Blocks和Grand Central Dispatch已经被iOS4支持,（Snow Leopard早就支持了）.
程序运行架构
Blocks
Blacks are available in C++ and Objective-C++.
Basic Blocks
我们在block中使用这个字符^ 因为这个字符很特别，在C++中不能作为操作符重载.
Block 词法
^ [返回类型][参数] { 内容 }
看起来就像一个没有名字的函数，前面加上^. 如果没有返回或者没有参数，可以把void省略。
Blocks as Data
定义Block pointer. 看起来像函数pointer: void (*callable)(void);
void (^callable)(void);
下面这是一个复杂的block pointer，它的参数也是一个block pointer.
char *(^worker)(char *a, BOOL(^done)(int));
可以使用typedef来简化：
typedef BOOL (^doneBlk_t)(int);
char *(^workB)(char *a, doneBlk_t d);
Blocks实践
Work Block Consumer
定义一个repeat函数，参数一个block pointer:
typedef void (^workBlk_t)(int i);
void repeat(int n, workBlk_t aBlock) {
for (int i = 0; i < n; ++i)
aBlock(i);
}
定义一个 block然后调用repeat函数:
int d = 2;
workBlk_t w = ^(int i) {
printf(“%d\n”, i * d);
};
repeat(5, w);
输出：
0
2
4
6
8
同步执行
[mySet objectsPassingTest: ^(id obj, BOOL *stop) {
return [obj testValue: value];
}];
[aDictionary enumerateKeysAndObjectsUsingBlock: ^(id k, id v, BOOL *stop) {
NSLog(@”%@ => %@”, k, v);
}];
Callbacks回调
有了block，我们可以直接定义callbacks，而不用去定义一个函数。这样会让代码简单点。
[application beginBackgroundTaskWithExpirationHandler: ^{
/* expiration handler callback code */
}];
[anOperation setCompletionBlock: ^{
/* handle operation completion */
}];
异步执行
[operationQueue addOperationWithBlock: ^{
/* hard work is hard */
}];
dispatch_async(main_queue, ^{
[viewController displayNewStuff];
}];
Lockless exclusion
// thread a
dispatch_async(queue, ^{ … });
// thread b
dispatch_async(queue, ^{ … });
// main thread
dispatch_async(queue, ^{ … });
Blocks详情
Block Object Details
1. Blocks（竟然）是Objective-C的一个对象
2. Block生成与栈上 (因为在栈上生成会很快)
- 可以用[aBlock copy] or Block_copy()复制到heap中 (相对代价大)
- 释放[aBlock release] or Block_release() (必须手动释放heap中的block)
3. Blocks have private const copy of stack (auto) variables
- Object references are retained.
4. Mutable variables must be declared with __block keyword
- Shared with all other blocks that use that variable
- Shared with the scope of declaration
- __block Object references are not retained
Block lifetime illustrated
1. Start on stack and capture state
2. Copies state to heap
3. block1 and block2/3 could finish first. No sequence.