iOS开发之-Concurrency Programming Guide - Dispatch Queues

Dispatch Queues

About Dispatch Queues

1. A task is simply some work that your application needs to perform. For example, you could define a task to perform some calculations, create or modify a data structure, process some data read from a file, or any number of things. You define tasks by placing the corresponding code inside either a function or a block object and adding it to a dispatch queue.

2. A dispatch queue is an object-like structure that manages the tasks you submit to it. All dispatch queues are first-in, first-out data structures. Thus, the tasks you add to a queue are always started in the same order that they were added. GCD provides some dispatch queues for you automatically, but others you can create for specific purposes.

3. When it comes to adding concurrency to an application, dispatch queues provide several advantages over threads. 

4. If you have two tasks that access the same shared resource but run on different threads, either thread could modify the resource first and you would need to use a lock to ensure that both tasks did not modify that resource at the same time. With dispatch queues, you could add both tasks to a serial dispatch queue to ensure that only one task modified the resource at any given time. 

5. Some other key points to remember about dispatch queues include the following:

• Dispatch queues execute their tasks concurrently with respect to other dispatch queues. The serialization of tasks is limited to the tasks in a single dispatch queue.
• The system determines the total number of tasks executing at any one time. 
• The system takes queue priority levels into account when choosing which new tasks to start. 
• Tasks in a queue must be ready to execute at the time they are added to the queue. 
• Private dispatch queues are reference-counted objects. In addition to retaining the queue in your own code, be aware that dispatch sources can also be attached to a queue and also increment its retain count. Thus, you must make sure that all dispatch sources are canceled and all retain calls are balanced with an appropriate release call. 

Queue-Related Technologies

Implementing Tasks Using Blocks

1. A block is actually represented by an underlying data structure that resembles an object and is created and managed for you by the compiler. The compiler packages up the code you provide (along with any related data) and encapsulates it in a form that can live in the heap and be passed around your application.

2. One of the key advantages of blocks is their ability to use variables from outside their own lexical scope. For example, a block can read the values of variables defined in the parent scope. Variables accessed by the block are copied to the block data structure on the heap so that the block can access them later. When blocks are added to a dispatch queue, these values must typically be left in a read-only format. However, blocks that are executed synchronously can also use variables that have the __block keyword prepended to return data back to the parent’s calling scope.