Archives for category: Animation

Full Source code:


The Twitter iPhone app pioneered the ability to swipe on a tweet and have a menu appear, letting you do things like reply or favorite the tweet.

Tweets in the Twitter app are table view cells in a table view. How do we recreate this feature and add the ability to side swipe on table view cells?


This feature has two distinct parts. The first is detecting that the user swiped on the table. The second is animating in and animating out the menu view.

Detecting swipes in iOS 4

iOS 4 introduced Gesture Recognizers which make gestures like swiping, tapping and pinching very east to detect. Specifically we can create a couple of UISwipeGestureRecognizer objects, one for the right direction and another for the left detection and attach them to the table view:

// Setup a right swipe gesture recognizer
UISwipeGestureRecognizer* rightSwipeGestureRecognizer = [[[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(swipeRight:)] autorelease];
rightSwipeGestureRecognizer.direction = UISwipeGestureRecognizerDirectionRight;
[tableView addGestureRecognizer:rightSwipeGestureRecognizer];

// Setup a left swipe gesture recognizer
UISwipeGestureRecognizer* leftSwipeGestureRecognizer = [[[UISwipeGestureRecognizer alloc] initWithTarget:self action:@selector(swipeLeft:)] autorelease];
leftSwipeGestureRecognizer.direction = UISwipeGestureRecognizerDirectionLeft;
[tableView addGestureRecognizer:leftSwipeGestureRecognizer];

Now when the user swipes left or right anywhere in the table, our swipeRight: or swipeLeft: methods will get called. The touch handling code that tracks the user’s finger and figures out their intent is blissfully handled for us.

Detecting swipes in iOS 3

When this feature was introduced in what was then the Tweetie app, it worked in iOS 3 well before iOS 4 was released. You might smartly argue that if iOS 3 currently makes up 1-2% of users out there it isn’t worth developing for and I admit this is a valid point. Still it’s an interesting technical mystery and that’s just the kind of thing we love solving!

You might have thought like I did, that the Twitter app must have implemented its own touch handling, guessing based on the location of your finger whether you were trying to do a swipe, but this is wrong.

In the Twitter app it doesn’t matter if you swipe left or swipe right, the animation of the menu always happens from left to right. This is the same behavior as the editing of table view cells and it turns out this is how the Twitter app does it: It hijacks the built in swipe to delete feature of table view cells. There are 3 parts to making this work:

1. Enabling swipe to delete

Per Apple’s documentation:

To enable the swipe-to-delete feature of table views (wherein a user swipes horizontally across a row to display a Delete button), you must implement the tableView:commitEditingStyle:forRowAtIndexPath: method

So the first step is to implement the tableView:commitEditingStyle:forRowAtIndexPath: method.

- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath

The method doesn’t have to do anything. Once it is implemented, you’ll be able to side swipe on a cell and the Delete button will appear.

2. Disabling the Delete button

Apple’s Inserting and Deleting Rows and Sections documentation indicates that when you explicitly put a table in editing mode by calling setEditing:animated:, the same message is then sent to each of the visible cells.

The documentation for a table view cell’s setEditing:animated: indicates that when this method is called, insertion/deletion control are animated in.

So disabling the Delete button turns out to be relatively simple: Override the table view cell’s setEditing:animated: and don’t call the superclass’s implementation.

- (void)setEditing:(BOOL)editing animated:(BOOL)animated
  // We suppress the Delete button by explicitly not calling
  // super's implementation
  if (supressDeleteButton)
    // Reset the editing state of the table back to NO
    UITableView* tableView = [self getTableView:self];
    tableView.editing = NO;
    [super setEditing:editing animated:animated];

3. Getting notified when a swipe occurred

Apple’s docs for tableView:willBeginEditingRowAtIndexPath: are crystal clear: This method is called when the user swipes horizontally across a row.

The implementation of this method in iOS 3 is a parallel to the swipeLeft: and swipeRight: methods we registered with UISwipeGestureRecognizers under iOS 4. When any of these methods are called, we know that a swipe happened and we are ready to animate in the menu.

Animating in the menu view

Before we animate in the menu, we first add it as a subview of the table view.

As you can see in the image at the top of this post, we are animating the existing cell content offscreen while simultaneously animating in the menu. Here is a rough illustration of how both the cell content and the menu have to animate in sync during a left to right animation:

So we first set the frame of the menu, placing it offscreen. Depending of the direction, we’d put it offscreen on the right or left side of the table. Next we’d start an animation block and set the frame of the menu to be at 0 x-offset. Inside the same animation block we also set the cell’s frame to be offscreen on the other side of the table.

- (void) addSwipeViewTo:(UITableViewCell*)cell direction:(UISwipeGestureRecognizerDirection)direction
  // Change the frame of the side swipe view to match the cell
  sideSwipeView.frame = cell.frame;

  // Add the side swipe view to the table
  [tableView addSubview:sideSwipeView];
  // Remember which cell the side swipe view is displayed on and the swipe direction
  self.sideSwipeCell = cell;
  sideSwipeDirection = direction;

  // Move the side swipe view offscreen either to the left or the right depending on the swipe direction
  CGRect cellFrame = cell.frame;
  sideSwipeView.frame = CGRectMake(direction == UISwipeGestureRecognizerDirectionRight ? -cellFrame.size.width : cellFrame.size.width, cellFrame.origin.y, cellFrame.size.width, cellFrame.size.height);

  // Animate in the side swipe view
  animatingSideSwipe = YES;
  [UIView beginAnimations:nil context:nil];
  [UIView setAnimationDuration:0.2];
  [UIView setAnimationDelegate:self];
  [UIView setAnimationDidStopSelector:@selector(animationDidStopAddingSwipeView:finished:context:)];
  // Move the side swipe view to offset 0
  sideSwipeView.frame = CGRectMake(0, cellFrame.origin.y, cellFrame.size.width, cellFrame.size.height);
  // While simultaneously moving the cell's frame offscreen
  // The net effect is that the side swipe view is pushing the cell offscreen
  cell.frame = CGRectMake(direction == UISwipeGestureRecognizerDirectionRight ? cellFrame.size.width : -cellFrame.size.width, cellFrame.origin.y, cellFrame.size.width, cellFrame.size.height);
  [UIView commitAnimations];

Animating out the menu view

When the menu is animated away, there is a little bounce of the cell content as it comes back into view.

Here is another rough illustration showing the three animations that make up a bounce, showing at each step where the cell content and menu are:

There are multiple ways we can achieve this animation. One is CAKeyframeAnimation where you specify a path and the animation follows that path.

Instead the code simply chains together the 3 separate animations. Since we might care about iOS 3, we don’t use animation blocks, but instead use begin/commit animation methods and register an animation stop selector where we start the next animation.

Just like we did when animating the menu in, at each step we animate both the menu as well as the cell to give the illusion that the cell content is pushing the menu out of view.

UPDATE: It was pointed out on Hacker News that the Twitter app actually puts the menu behind the cell and then only animates the cell content in and out. The menu isn’t animated at all. I’ve updated the code so that by default it now does this style of animation. If you really liked the pushing behavior where both the menu and cell content are animated, there is a PUSH_STYLE_ANIMATION #define that you can set to YES to get it back.

Full Source code:

Tweet This!Hacker NewsShare on Facebook


Full Source code:


When the Instagram app wants to let you know that you have new comments, likes or followers, it doesn’t use standard badge values on tab bar items:

Instead it uses a custom tab bar notification view. How do we build a similar custom notification view?


Just like in our recipe for the Twitter app’s current tab bar indicator we will add a view on top of the tab bar to notify the user. Instead of a static image, we will add a slightly more complicated view. This view has the background, icons for each of the activities (comments, likes, followers), and labels with the number value of each of these activities. Here is what it looks like laid out in the NIB:

Vertical/Horizontal Location

Again we need to figure out the horizontal and vertical location of our custom view and we essentially do the same thing we did in our Twitter tab bar indicator recipe:

To calculate the vertical location, we start at the top of the tab bar (0), go up by the height of the notification view, then go up another 2 pixels so our view is slightly above the tab bar

For the horizontal location we divide the width of the tab bar by the number of items to calculate the width of a single item. We then multiply the index by the width of single item and add half the width of an item to land in the middle.

Showing/Hiding the custom notification view

In a real app we would hit a web service, get the data and then notify the user by showing the custom notification UI.

In the sample app we have a couple of buttons that show and hide the notification view. We also do a very simple fade in/fade out animation of the notification view.

Showing the custom notification view:

- (void) showNotificationViewFor:(NSUInteger)tabIndex
  // To get the vertical location we start at the top of the tab bar (0), go up by the height of the notification view, then go up another 2 pixels so our view is slightly above the tab bar
  CGFloat verticalLocation = - notificationView.frame.size.height - 2.0;
  notificationView.frame = CGRectMake([self horizontalLocationFor:tabIndex], verticalLocation, notificationView.frame.size.width, notificationView.frame.size.height);

  if (!notificationView.superview)
    [self.tabBar addSubview:notificationView];

  notificationView.alpha = 0.0;

  [UIView beginAnimations:nil context:nil];
  [UIView setAnimationDuration:0.5];
  notificationView.alpha = 1.0;
  [UIView commitAnimations];

Hiding the custom notification view:

- (IBAction)hideNotificationView:(id)sender
  [UIView beginAnimations:nil context:nil];
  [UIView setAnimationDuration:0.5];
  notificationView.alpha = 0.0;
  [UIView commitAnimations];

In the end the solution is quite simple: A custom view laid out in a NIB, added to the view hierarchy at the right location. But it definitely looks sexier than the built in red badge.

What do you think? Can you make this code better? Let us know in the comments!

Full Source code:

Tweet This!Hacker NewsShare on Facebook

Full Source code:


The Twitter iPhone App has a custom tab bar that is shorter than the standard tab bar, doesn’t have titles for the tab bar items and has a blue glow indicating when a section has new content. We want to recreate this custom tab bar.


Just like segmented controls, the best way to customize the tab bar is to build it from scratch. In fact we’re going to start by using a recipe similar to what we used for custom segment controls:

  • Create a button for every tab bar item.
  • Manage the touches on the buttons so when one is selected, the others are deselected.

But how do we recreate the look of the buttons and how about that nice background for the tab bar?

The tab bar background

Looking at the images of the Twitter app, we find the TabBarGradient.png image which is 22px, exactly half the 44px height of this custom tab bar.

Taking a screenshot of the Twitter app and looking at it carefully reveals how the background is built:

  • The top half is a stretchable image of TabBarGradient.png
  • The bottom half is simply solid black

The custom tab bar asks its delegate for the background image and here is how we build it:

// Get the image that will form the top of the background
UIImage* topImage = [UIImage imageNamed:@"TabBarGradient.png"];

// Create a new image context
UIGraphicsBeginImageContextWithOptions(CGSizeMake(width, topImage.size.height*2), NO, 0.0);

// Create a stretchable image for the top of the background and draw it
UIImage* stretchedTopImage = [topImage stretchableImageWithLeftCapWidth:0 topCapHeight:0];
[stretchedTopImage drawInRect:CGRectMake(0, 0, width, topImage.size.height)];

// Draw a solid black color for the bottom of the background
[[UIColor blackColor] set];
CGContextFillRect(UIGraphicsGetCurrentContext(), CGRectMake(0, topImage.size.height, width, topImage.size.height));

// Generate a new image
UIImage* resultImage = UIGraphicsGetImageFromCurrentImageContext();

return resultImage;

The buttons

The buttons have the following visual states:

  • Drawn in gray when unselected
  • Drawn with blue gradient when selected
  • Embossed border is drawn around the selected item

A button has both an image and a background image and they can both be set for the various control states. When a button is selected, the blue gradient appears to be on top and the embossed border is behind it. So here is how we’ll setup the button:

  • The button’s image for the normal state will be gray
  • The button’s image for the selected/highlighted state will be blue
  • The button’s background image for the selected/highlighted state will be the embossed border

The images for the tab bar items

A standard UITabBar only uses the alpha values of the tab bar item images. It doesn’t matter what color the images are, they will always appear in gray and blue. For our custom tab bar to be truly reusable, it will need to do the same thing.

But how exactly do we do this? It takes several steps:

  1. First we take the image and use CGContextClipToMask to generate a new image that has a white background and black content:
  2. Next we take this black and white image and use CGImageMaskCreate to create an image mask.
  3. Finally we combine the image mask with a background color.

For every tab bar item we generate two images: one with a solid gray background and another with a blue gradient background.

The blue glow

The blue glow is an image that is simply added to each button as a subview. In the Twitter app, a tab bar item will get a blue glow after the app has downloaded new content. It is a visual cue that there is more content in that section.

Our custom tab bar asks its delegate for the glowImage and it exposes a couple of methods to manage the glow: glowItemAtIndex and removeGlowAtIndex.

The current tab bar indicator

When a tab bar item is selected, a triangle at the top of the tab bar animates into place. We covered this animation in an earlier post. We use the code from that post to get the same animation for the custom tab bar.

Full Source code:

Tweet This!Hacker NewsShare on Facebook

Full Source code:


The Reeder iPhone App lets you pull up to see the title of the next article. If you pull up far enough the arrow rotates and the next article animates into view. We want to recreate this UI.


If you don’t pull up far enough, the article bounces back into view and that is a very strong clue that we are dealing with a UIScrollView.

A UIScrollView is used to display content that is larger than the application’s window. You tell it the contentSize of your content and it manages scrolling within the content. When you get to the edge of the content, the scroll view bounces to let you know you’ve reached the edge.

The header and footer views

Normally when you pull up at the edge of a scroll view empty space appears, but in the Reeder app, the title of the next article appears along with an arrow. To recreate this we’ll create a scroll view with a contentSize that is the same as the scroll view. Then we’ll tell the scroll view to alwaysBounceVertical. This causes a view that bounces vertically when you pull up or down.

Next we’ll add a header view as the subview of the scroll view and set it’s frame to be right above the scroll view and we’ll add a footer view as the subview of the scroll view and set it’s frame to be right below the scroll view. The header and footer are offscreen but when you pull up or down, they get pulled into view.

Subclassing UIScrollView

In addition to trying to figure out how to recreate the feature, we need to also figure out how to structure our code. The most reusable part of this feature is the ability to swipe up and down to see another article while seeing a preview of the previous/next article. We’ve already determined that we’ll be using a scroll view that we’ve customized so it seems logical that we would create a subclass of UIScrollView.

Animating the header and footer views

The arrows in the header and footer rotate to let the user know that when they lift their finger, the previous/next article will be shown. When the view has been scrolled past some distance we need to trigger this arrow rotation.

To accomplish this we will listen to the UIScrollViewDelegate’s scrollViewDidScroll message and check the scroll view’s contentOffset. This means that our UIScrollView subclass will have itself as its delegate. This sounds odd but works just fine.

Our subclass will send out 4 messages:

  • headerLoadedInScrollView
  • headerUnloadedInScrollView
  • footerLoadedInScrollView
  • footerUnloadedInScrollView

A header/footer is loaded when the user pulls down or up past the height of the header/footer. It is unloaded when they pull back and hide part of the header/footer. So with one arrow image, this is how we animate the arrow rotation:

- (void) rotateImageView:(UIImageView*)imageView angle:(CGFloat)angle
  [UIView beginAnimations:nil context:nil];
  [UIView setAnimationDuration:0.2];
  imageView.transform = CGAffineTransformMakeRotation(DegreesToRadians(angle));
  [UIView commitAnimations];
-(void) headerLoadedInScrollView:(VerticalSwipeScrollView*)scrollView
  [self rotateImageView:headerImageView angle:0];

-(void) headerUnloadedInScrollView:(VerticalSwipeScrollView*)scrollView
  [self rotateImageView:headerImageView angle:180];

-(void) footerLoadedInScrollView:(VerticalSwipeScrollView*)scrollView
  [self rotateImageView:footerImageView angle:180];

-(void) footerUnloadedInScrollView:(VerticalSwipeScrollView*)scrollView
  [self rotateImageView:footerImageView angle:0];

Animating the previous and next page

The UIScrollViewDelegate’s scrollViewDidEndDragging message lets us know when the user has lifted their finger after dragging. To animate the next page, we place the page below the footer and inside of an animation block place it on screen. This results in a nice up animation.

if (_footerLoaded) // If the footer is loaded, then the user wants to go to the next page
  // Ask the delegate for the next page
  UIView* nextPage = [externalDelegate viewForScrollView:self atPage:currentPageIndex+1];
  // We want to animate this new page coming up, so we first
  // Set its frame to the bottom of the scroll view
  nextPage.frame = CGRectMake(0, nextPage.frame.size.height + self.contentOffset.y, self.frame.size.width, self.frame.size.height);
  [self addSubview:nextPage];

  // Start the page up animation
  [UIView beginAnimations:nil context:nextPage];
  [UIView setAnimationDuration:0.2];
  [UIView setAnimationDelegate:self];
  [UIView setAnimationDidStopSelector:@selector(pageAnimationDidStop:finished:context:)];
  // When the animation is done, we want the next page to be front and center
  nextPage.frame = self.frame;
  // We also want the existing page to animate to the top of the scroll view
  currentPageView.frame = CGRectMake(0, -(self.frame.size.height + headerView.frame.size.height), self.frame.size.width, self.frame.size.height);
  // And we also animate the footer view to animate off the top of the screen
  footerView.frame = CGRectMake(0, -footerView.frame.size.height, footerView.frame.size.width, footerView.frame.size.height);
  [UIView commitAnimations];

  // Increment our current page

We also register a callback for when this animation is done and make sure our header and footer are in place for the next time the user pulls the scroll view up or down.

UIWebViews are unique

Our UIScrollView subclass calls the delegate’s viewForScrollView:atPage to get the actual pages. Life would be simple if we could return a static page like say an image, but in the real world it is more likely that you will be returning a UIWebView to accommodate things like titles that may wrap.

The sample app uses a JSON feed of the top paid apps in the App Store and uses a UIWebView to display each page.

No matter how simple the html that you are displaying in a UIWebView, the rendering will not be instantaneous and there will always be an overhead of setting up the UIWebView. If every time viewForScrollView:atPage is called you created a new  UIWebView with html, then as this page is getting animated into view, the rendering will not have completed. The net result will be that the scroll animation will show a blank white page instead of the actual content.

To deal with this the sample app keeps around a previousPage and nextPage UIWebViews. When asked for page 1, the sample preloads previousPage with page 0 and nextPage with page 2. If there are other caching techniques you think would work here, please share your thoughts in the comments.

Full Source code:

Tweet This!Hacker NewsShare on Facebook

Full Source code:


The Twitter iPhone App has a small arrow indicator above the tab bar that animates when a tab is selected. We want to recreate this animation.


The arrow is simply another image added on top of the tab bar which is animated every time the tab selection changes. So we have two tasks:

  • Add the arrow on top of the tab bar. This is similar to what we did in our last recipe.
  • Animate the arrow when a new tab is selected.

To add arrow on top of the tab, we have to figure out the proper horizontal and vertical locations.

Vertical Location

The vertical location is always the same so we’ll figure it out just once. To calculate the vertical location, we start at the bottom of the window, go up by height of the tab bar, go up again by the height of arrow and then come back down 2 pixels so the arrow is slightly on top of the tab bar:

CGFloat verticalLocation = self.window.frame.size.height - tabBarController.tabBar.frame.size.height - tabBarArrowImage.size.height + 2;

Horizontal Location

The horizontal location will change depending on which tab bar is currently selected. So we’ll write a method that given a tab index figures out the horizontal location.
There is nothing too complicated here: We divide the width of the tab bar by the number of items to calculate the width of a single item. We then multiply the index by the width of single item and add half the width of an item so the arrow lands in the middle:

- (CGFloat) horizontalLocationFor:(NSUInteger)tabIndex
  // A single tab item's width is the entire width of the tab bar divided by number of items
  CGFloat tabItemWidth = tabBarController.tabBar.frame.size.width / tabBarController.tabBar.items.count;
  // A half width is tabItemWidth divided by 2 minus half the width of the arrow
  CGFloat halfTabItemWidth = (tabItemWidth / 2.0) - (tabBarArrow.frame.size.width / 2.0);

  // The horizontal location is the index times the width plus a half width
  return (tabIndex * tabItemWidth) + halfTabItemWidth;

Add the arrow on top of the tab bar

On app startup we add the arrow on top of the selected tab. Our sample app doesn’t remember which tab you had selected before you quit, so we always start at index 0 ([self horizontalLocationFor:0]):

- (void) addTabBarArrow
  UIImage* tabBarArrowImage = [UIImage imageNamed:@"TabBarNipple.png"];
  self.tabBarArrow = [[[UIImageView alloc] initWithImage:tabBarArrowImage] autorelease];
  // To get the vertical location we start at the bottom of the window, go up by height of the tab bar, go up again by the height of arrow and then come back down 2 pixels so the arrow is slightly on top of the tab bar.
  CGFloat verticalLocation = self.window.frame.size.height - tabBarController.tabBar.frame.size.height - tabBarArrowImage.size.height + 2;
  tabBarArrow.frame = CGRectMake([self horizontalLocationFor:0], verticalLocation, tabBarArrowImage.size.width, tabBarArrowImage.size.height);

  [self.window addSubview:tabBarArrow];

Animate the arrow when a new tab is selected

A UITabBarController delegate gets notified every time a view controller was selected. We’ll use this as the trigger for starting the animation.

The actual animation is very simple. We use animation blocks available on every view.

If you haven’t used animation blocks before, here is a simple description:

  • Before you start the animation block set the frame of the item you want to animate to the start location.
  • Inside the animation block set the frame of the item you want to animate to the end location.

That’s all you have to do. The OS figures out the intermediate frames and does the actual animation for you. Doesn’t get simpler than that.

The arrow is already at the location we want it to animate from, so we don’t have to do anything before we start the animation block.

Inside the animation block block, all we have to do is set the final location of the arrow. So we take the existing frame of the arrow and change its horizontal location based on the newly selected tab index:

- (void)tabBarController:(UITabBarController *)theTabBarController didSelectViewController:(UIViewController *)viewController
  [UIView beginAnimations:nil context:nil];
  [UIView setAnimationDuration:0.2];
  CGRect frame = tabBarArrow.frame;
  frame.origin.x = [self horizontalLocationFor:tabBarController.selectedIndex];
  tabBarArrow.frame = frame;
  [UIView commitAnimations];

There a couple of things you can play around with to customize the animation:

  • The value you pass to setAnimationDuration will speed up or slow down the animation.
  • You can also set an animation curve. The default curve is UIViewAnimationCurveEaseInOut which causes the animation to start slowly, get faster in the middle and then slow before the animation is complete. Other curves like UIViewAnimationCurveEaseIn cause the animation to start slowly and then get faster until completion.

Full Source code:

Tweet This!Hacker NewsShare on Facebook