Getting Started with Blocks
Declaring and Using a Block
You use the ^ operator to declare a block variable and to indicate the beginning of a block literal. The body of the block itself is contained within{}
int multiplier = 7;
int (^myBlock)(int) = ^(int num) {
return num * multiplier;
};
int multiplier = 7;
int (^myBlock)(int) = ^(int num) {
return num * multiplier;
};
printf("%d", myBlock(3));
// prints "21"
Using a Block Directly
char *myCharacters[3] = { "TomJohn", "George", "Charles Condomine" };
qsort_b(myCharacters, 3, sizeof(char *), ^(const void *l, const void *r) {
char *left = *(char **)l;
char *right = *(char **)r;
return strncmp(left, right, 1);
});
// myCharacters is now { "Charles Condomine", "George", "TomJohn" }
Blocks with Cocoa
NSArray *stringsArray = [NSArray arrayWithObjects:
@"string 1",
@"String 21",
@"string 12",
@"String 11",
@"String 02", nil];
static NSStringCompareOptions comparisonOptions = NSCaseInsensitiveSearch | NSNumericSearch |
NSWidthInsensitiveSearch | NSForcedOrderingSearch;
NSLocale *currentLocale = [NSLocale currentLocale];
NSComparator finderSortBlock = ^(id string1, id string2) {
NSRange string1Range = NSMakeRange(0, [string1 length]);
return [string1 compare:string2 options:comparisonOptions range:string1Range locale:currentLocale];
};
NSArray *finderSortArray = [stringsArray sortedArrayUsingComparator:finderSortBlock];
NSLog(@"finderSortArray: %@", finderSortArray);
/*
Output:
finderSortArray: (
"string 1",
"String 02",
"String 11",
"string 12",
"String 21"
)
*/
__block Variables
A powerful feature of blocks is that they can modify variables in the same lexical scope. You signal that a block can modify a variable using the__block storage type modifier.
NSArray *stringsArray = [NSArray arrayWithObjects:
@"string 1",
@"String 21", // <-
@"string 12",
@"String 11",
@"Strîng 21", // <-
@"Striñg 21", // <-
@"String 02", nil];
NSLocale *currentLocale = [NSLocale currentLocale];
__block NSUInteger orderedSameCount = 0;
NSArray *diacriticInsensitiveSortArray = [stringsArray sortedArrayUsingComparator:^(id string1, id string2) {
NSRange string1Range = NSMakeRange(0, [string1 length]);
NSComparisonResult comparisonResult = [string1 compare:string2 options:NSDiacriticInsensitiveSearch range:string1Range locale:currentLocale];
if (comparisonResult == NSOrderedSame) {
orderedSameCount++;
}
return comparisonResult;
}];
NSLog(@"diacriticInsensitiveSortArray: %@", diacriticInsensitiveSortArray);
NSLog(@"orderedSameCount: %d", orderedSameCount);
/*
Output:
diacriticInsensitiveSortArray: (
"String 02",
"string 1",
"String 11",
"string 12",
"String 21",
"Str\U00eeng 21",
"Stri\U00f1g 21"
)
orderedSameCount: 2
*/
Conceptual
Block Functionality
A block is an anonymous inline collection of code that:
- Has a typed argument list just like a function
- Has an inferred or declared return type
- Can capture state from the lexical scope within which it is defined
- Can optionally modify the state of the lexical scope
- Can share the potential for modification with other blocks defined within the same lexical scope
- Can continue to share and modify state defined within the lexical scope (the stack frame) after the lexical scope (the stack frame) has been destroyed
You can copy a block and even pass it to other threads for deferred execution (or, within its own thread, to a runloop). The compiler and runtime arrange that all variables referenced from the block are preserved for the life of all copies of the block. Although blocks are available to pure C and C++, a block is also always an Objective-C object.
Declaring and Creating Blocks
Declaring a Block Reference
Block variables hold references to blocks. You declare them using syntax similar to that you use to declare a pointer to a function, except that you use^ instead of *.
void (^blockReturningVoidWithVoidArgument)(void);
int (^blockReturningIntWithIntAndCharArguments)(int, char);
void (^arrayOfTenBlocksReturningVoidWithIntArgument[10])(int);
Blocks also support variadic (...) arguments. A block that takes no arguments must specifyvoid in the argument list.
You can also create types for blocks—doing so is generally considered to be best practice when you use a block with a given signature in multiple places:
typedef float (^MyBlockType)(float, float);
MyBlockType myFirstBlock = // ... ;
MyBlockType mySecondBlock = // ... ;
Creating a Block
You use the ^ operator to indicate the beginning of a block literal expression. It may be followed by an argument list contained within(). The body of the block is contained within {}.
int (^oneFrom)(int);
oneFrom = ^(int anInt) {
return anInt - 1;
};
Global Blocks
#import <stdio.h>
int GlobalInt = 0;
int (^getGlobalInt)(void) = ^{ return GlobalInt; };
Blocks and Variables
Types of Variable
1. Within the block object’s body of code, variables may be treated in five different ways.
You can reference three standard types of variable, just as you would from a function:
- Global variables, including static locals
- Global functions (which aren’t technically variables)
- Local variables and parameters from an enclosing scope
2. Blocks also support two other types of variable:
- At function level are __block variables. These are mutable within the block (and the enclosing scope) and are preserved if any referencing block is copied to the heap.
- const imports.
3. Finally, within a method implementation, blocks may reference Objective-C instance variables
4.The following rules apply to variables used within a block:
- Global variables are accessible, including static variables that exist within the enclosing lexical scope.
- Parameters passed to the block are accessible.
- Stack (non-static) variables local to the enclosing lexical scope are captured asconst variables.
- Variables local to the enclosing lexical scope declared with the __block storage modifier are provided by reference and so are mutable.Any changes are reflected in the enclosing lexical scope, including any other blocks defined within the same enclosing lexical scope.
- Local variables declared within the lexical scope of the block, which behave exactly like local variables in a function. Each invocation of the block provides a new copy of that variable. These variables can in turn be used asconst or by-reference variables in blocks enclosed within the block.
int x = 123;
void (^printXAndY)(int) = ^(int y) {
printf("%d %d\n", x, y);
};
printXAndY(456); // prints: 123 456
As noted, trying to assign a new value to x within the block would result in an error. To allow a variable to be changed within a block, you use the __block storage type modifier.
int x = 123;
void (^printXAndY)(int) = ^(int y) {
x = x + y; // error
printf("%d %d\n", x, y);
};
The __block Storage Type
1. __block variables live in storage that is shared between the lexical scope of the variable and all blocks and block copies declared or created within the variable’s lexical scope. Thus, the storage will survive the destruction of the stack frame if any copies of the blocks declared within the frame survive beyond the end of the frame (for example, by being enqueued somewhere for later execution). Multiple blocks in a given lexical scope can simultaneously use a shared variable.
2. As an optimization, block storage starts out on the stack—just like blocks themselves do. If the block is copied usingBlock_copy (or in Objective-C when the block is sent a copy), variables are copied to the heap. Thus, the address of a__block variable can change over time.
3. There are two further restrictions on__block variables: they cannot be variable length arrays, and cannot be structures that contain C99 variable-length arrays.
__block int x = 123; // x lives in block storage
void (^printXAndY)(int) = ^(int y) {
x = x + y;
printf("%d %d\n", x, y);
};
printXAndY(456); // prints: 579 456
// x is now 579
extern NSInteger CounterGlobal;
static NSInteger CounterStatic;
{
NSInteger localCounter = 42;
__block char localCharacter;
void (^aBlock)(void) = ^(void) {
++CounterGlobal;
++CounterStatic;
CounterGlobal = localCounter; // localCounter fixed at block creation
localCharacter = 'a'; // sets localCharacter in enclosing scope
};
++localCounter; // unseen by the block
localCharacter = 'b';
aBlock(); // execute the block
// localCharacter now 'a'
}
Object and Block Variables
1. In a reference-counted environment, by default when you reference an Objective-C object within a block, it is retained. This is true even if you simply reference an instance variable of the object. Object variables marked with the __block storage type modifier, however, are not retained.
2. If you use a block within the implementation of a method, the rules for memory management of object instance variables are more subtle:
- If you access an instance variable by reference, self is retained;
- If you access an instance variable by value, the variable is retained.
dispatch_async(queue, ^{
// instanceVariable is used by reference, self is retained
doSomethingWithObject(instanceVariable);
});
id localVariable = instanceVariable;
dispatch_async(queue, ^{
// localVariable is used by value, localVariable is retained (not self)
doSomethingWithObject(localVariable);
});
3. When you copy a block, any references to other blocks from within that block are copied if necessary—an entire tree may be copied (from the top). If you have block variables and you reference a block from within the block, that block will be copied.
When you copy a stack-based block, you get a new block. If you copy a heap-based block, however, you simply increment the retain count of that block and get it back as the returned value of the copy function or method.
Invoking a Block
int (^oneFrom)(int) = ^(int anInt) {
return anInt - 1;
};
printf("1 from 10 is %d", oneFrom(10));
// Prints "1 from 10 is 9"
float (^distanceTraveled) (float, float, float) =
^(float startingSpeed, float acceleration, float time) {
float distance = (startingSpeed * time) + (0.5 * acceleration * time * time);
return distance;
};
float howFar = distanceTraveled(0.0, 9.8, 1.0);
// howFar = 4.9
Using a Block as a Function Argument
char *myCharacters[3] = { "TomJohn", "George", "Charles Condomine" };
qsort_b(myCharacters, 3, sizeof(char *), ^(const void *l, const void *r) {
char *left = *(char **)l;
char *right = *(char **)r;
return strncmp(left, right, 1);
});
// Block implementation ends at "}"
// myCharacters is now { "Charles Condomine", "George", "TomJohn" }
Using a Block as a Method Argument
NSArray *array = [NSArray arrayWithObjects: @"A", @"B", @"C", @"A", @"B", @"Z",@"G", @"are", @"Q", nil];
NSSet *filterSet = [NSSet setWithObjects: @"A", @"Z", @"Q", nil];
BOOL (^test)(id obj, NSUInteger idx, BOOL *stop);
test = ^ (id obj, NSUInteger idx, BOOL *stop) {
if (idx < 5) {
if ([filterSet containsObject: obj]) {
return YES;
}
}
return NO;
};
NSIndexSet *indexes = [array indexesOfObjectsPassingTest:test];
NSLog(@"indexes: %@", indexes);
/*
Output:
indexes: <NSIndexSet: 0x10236f0>[number of indexes: 2 (in 2 ranges), indexes: (0 3)]
*/
__block BOOL found = NO;
NSSet *aSet = [NSSet setWithObjects: @"Alpha", @"Beta", @"Gamma", @"X", nil];
NSString *string = @"gamma";
[aSet enumerateObjectsUsingBlock:^(id obj, BOOL *stop) {
if ([obj localizedCaseInsensitiveCompare:string] == NSOrderedSame) {
*stop = YES;
found = YES;
}
}];
// At this point, found == YES
Copying Blocks
Typically, you shouldn’t need to copy (or retain) a block. You only need to make a copy when you expect the block to be used after destruction of the scope within which it was declared. Copying moves a block to the heap.
If you are using Objective-C, you can send a block copy, retain, and release (and autorelease) messages.
To avoid a memory leak, you must always balance a Block_copy() with Block_release(). You must balance copy or retain with release (or autorelease)—unless in a garbage-collected environment.
Block_copy();
Block_release();
Patterns to Avoid
A block literal (that is, ^{ ... }) is the address of a stack-local data structure that represents the block. The scope of the stack-local data structure is therefore the enclosing compound statement, so you should avoid the patterns shown in the following examples:
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