Cocoa Touch Tutorial: Using Grand Central Dispatch for Asynchronous Table View Cells
One of the problems that an iOS developer will often face is the performance of table view cells. Table view cells are loaded on-demand by the UITableView that they’re a part of; the system calls ‑cellForRowAtIndexPath: on the table view’s dataSource property to fetch a new cell in order to display it. Since this method is called (several times) while scrolling a table view, it needs to be very performant. You don’t have very much time to provide the system with a table view cell; take too long, and the application will appear to stutter to your users. This kills the immersion of your application and is an instant sign to users that the application is poorly-written. I guess what I’m saying is that this code needs to be fast. But what if something you need to do to display the table view cell takes a long time—say, loading an image?
In my MobiDevDay presentation a couple of weeks ago, I illustrated a solution to this problem: Grand Central Dispatch. GCD, Apple’s new multiprocessing API in Mac OS X Snow Leopard and iOS 4, is the perfect solution for this problem. Let’s take a look at how it works.
Grand Central Dispatch operates using queues. Queues are a C typedef: dispatch_queue_t. To get a new global queue, we call dispatch_get_global_queue(), which takes two arguments: a long for priority and an unsigned long for options, which is unused, so we’ll pass 0ul. Here’s how we get a high-priority queue:
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
It’s pretty straightforward. To use this queue, we add blocks of code onto it. Typically this is done with blocks (Apple’s new code encapsulation extension to the C language), though it can be done with C functions. To submit a block onto a queue for execution, use the functions dispatch_sync and dispatch_async. They both take a queue and a block as parameters. dispatch_async returns immediately, running the block asynchronously, while dispatch_sync blocks execution until the provided block returns (though you cannot use its return value). Here’s how we schedule some code onto a queue (we’ll assume this code runs after our previous example, so queue is already defined):
dispatch_async(queue, ^{
NSLog(@"Hello, World!");
});
It’s very easy to forget the ); at the end of that line, so be careful.
How does this apply to table view cells? Let’s take a look at a typical scenario for loading images from disk:
- (UITableViewCell *)tableView:(UITableView *)tableView
cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"ExampleCell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
reuseIdentifier:CellIdentifier] autorelease];
}
// Get the filename to load.
NSString *imageFilename = [imageArray objectAtIndex:[indexPath row]];
NSString *imagePath = [imageFolder stringByAppendingPathComponent:imageFilename];
[[cell textLabel] setText:imageFilename];
UIImage *image = [UIImage imageWithContentsOfFile:imagePath];
[[cell imageView] setImage:image];
return cell;
}
The problem with that code is that creating image blocks until ‑imageWithContentsOfFile: returns. If the images are especially large, this is catastrophic. Modifying this code to use Grand Central Dispatch is simple:
- (UITableViewCell *)tableView:(UITableView *)tableView
cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = @"Cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault
reuseIdentifier:CellIdentifier] autorelease];
}
// Get the filename to load.
NSString *imageFilename = [imageArray objectAtIndex:[indexPath row]];
NSString *imagePath = [imageFolder stringByAppendingPathComponent:imageFilename];
[[cell textLabel] setText:imageFilename];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0ul);
dispatch_async(queue, ^{
UIImage *image = [UIImage imageWithContentsOfFile:imagePath];
dispatch_sync(dispatch_get_main_queue(), ^{
[[cell imageView] setImage:image];
[cell setNeedsLayout];
});
});
return cell;
}
First, we create our image asynchronously by using dispatch_async(). Once we have it, however, we have to come back to the main thread in order to update our table view cell’s UI (all UI updates should be on the main thread, unless you like reading crash reports). GCD has a function to get the main queue—analogous to the main thread—called dispatch_get_main_queue(). We can dispatch a block to that thread to update the UI.
By making this simple modification, we can very easily improve the performance of our table view. There are a few steps remaining, however, and this method has one serious shortcoming: if the cell is re-used by the time the image loads, it can load the wrong image into the cell. To get around this, it would be better to cache the images in an array or a dictionary (just be sure to release it in your view controller’s ‑didReceiveMemoryWarning: method). That said, this is an example of something you can do quite easily to improve the performance of your application. The better it performs, the more your users will like it, and that’s the ultimate goal.
The code used in this post is available as a GitHub repository.
Cocoa Touch Tutorial: Stripping Non-Alphanumeric Characters on Entry in a UITextField
In a previous post, I showed you how to trim non-alphanumeric characters from a string. Here I’ll go more in-depth and show a method that I wrote to restrict text entry in a UITextField to alphanumeric characters. Since I also wanted the characters to be uppercase, I’ll also ensure that only uppercase characters are allowed.
This should all happen in the - (BOOL)textField:(UITextField *)textField shouldChangeCharactersInRange:(NSRange)range replacementString:(NSString *)string method of your UITextField’s delegate (which, of course, must implement the UITextFieldDelegate protocol). I’ve implemented it as follows:
- ( BOOL )textField:( UITextField * )textField
shouldChangeCharactersInRange:( NSRange )range
replacementString:( NSString * )string
{
/*
* We only want uppercase letters and numbers in this text field, so if
* this method is adding something else, we don't want it. But we also
* want to support copy-and-paste, so it's not always going to be one
* character added.
*/
BOOL shouldAllowChange = YES;
The shouldAllowChange variable is set to YES initially because we want to allow this change when possible. The method will test the string to see if it meets criteria for rejection as we move forward.
NSMutableString *newReplacement =
[[ NSMutableString alloc ] initWithString:[ string uppercaseString ]];
if ( ! [ string isEqualToString:newReplacement ]) {
shouldAllowChange = NO;
}
First, we define newReplacement. It’s an NSMutableString so that if we discover non-alphanumeric characters in it, we can remove them on-the-fly. It also serves as a convenient string against which we can test to see if string is already uppercase.
NSCharacterSet *desiredCharacters =
[ NSCharacterSet alphanumericCharacterSet ];
for ( NSUInteger i = 0; i < [ newReplacement length ]; i++ ) {
unichar currentCharacter = [ newReplacement characterAtIndex:i ];
if ( ! [ desiredCharacters characterIsMember:currentCharacter ]) {
shouldAllowChange = NO;
[ newReplacement deleteCharactersInRange:NSMakeRange( i, 1 )];
i--;
}
}
In this section, we define the NSCharacterSet that we want to work with - in this case, the alphanumeric character set. We go through one character by a time and if the current character isn’t alphanumeric, we remove it from the NSMutableString (decrementing i so that we don’t inadvertently skip a character) and set our shouldAllowChange flag accordingly.
if ( shouldAllowChange ) {
[ newReplacement release ];
return YES;
} else {
[ textField setText:[[ textField text ]
stringByReplacingCharactersInRange:range
withString:newReplacement ]];
[ newReplacement release ];
return NO;
}
}
To finish, if shouldAllowChange is still true, we return YES and allow the replacement characters to be added. Otherwise, we return NO, but not before using our replacement replacement string (say that ten times fast) to manually edit the text field’s text. The end result is a text field that will consist only of uppercase letters and numbers.
Cocoa Tutorial: Strip Non-Alphanumeric Characters from an NSString
Let’s say you have an NSString that contains both alphanumeric and non-alphanumeric characters and you want to strip the non-alphanumeric characters out of it. The hard way is to manually go through, character-by-character, and put the character in a new string if it matches certain criteria. But why do it the hard way?
Apple provides a class that we can use for this to great effect: NSCharacterSet. We want alphanumeric characters, so we can create a character set of the characters we want using this method:
NSCharacterSet *alphanumericSet = [ NSCharacterSet alphanumericCharacterSet ];
Now we have a character set like we want. We just need a way to turn our string into a string that contains only those characters. Unfortunately, the closest thing in NSString’s implementation is the -stringByTrimmingCharactersInSet: method. But that seems to do the opposite of what we want. Fortunately NSCharacterSet has our back here. We can use the -invertedSet method. So here is our final code:
NSString *beginningString = @"Some string with non-alphanumeric characters. !@#$%^&*()";
NSCharacterSet *nonalphanumericSet = [[ NSCharacterSet alphanumericCharacterSet ] invertedSet ];
NSString *endingString = [ beginningString stringByTrimmingCharactersInSet:nonalphanumericSet ];
In this example, endingString will be equal to “Somestringwithnonalphanumericcharacters”.
UPDATE: As it turns out, this only works if the non-alphanumeric characters are at the beginning or end of the NSString. Whoops.
Cocoa Touch Tutorial: Extract Address Book Address Values on iPhone OS
This is the first of what I hope to be several Cocoa Touch tutorials on this site. I was doing some furious Googling last night trying to find out how to get a contact’s street address from the Address Book for an upcoming update to Take Me Home, and I realized that it’s complicated and there aren’t any good tutorials online. So, after I figured it out, I commented it up so that hopefully, if you’re reading this, you’ll save some time that I didn’t.
Before you read this tutorial, you should go through Apple’s excellent Address Book Programming Guide for iPhone OS. This tutorial will rely on the QuickStart application you write in the guide, so do that first.
The first thing we need to do is add an address field to the QuickStart application. Use Interface Builder to add a new UILabel underneath the two you already have. You may want to stretch it to fill the entire width of the screen, like so:
Add a new UILabel underneath the exisiting two.
Now, add the information about this label to QuickStartViewController.h:
// // QuickStartViewController.h // QuickStart // #import <UIKit/UIKit.h> #import <AddressBook/AddressBook.h> #import <AddressBookUI/AddressBookUI.h> @interface QuickStartViewController : UIViewController <ABPeoplePickerNavigationControllerDelegate> { IBOutlet UILabel *firstName; IBOutlet UILabel *lastName; IBOutlet UILabel *addressLabel; } @property (nonatomic, retain) UILabel *firstName; @property (nonatomic, retain) UILabel *lastName; @property (nonatomic, retain) UILabel *addressLabel; - (IBAction)showPicker:(id)sender; @end
Be sure to go back into Interface Builder and connect File’s Owner in QuickStartViewController.xib to addressLabel.
Now, we have to change the method that gets called when you click on a person in the ABPeoplePicker. As it is at the end of the QuickStart tutorial, once you select a person the picker is dismissed. So, we do the following in QuickStartViewController.m:
- (BOOL)peoplePickerNavigationController:(ABPeoplePickerNavigationController *)peoplePicker shouldContinueAfterSelectingPerson:(ABRecordRef)person { NSString *name = (NSString *)ABRecordCopyValue(person, kABPersonFirstNameProperty); self.firstName.text = name; [name release]; name = (NSString *)ABRecordCopyValue(person, kABPersonLastNameProperty); self.lastName.text = name; [name release];[self dissmissModalViewControllerAnimated:YES];return YES; }
Note that you have to delete the line that dismisses the modal view controller; if you don’t, the people picker is dismissed before you have a chance to get the address. When you delete it, the people picker will continue when you select a person. Next up, we have to write the method for when someone selects an address on the next screen. Here’s the method:
- (BOOL)peoplePickerNavigationController:(ABPeoplePickerNavigationController *)peoplePicker shouldContinueAfterSelectingPerson:(ABRecordRef)person property:(ABPropertyID)property identifier:(ABMultiValueIdentifier)identifier { // Only inspect the value if it's an address. if (property == kABPersonAddressProperty) { /* * Set up an ABMultiValue to hold the address values; copy from address * book record. */ ABMultiValueRef multi = ABRecordCopyValue(person, property); // Set up an NSArray and copy the values in. NSArray *theArray = [(id)ABMultiValueCopyArrayOfAllValues(multi) autorelease]; // Figure out which values we want and store the index. const NSUInteger theIndex = ABMultiValueGetIndexForIdentifier(multi, identifier); // Set up an NSDictionary to hold the contents of the array. NSDictionary *theDict = [theArray objectAtIndex:theIndex]; // Set up NSStrings to hold keys and values. First, how many are there? const NSUInteger theCount = [theDict count]; NSString *keys[theCount]; NSString *values[theCount]; // Get the keys and values from the CFDictionary. Note that because // we're using the "GetKeysAndValues" function, you don't need to // release keys or values. It's the "Get Rule" and only applies to // CoreFoundation objects. [theDict getObjects:values andKeys:keys]; // Set the address label's text. NSString *address; address = [NSString stringWithFormat:@"%@, %@, %@, %@ %@", [theDict objectForKey:(NSString *)kABPersonAddressStreetKey], [theDict objectForKey:(NSString *)kABPersonAddressCityKey], [theDict objectForKey:(NSString *)kABPersonAddressStateKey], [theDict objectForKey:(NSString *)kABPersonAddressZIPKey], [theDict objectForKey:(NSString *)kABPersonAddressCountryKey]]; self.addressLabel.text = address; // Return to the main view controller. [ self dismissModalViewControllerAnimated:YES ]; return NO; } // If they didn't pick an address, return YES here to keep going. return YES; }
Let’s go through that in more detail. The method gives us the following information: an ABRecordRef of the person we’ve selected, an ABPropertyID of the property slected (in this case, we ensure that it’s the address) and an ABMultiValueIdentifier of which address we’ve selected. It is important to note that the ABPropertyID is equal to kABPersonAddressProperty when you select any address; that is, there is only one address property. This one address property holds the values in an ABMultiValue, each at a specific index. Here are the steps we take in the code:
- The first thing we do is define our ABMultiValue, multi, and copy the contents of the selected value into it.
- Then we define an NSArray, theArray, into which to copy the multiple values. But which one do we want?
- Each address has an identifier, which the method gives to us as identifier, but we reference them by index when getting them out of the array. So, we need to create an index (which we’ll store as an unsigned integer), theIndex, and set it to the return value of the
ABMultiValueGetIndexForIdentifierfunction. Now that we have the index, we know which value of the array to store . They’re stored as type CFDictionary, which have key-value pairs for us to use, so we define an NSDictionary, theDict to put them into. - First, we need to know how many key-value pairs there are, so we use the
countmethod and store the return value in an unsigned integer, theCount. Be sure that this variable doesn&rquo;t change—you don’t want to assume that there are more members in the array than there actually are, as that can lead to nasty memory problems. For that reason I’ve defined it as a constant. - Now, we define two NSString arrays, keys[theCount] and values[theCount], and then we’re ready for action.
- Next we use the NSDictionary
getObjects: andKeys:function to copy the keys and values. The function copies the data, and we can construct our street address. For the purpose of this example, I’m going to make the address a single line, but you do with it what you want. - Finally, we create a final NSString to put the formatted address into, pull the values out of the dictionary into the appropriate place, and we’re all done!
Update 2010-07-27: Removed [ theDict release ];, some bad memory management.