped_ptr.h"
+#import "ios/chrome/browser/memory/memory_metrics.h"
+#include "ios/chrome/browser/ui/ui_util.h"
+#import "ios/chrome/browser/ui/uikit_ui_util.h"
+
+namespace {
+// The number of bytes in a megabyte.
+const CGFloat kNumBytesInMB = 1024 * 1024;
+// The horizontal and vertical padding between subviews.
+const CGFloat kPadding = 10;
+}  // namespace
+
+@implementation MemoryDebugger {
+  // A timer to trigger refreshes.
+  base::scoped_nsobject<NSTimer> _refreshTimer;
+
+  // A timer to trigger continuous memory warnings.
+  base::scoped_nsobject<NSTimer> _memoryWarningTimer;
+
+  // The font to use.
+  base::scoped_nsobject<UIFont> _font;
+
+  // Labels for memory metrics.
+  base::scoped_nsobject<UILabel> _physicalFreeMemoryLabel;
+  base::scoped_nsobject<UILabel> _realMemoryUsedLabel;
+  base::scoped_nsobject<UILabel> _xcodeGaugeLabel;
+  base::scoped_nsobject<UILabel> _dirtyVirtualMemoryLabel;
+
+  // Inputs for memory commands.
+  base::scoped_nsobject<UITextField> _bloatField;
+  base::scoped_nsobject<UITextField> _refreshField;
+  base::scoped_nsobject<UITextField> _continuousMemoryWarningField;
+
+  // A place to store the artifical memory bloat.
+  scoped_ptr<uint8> _bloat;
+
+  // Distance the view was pushed up to accomodate the keyboard.
+  CGFloat _keyboardOffset;
+
+  // The current orientation of the device.
+  BOOL _currentOrientation;
+}
+
+- (instancetype)init {
+  self = [super initWithFrame:CGRectZero];
+  if (self) {
+    _font.reset([[UIFont systemFontOfSize:14] retain]);
+    self.backgroundColor = [UIColor colorWithWhite:0.8f alpha:0.9f];
+    self.opaque = NO;
+
+    [self addSubviews];
+    [self adjustForOrientation:nil];
+    [self sizeToFit];
+    [self registerForNotifications];
+  }
+  return self;
+}
+
+// NSTimers create a retain cycle so they must be invalidated before this
+// instance can be deallocated.
+- (void)invalidateTimers {
+  [_refreshTimer invalidate];
+  [_memoryWarningTimer invalidate];
+}
+
+- (void)dealloc {
+  [[NSNotificationCenter defaultCenter] removeObserver:self];
+  [super dealloc];
+}
+
+#pragma mark UIView methods
+
+- (CGSize)sizeThatFits:(CGSize)size {
+  CGFloat width = 0;
+  CGFloat height = 0;
+  for (UIView* subview in self.subviews) {
+    width = MAX(width, CGRectGetMaxX(subview.frame));
+    height = MAX(height, CGRectGetMaxY(subview.frame));
+  }
+  return CGSizeMake(width + kPadding, height + kPadding);
+}
+
+#pragma mark initialization helpers
+
+- (void)addSubviews {
+  // |index| is used to calculate the veritical position of each element in

[sdefresne, 2015/04/03 08:21:49]

nit: s/veritical/vertical/

+  // the debugger view.
+  NSUInteger index = 0;
+
+  // Display some metrics.
+  _physicalFreeMemoryLabel.reset([[UILabel alloc] initWithFrame:CGRectZero]);
+  [self addMetricWithName:@"Physical Free"
+                  atIndex:index++
+               usingLabel:_physicalFreeMemoryLabel];
+  _realMemoryUsedLabel.reset([[UILabel alloc] initWithFrame:CGRectZero]);
+  [self addMetricWithName:@"Real Memory Used"
+                  atIndex:index++
+               usingLabel:_realMemoryUsedLabel];
+  _xcodeGaugeLabel.reset([[UILabel alloc] initWithFrame:CGRectZero]);
+  [self addMetricWithName:@"Xcode Gauge"
+                  atIndex:index++
+               usingLabel:_xcodeGaugeLabel];
+  _dirtyVirtualMemoryLabel.reset([[UILabel alloc] initWithFrame:CGRectZero]);
+  [self addMetricWithName:@"Dirty VM"
+                  atIndex:index++
+               usingLabel:_dirtyVirtualMemoryLabel];
+
+// Since _performMemoryWarning is a private API it can't be compiled into
+// stable builds.

[sdefresne, 2015/04/03 08:21:48]

nit: s/stable/official/

+// TODO(lliabraa): Figure out how to support memory warnings (or something
+// like them) in official builds.
+#if CHROMIUM_BUILD
+  [self addButtonWithTitle:@"Trigger Memory Warning"
+                    target:[UIApplication sharedApplication]
+                    action:@selector(_performMemoryWarning)
+                withOrigin:[self originForSubviewAtIndex:index++]];
+#endif  // CHROMIUM_BUILD
+
+  // Display a text input to set the amount of artificial memory bloat and a
+  // button to reset the bloat to zero.
+  _bloatField.reset([[UITextField alloc] initWithFrame:CGRectZero]);
+  [self addLabelWithText:@"Set bloat (MB)"
+                   input:_bloatField
+             inputTarget:self
+             inputAction:@selector(updateBloat)
+         buttonWithTitle:@"Clear"
+            buttonTarget:self
+            buttonAction:@selector(clearBloat)
+                 atIndex:index++];
+  [_bloatField setText:@"0"];
+  [self updateBloat];
+
+// Since _performMemoryWarning is a private API it can't be compiled into
+// stable builds.

[sdefresne, 2015/04/03 08:21:48]

nit: s/stable/official/

+// TODO(lliabraa): Figure out how to support memory warnings (or something
+// like them) in official builds.
+#if CHROMIUM_BUILD
+  // Display a text input to control the rate of continuous memory warnings.
+  _continuousMemoryWarningField.reset(
+      [[UITextField alloc] initWithFrame:CGRectZero]);
+  [self addLabelWithText:@"Set memory warning interval (secs)"
+                   input:_continuousMemoryWarningField
+             inputTarget:self
+             inputAction:@selector(updateMemoryWarningInterval)
+                 atIndex:index++];
+  [_continuousMemoryWarningField setText:@"0.0"];
+#endif  // CHROMIUM_BUILD
+
+  // Display a text input to control the refresh rate of the memory debugger.
+  _refreshField.reset([[UITextField alloc] initWithFrame:CGRectZero]);
+  [self addLabelWithText:@"Set refresh interval (secs)"
+                   input:_refreshField
+             inputTarget:self
+             inputAction:@selector(updateRefreshInterval)
+                 atIndex:index++];
+  [_refreshField setText:@"0.5"];
+  [self updateRefreshInterval];
+}
+
+- (void)registerForNotifications {
+  // On iOS 7, the screen coordinate system is not dependent on orientation so
+  // the debugger has to handle its own rotation.
+  if (!base::ios::IsRunningOnIOS8OrLater()) {
+    // Register to receive orientation notifications.
+    [[NSNotificationCenter defaultCenter]
+        addObserver:self
+           selector:@selector(adjustForOrientation:)
+               name:UIDeviceOrientationDidChangeNotification
+             object:nil];
+  }
+
+  // Register to receive memory warning.
+  [[NSNotificationCenter defaultCenter]
+      addObserver:self
+         selector:@selector(lowMemoryWarningReceived:)
+             name:UIApplicationDidReceiveMemoryWarningNotification
+           object:nil];
+
+  // Register to receive keyboard will show notification.
+  [[NSNotificationCenter defaultCenter]
+      addObserver:self
+         selector:@selector(keyboardWillShow:)
+             name:UIKeyboardWillShowNotification
+           object:nil];
+
+  // Register to receive keyboard will hide notification.
+  [[NSNotificationCenter defaultCenter]
+      addObserver:self
+         selector:@selector(keyboardWillHide:)
+             name:UIKeyboardWillHideNotification
+           object:nil];
+}
+
+// Adds subviews for the specified metric, the value of which will be displayed
+// in |label|.
+- (void)addMetricWithName:(NSString*)name
+                  atIndex:(NSUInteger)index
+               usingLabel:(UILabel*)label {
+  // The width of the view for the metric's name.
+  const CGFloat kNameWidth = 150;
+  // The width of the view for each metric.
+  const CGFloat kMetricWidth = 100;
+  CGPoint nameOrigin = [self originForSubviewAtIndex:index];
+  CGRect nameFrame =
+      CGRectMake(nameOrigin.x, nameOrigin.y, kNameWidth, [_font lineHeight]);
+  base::scoped_nsobject<UILabel> nameLabel(
+      [[UILabel alloc] initWithFrame:nameFrame]);
+  [nameLabel setText:[NSString stringWithFormat:@"%@: ", name]];
+  [nameLabel setFont:_font];
+  [self addSubview:nameLabel];
+  label.frame = CGRectMake(CGRectGetMaxX(nameFrame), nameFrame.origin.y,
+                           kMetricWidth, [_font lineHeight]);
+  [label setFont:_font];
+  [label setTextAlignment:NSTextAlignmentRight];
+  [self addSubview:label];
+}
+
+// Adds a subview for a button with the given title and target/action.
+- (void)addButtonWithTitle:(NSString*)title
+                    target:(id)target
+                    action:(SEL)action
+                withOrigin:(CGPoint)origin {
+  base::scoped_nsobject<UIButton> button(
+      [[UIButton buttonWithType:UIButtonTypeSystem] retain]);
+  [button setTitle:title forState:UIControlStateNormal];
+  [button titleLabel].font = _font;
+  [[button titleLabel] setTextAlignment:NSTextAlignmentCenter];
+  [button sizeToFit];
+  [button setFrame:CGRectMake(origin.x, origin.y, [button frame].size.width,
+                              [_font lineHeight])];
+  [button addTarget:target
+                action:action
+      forControlEvents:UIControlEventTouchUpInside];
+  [self addSubview:button];
+}
+
+// Adds subviews for a UI component with label and input text field.
+//
+// -------------------------
+// | labelText  | <input>  |
+// -------------------------
+//
+// The inputTarget/inputAction will be invoked when the user finishes editing
+// in |input|.
+- (void)addLabelWithText:(NSString*)labelText
+                   input:(UITextField*)input
+             inputTarget:(id)inputTarget
+             inputAction:(SEL)inputAction
+                 atIndex:(NSUInteger)index {
+  [self addLabelWithText:labelText
+                   input:input
+             inputTarget:inputTarget
+             inputAction:inputAction
+         buttonWithTitle:nil
+            buttonTarget:nil
+            buttonAction:nil
+                 atIndex:index];
+}
+
+// Adds subviews for a UI component with label, input text field and button.
+//
+// -------------------------------------
+// | labelText  | <input>  |  <button> |
+// -------------------------------------
+//
+// The inputTarget/inputAction will be invoked when the user finishes editing
+// in |input|.
+- (void)addLabelWithText:(NSString*)labelText
+                   input:(UITextField*)input
+             inputTarget:(id)inputTarget
+             inputAction:(SEL)inputAction
+         buttonWithTitle:(NSString*)buttonTitle
+            buttonTarget:(id)buttonTarget
+            buttonAction:(SEL)buttonAction
+                 atIndex:(NSUInteger)index {
+  base::scoped_nsobject<UILabel> label(
+      [[UILabel alloc] initWithFrame:CGRectZero]);
+  if (labelText) {
+    [label setText:[NSString stringWithFormat:@"%@: ", labelText]];
+  }
+  [label setFont:_font];
+  [label sizeToFit];
+  CGPoint labelOrigin = [self originForSubviewAtIndex:index];
+  [label setFrame:CGRectOffset([label frame], labelOrigin.x, labelOrigin.y)];
+  [self addSubview:label];
+  if (input) {
+    // The width of the views for each input text field.
+    const CGFloat kInputWidth = 50;
+    input.frame =
+        CGRectMake(CGRectGetMaxX([label frame]) + kPadding,
+                   [label frame].origin.y, kInputWidth, [_font lineHeight]);
+    input.font = _font;
+    input.backgroundColor = [UIColor whiteColor];
+    input.delegate = self;
+    input.keyboardType = UIKeyboardTypeNumbersAndPunctuation;
+    input.adjustsFontSizeToFitWidth = YES;
+    input.textAlignment = NSTextAlignmentRight;
+    [input addTarget:inputTarget
+                  action:inputAction
+        forControlEvents:UIControlEventEditingDidEnd];
+
+    [self addSubview:input];
+  }
+
+  if (buttonTitle) {
+    const CGFloat kButtonXOffset =
+        input ? CGRectGetMaxX(input.frame) : CGRectGetMaxX([label frame]);
+    CGPoint origin =
+        CGPointMake(kButtonXOffset + kPadding, [label frame].origin.y);
+    [self addButtonWithTitle:buttonTitle
+                      target:buttonTarget
+                      action:buttonAction
+                  withOrigin:origin];
+  }
+}
+
+// Returns the CGPoint of the origin of the subview at |index|.
+- (CGPoint)originForSubviewAtIndex:(NSUInteger)index {
+  return CGPointMake(kPadding,
+                     (index + 1) * kPadding + index * [_font lineHeight]);
+}
+
+#pragma mark Refresh callback
+
+// Updates content and ensures the view is visible.
+- (void)refresh:(NSTimer*)timer {
+  [self.superview bringSubviewToFront:self];
+  [self updateMemoryInfo];
+}
+
+#pragma mark Memory inspection
+
+// Updates the memory metrics shown.
+- (void)updateMemoryInfo {
+  CGFloat value = memory_util::GetFreePhysicalBytes() / kNumBytesInMB;
+  [_physicalFreeMemoryLabel
+      setText:[NSString stringWithFormat:@"%.2f MB", value]];
+  value = memory_util::GetRealMemoryUsedInBytes() / kNumBytesInMB;
+  [_realMemoryUsedLabel setText:[NSString stringWithFormat:@"%.2f MB", value]];
+  value = memory_util::GetInternalVMBytes() / kNumBytesInMB;
+  [_xcodeGaugeLabel setText:[NSString stringWithFormat:@"%.2f MB", value]];
+  value = memory_util::GetDirtyVMBytes() / kNumBytesInMB;
+  [_dirtyVirtualMemoryLabel
+      setText:[NSString stringWithFormat:@"%.2f MB", value]];
+}
+
+#pragma mark Memory Warning notification callback
+
+// Flashes the debugger to indicate memory warning.
+- (void)lowMemoryWarningReceived:(NSNotification*)notification {
+  UIColor* originalColor = self.backgroundColor;
+  self.backgroundColor =
+      [UIColor colorWithRed:1.0 green:0.0 blue:0.0 alpha:0.9];
+  [UIView animateWithDuration:1.0
+                        delay:0.0
+                      options:UIViewAnimationOptionAllowUserInteraction
+                   animations:^{
+                     self.backgroundColor = originalColor;
+                   }
+                   completion:nil];
+}
+
+#pragma mark Rotation notification callback
+
+- (void)didMoveToSuperview {
+  UIView* superview = [self superview];
+  if (superview)
+    [self setCenter:[superview center]];
+}
+
+- (void)adjustForOrientation:(NSNotification*)notification {
+  if (base::ios::IsRunningOnIOS8OrLater()) {
+    return;
+  }
+  UIInterfaceOrientation orientation =
+      [[UIApplication sharedApplication] statusBarOrientation];
+  if (orientation == _currentOrientation) {
+    return;
+  }
+  _currentOrientation = orientation;
+  CGFloat angle;
+  switch (orientation) {
+    case UIInterfaceOrientationPortrait:
+      angle = 0;
+      break;
+    case UIInterfaceOrientationPortraitUpsideDown:
+      angle = M_PI;
+      break;
+    case UIInterfaceOrientationLandscapeLeft:
+      angle = -M_PI_2;
+      break;
+    case UIInterfaceOrientationLandscapeRight:
+      angle = M_PI_2;
+      break;
+    case UIInterfaceOrientationUnknown:
+    default:
+      angle = 0;
+  }
+
+  // Since the debugger view is in screen coordinates and handles its own
+  // rotation via the |transform| property, the view's position after rotation
+  // can be unexpected and partially off-screen. Centering the view before
+  // rotating it ensures that the view remains within the bounds of the screen.
+  // TODO(lliabraa): Correctly handle rotation (crbug/368263).

[sdefresne, 2015/04/03 08:21:48]

nit: Remove this comment as the bug has been marked as WontFix

+  if (self.superview) {
+    self.center = self.superview.center;
+  }
+  self.transform = CGAffineTransformMakeRotation(angle);
+}
+
+#pragma mark Keyboard notification callbacks
+
+// Ensures the debugger is visible by shifting it up as the keyboard animates
+// in.
+- (void)keyboardWillShow:(NSNotification*)notification {
+  NSDictionary* userInfo = [notification userInfo];
+  NSValue* keyboardFrameValue =
+      [userInfo valueForKey:UIKeyboardFrameEndUserInfoKey];
+  CGFloat keyboardHeight = CurrentKeyboardHeight(keyboardFrameValue);
+
+  // Get the coord of the bottom of the debugger's frame. This is orientation
+  // dependent on iOS 7 because the debugger is in screen coords.
+  CGFloat bottomOfFrame = CGRectGetMaxY(self.frame);
+  if (!base::ios::IsRunningOnIOS8OrLater() && IsLandscape())
+    bottomOfFrame = CGRectGetMaxX(self.frame);
+
+  // Shift the debugger up by the "height" of the keyboard, but since the
+  // keyboard rect is in screen coords, use the orientation to find the height.
+  CGFloat distanceFromBottom = CurrentScreenHeight() - bottomOfFrame;
+  _keyboardOffset = -1 * fmax(0.0f, keyboardHeight - distanceFromBottom);
+  [self animateForKeyboardNotification:notification
+                            withOffset:CGPointMake(0, _keyboardOffset)];
+}
+
+// Shifts the debugger back down when the keyboard is hidden.
+- (void)keyboardWillHide:(NSNotification*)notification {
+  [self animateForKeyboardNotification:notification
+                            withOffset:CGPointMake(0, -_keyboardOffset)];
+}
+
+- (void)animateForKeyboardNotification:(NSNotification*)notification
+                            withOffset:(CGPoint)offset {
+  // Account for orientation.
+  offset = CGPointApplyAffineTransform(offset, self.transform);
+  // Normally this would use an animation block, but there is no API to
+  // convert the UIKeyboardAnimationCurveUserInfoKey's value from a
+  // UIViewAnimationCurve to a UIViewAnimationOption. Awesome!
+  NSDictionary* userInfo = [notification userInfo];
+  [UIView beginAnimations:nil context:NULL];
+  [UIView setAnimationDuration:
+              [userInfo[UIKeyboardAnimationDurationUserInfoKey] doubleValue]];
+  NSInteger animationCurveKeyValue =
+      [userInfo[UIKeyboardAnimationCurveUserInfoKey] integerValue];
+  UIViewAnimationCurve animationCurve =
+      (UIViewAnimationCurve)animationCurveKeyValue;
+  [UIView setAnimationCurve:animationCurve];
+  [UIView setAnimationBeginsFromCurrentState:YES];
+  self.frame = CGRectOffset(self.frame, offset.x, offset.y);
+  [UIView commitAnimations];
+}
+
+#pragma mark Artificial memory bloat methods
+
+- (void)updateBloat {
+  double bloatSizeMB;
+  NSScanner* scanner = [NSScanner scannerWithString:[_bloatField text]];
+  if (![scanner scanDouble:&bloatSizeMB] || bloatSizeMB < 0.0) {
+    bloatSizeMB = 0;
+    NSString* errorMessage =
+        [NSString stringWithFormat:@"Invalid value \"%@\" for bloat size.\n"
+                                   @"Must be a positive number.\n"
+                                   @"Resetting to %.1f MB",
+                                   [_bloatField text], bloatSizeMB];
+    [self alert:errorMessage];
+    [_bloatField setText:[NSString stringWithFormat:@"%.1f", bloatSizeMB]];
+  }
+  const CGFloat kBloatSizeBytes = ceil(bloatSizeMB * kNumBytesInMB);
+  const uint64 kNumberOfBytes = static_cast<uint64>(kBloatSizeBytes);
+  _bloat.reset(kNumberOfBytes ? (uint8*)malloc(kNumberOfBytes) : NULL);

[sdefresne, 2015/04/03 08:21:49]

style: C-style cast are forbidden

Moreover _bloat is a scoped_ptr<uint8> so the destructor will use delete, not
free. This is undefined behaviour (mixing new/delete & malloc/free). Please
change to (which does not require a cast):

_bloat.reset(kNumberOfBytes ? new uint8[kNumberOfBytes] : nullptr);

+  if (_bloat) {
+    memset(_bloat.get(), -1, kNumberOfBytes);  // Occupy memory.
+  } else {
+    if (kNumberOfBytes) {
+      [self alert:@"Could not allocate memory."];
+    }
+  }
+}
+
+- (void)clearBloat {
+  [_bloatField setText:@"0"];
+  [_bloatField resignFirstResponder];
+  [self updateBloat];
+}
+
+#pragma mark Refresh interval methods
+
+- (void)updateRefreshInterval {
+  double refreshTimerValue;
+  NSScanner* scanner = [NSScanner scannerWithString:[_refreshField text]];
+  if (![scanner scanDouble:&refreshTimerValue] || refreshTimerValue < 0.0) {
+    refreshTimerValue = 0.5;
+    NSString* errorMessage = [NSString
+        stringWithFormat:@"Invalid value \"%@\" for refresh interval.\n"
+                         @"Must be a positive number.\n" @"Resetting to %.1f",
+                         [_refreshField text], refreshTimerValue];
+    [self alert:errorMessage];
+    [_refreshField
+        setText:[NSString stringWithFormat:@"%.1f", refreshTimerValue]];
+    return;
+  }
+  [_refreshTimer invalidate];
+  _refreshTimer.reset(
+      [[NSTimer scheduledTimerWithTimeInterval:refreshTimerValue
+                                        target:self
+                                      selector:@selector(refresh:)
+                                      userInfo:nil
+                                       repeats:YES] retain]);
+}
+
+#pragma mark Memory warning interval methods
+
+// Since _performMemoryWarning is a private API it can't be compiled into
+// stable builds.

[sdefresne, 2015/04/03 08:21:48]

nit: s/stable/official/

+// TODO(lliabraa): Figure out how to support memory warnings (or something
+// like them) in official builds.
+#if CHROMIUM_BUILD
+- (void)updateMemoryWarningInterval {
+  [_memoryWarningTimer invalidate];
+  double timerValue;
+  NSString* text = [_continuousMemoryWarningField text];
+  NSScanner* scanner = [NSScanner scannerWithString:text];
+  BOOL valueFound = [scanner scanDouble:&timerValue];
+  // If the text field is empty or contains 0, return early to turn off
+  // continuous memory warnings.
+  if (![text length] || timerValue == 0.0) {
+    return;
+  }
+  // If no value could be parsed or a non-positive value was found, throw up an
+  // error message and return early to turn off continuous memory warnings.
+  if (!valueFound || timerValue <= 0.0) {
+    NSString* errorMessage = [NSString
+        stringWithFormat:@"Invalid value \"%@\" for memory warning interval.\n"
+                         @"Must be a positive number.\n"
+                         @"Turning off continuous memory warnings",
+                         text];
+    [self alert:errorMessage];
+    [_continuousMemoryWarningField setText:@""];
+    return;
+  }
+  // If a valid value was found have the timer start triggering continuous
+  // memory warnings.
+  _memoryWarningTimer.reset(
+      [[NSTimer scheduledTimerWithTimeInterval:timerValue
+                                        target:[UIApplication sharedApplication]
+                                      selector:@selector(_performMemoryWarning)
+                                      userInfo:nil
+                                       repeats:YES] retain]);
+}
+#endif  // CHROMIUM_BUILD
+
+#pragma mark UITextViewDelegate methods
+
+// Dismisses the keyboard if the user hits return.
+- (BOOL)textFieldShouldReturn:(UITextField*)textField {
+  [textField resignFirstResponder];
+  return YES;
+}
+
+#pragma mark UIResponder methods
+
+// Allows the debugger to be dragged around the screen.
+- (void)touchesMoved:(NSSet*)touches withEvent:(UIEvent*)event {
+  UITouch* touch = [touches anyObject];
+  CGPoint start = [touch previousLocationInView:self];
+  CGPoint end = [touch locationInView:self];
+  CGPoint offset = CGPointMake(end.x - start.x, end.y - start.y);
+  offset = CGPointApplyAffineTransform(offset, self.transform);
+  self.frame = CGRectOffset(self.frame, offset.x, offset.y);
+}
+
+#pragma mark Error handling
+
+// Shows an alert with the given |errorMessage|.
+- (void)alert:(NSString*)errorMessage {
+  UIAlertView* alert = [[UIAlertView alloc] initWithTitle:@"Error"
+                                                  message:errorMessage
+                                                 delegate:self
+                                        cancelButtonTitle:@"OK"
+                                        otherButtonTitles:nil, nil];
+  [alert show];
+}
+
+@end