Monday, 10 October 2011

Using Blocks in Objective-C


iOS 4 introduces one new feature that will fundamentally change the way you program in general: blocks. Blocks are an extension to the C language and thus fully supported in Objective-C. If you’re coming from a programming language such as Ruby, Python, or Lisp, then you know the power of blocks. Simply put, blocks let you encapsulate chunks of code and pass them around like any other object. It’s a different style of programming that you’ll want to become familiar with to take advantage of new APIs in iOS 4.

Let’s start by taking a look at two examples of where you might use blocks in iOS 4: view animations and enumeration.

Blocks By Example

As our first example, suppose we’re creating a card game and we want to animate sliding a card from the dealer’s hand to a player’s position. Fortunately, the UIKit framework does all the heavy lifting when it comes to performing animations. What gets animated, however, is specific to your application. You specify what will be animated in a block, and toss it over to theanimateWithDuration:animations: method, like so:
[UIView animateWithDuration:2.0
    animations:^ {
        self.cardView.alpha = 1.0;
        self.cardView.frame = CGRectMake(176.0, 258.0, 72.0, 96.0);
        self.cardView.transform = CGAffineTransformMakeRotation(M_PI);
When this animation block is run, our card view will animate in three ways: change its alpha to fade in the card, change its position to the lower-right of the frame (the player’s position), and rotate itself 180 degrees (to give the dealer style points).

Our second example of blocks is to enumerate over a collection of cards and print the name and index of each card. You could use a for loop for this, but in iOS 4 the NSArray class has a handy enumerateObjectsUsingBlock: method that takes a block. Here’s how to use it:
NSArray *cards = [NSArray arrayWithObjects:@"Jack", @"Queen", @"King", @"Ace", nil];
[cards enumerateObjectsUsingBlock:^(id object, NSUInteger index, BOOL *stop) {
    NSLog(@"%@ card at index %d", object, index);
As we’ll explore a bit more later, this block takes three parameters: the current element in the array, its index, and a flag to signal whether enumeration should stop (which we’ve ignored). The enumerateObjectsUsingBlock: method calls the block once for each element in the array and supplies the parameters.

So the upshot of using blocks in your Mac and iOS apps is that they allow you to attach arbitrary code to Apple-provided methods. Although similar in concept to delegation, passing short inline blocks of code to methods is often more convenient and elegant.

That’s a good start, but it’s important to understand what’s going on. Whenever I’m learning anything new, I like to break it down into its simplest elements, get comfortable with how they work, and then (hopefully) put everything back together again. That way I feel confident with the code I write and can quickly debug any problems. So let’s step back for a minute and learn how to declare and call basic blocks.

Block Basics

A block is simply a chunk of executable code. For example, here’s a block that prints the current date and time:
^ {
    NSDate *date = [NSDate date];
    NSLog(@"The date and time is %@", date);
The caret (^) introduces a block literal and the curly braces enclose statements that make up the body of the block. You can think of a block as being similar to an anonymous function.

So if it’s anonymous, how exactly do we use this block? The most common way to use a block is to pass it to a method that in turn calls the block. We saw how to do that earlier with animations and enumeration. The other way to use a block is to assign it to a block variable and call the block directly. Here’s how to assign our block to a block variable called now:
void (^now)(void) = ^ {
    NSDate *date = [NSDate date];
    NSLog(@"The date and time is %@", date);
Here’s where things get funky. The syntax for declaring a block variable takes some getting used to. If you’ve used function pointers, block variables will look familiar. On the right-hand side of the assignment we have our block literal (nothing new there). On the left-hand side of the assignment we’ve declared a block variable called now.

Block Syntax

The name of the block variable is always preceded by a ^ and in parentheses. Block variables have an associated type. In this case, the nowvariable can reference any block that returns no value (the first void) and takes no parameters (the void in parentheses). Our block conforms to this type, so we can safely assign it to the now variable.

Once we have a block variable in scope, calling the block is just like calling a function. Here’s how we call our block:
You could declare the block variable in a C function or Objective-C method, for example, and then call it in the same scope. When the block executes, it prints the current date and time. So far, so good.

Blocks Are Closures

If that’s all there was to blocks, they’d be just like functions. But it turns out that blocks are more than just chunks of executable code. Blocks also capture their surrounding state. That is, blocks are closures: they close around variables that are in scope at the time the block is declared. To illustrate, let’s change the previous example around a bit by moving the initialization of the date outside the block:
NSDate *date = [NSDate date];
void (^now)(void) = ^ {
    NSLog(@"The date and time is %@", date);
When you call this block the first time, it behaves exactly like the previous version: it prints the current date and time. But there’s a significant difference here. It becomes evident when we change the date and then call the block again:
date = [NSDate date];
Even though we’ve changed the date variable referenced by the block, when the block is called it still prints the original date and time. It’s as if time stood still when the block was declared. And that’s effectively what happens. As execution passes over the point where the block is declared, the block takes a (read-only) snapshot of all the variables in scope that the block uses. You can think of the value of the datevariable as being frozen inside the block. Therefore, whenever the block is called—immediately, 5 seconds later, or just before the app quits—it always prints the original date and time.

Now, the fact that blocks are closures is not particularly interesting in this example. After all, you could have just passed the date as a parameter to the block (more on that next). But closures become really useful when you start passing blocks around to methods because the captured state goes along for the ride.

Block Parameters

Just like functions, blocks can take parameters and return values. Say, for example, we want a block that takes a given number and returns the result of tripling that number. Here’s the block literal:
^(int number) {
    return number * 3;
Assigning this block to a block variable called triple looks like this:
int (^triple)(int) = ^(int number) {
    return number * 3;
Again, the tricky part is getting comfortable with the block variable syntax on the left-hand side of the assignment. Let’s break it down from left to right.

Block Syntax

The first int is the return type. Then in parentheses comes the caret introducing the block variable called triple. Finally we have a list of parameter types in parentheses (one int in this case). The block literal on the right-hand side of the assignment conforms to this type. Note, however, that as a matter of convenience there’s no need to declare the return type of the block literal. The compiler can infer it from the return statement.

To call the block, you need to pass the number to be tripled and (ideally) do something with the return value, like so:
int result = triple(2);
By way of comparison, here’s how you would declare and create a block that takes two int parameters, multiplies them together, and returns the result as an int value:
int (^multiply)(int, int) = ^(int x, int y) {
    return x * y;
And here’s how you’d call this block:
int result = multiply(2, 3);
Declaring block variables gave us an opportunity to explore block types and how to call blocks. The block variable looks like a function pointer and calling the block is similar to calling a function. But unlike function pointers, blocks are actually Objective-C objects. And that means we can pass them around like other objects.

Methods Can Take Blocks

Now, in practice blocks are most useful when you pass them as parameters to methods that in turn call the block. And when you’re passing a block to a method, it’s usually more convenient to use inline blocks rather than assigning the block to a typed variable and then passing it to the method. For instance, we used inline blocks in the animation and enumeration examples we saw earlier.

Apple has added methods to their frameworks that take blocks, and you can write APIs that take blocks, too. For example, suppose we want to create a Worker class method that takes a block and repeatedly calls it a given number of times, passing in the repeat count each time. Here’s how we might call that method with an inline block that triples each number (1 through 10):
[Worker repeat:10 withBlock:^(int number) {
    return number * 3;
The method could handle any block that takes a single int parameter and returns an int result. Want to double all the numbers? Just give the method a different block.

Your Turn

OK, so how would you implement the repeat:withBlock: method above to accept and call a passed block? Give it some thought, and we’ll tackle it in the next installment. In the meantime, practice using blocks by calling the enumerateKeysAndObjectsUsingBlock: method with a block that prints the keys and values of this NSDictionary:
NSDictionary *cards =
    [NSDictionary dictionaryWithObjectsAndKeys:@"Queen"@"card",
                                               @"Hearts", @"suit",
                                               @"10",     @"value", nil];
Have fun.

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