Use grad Central Dispatch to simplify iPhone Development

Source: Internet
Author: User



GCD Overview

1. GCD included in libsystem. dylib

2. Available for allProgramUse.

-# Include <dispatch/dispatch. h>

3. gcd api provides block-based and function-based variants

-Currently, only block-based APIs are provided.

GCD Summary

1. Blocks

-Dispatch_async ()

2. Queues

-Lightweight list of Blocks

-Enqueue/dequeue is FIFO

3. dispatch_get_main_queue ()

-Main thread/main runloop

4. dispatch_queue_create ()

-Automatic helper thread

Benefits of GCD

1. Efficient-more CPU cycles available for your code

2. ease of use

-Blocks are easy to use

-Queues are inherently producer/consumer

3. Systemwide perspective

-Only the OS can balance unrelated subsystems


1. Existing threading and synchronization primitives are 100% compatible

2. GCD threads are wrapped POSIX Threads

-Do not cancel, exit, kill, join, or detach GCD threads

3. GCD reuses threads

-Restore any per-thread state changed within a block


Session 206 has mentioned this content.

Lock Resources

1. mutex access to key resources.

2. Access shared resources in sequence in the thread.

3. ensure data integrity

No GCDCode

-(Void) updateimagecachewithimg :( uiimage *) IMG {

Nslock * l = self. imagecachelock;

[L lock];

// Critical section

If ([self. imagecache containsobj: img]) {

[L unlock]; // don't forget to unlock



[Self. imagecache addobj: img];

[L unlock];


GCD code

-(Void) updateimagecachewithimg :( nsimage *) IMG {

Dispatch_queue_t queue = self. imagecachequeue;

Dispatch_sync (queue, ^ {// you can change dispatch_sync to dispatch_async to defer critical section

// Critical section

If ([self. imagecache containsobj: img]) {



[Self. imagecache addobj: img];



Inter-thread Communication

Use the following method (without gcd)

-Receivmselecdomainmainthread: withobject: waituntildone:

-Receivmselector: onthread: withobject: waituntildone:

-Receivmselector: withobject: afterdelay:

-Optional mselectorinbackground: withobject:

GCD code, equivalentPerformselector: onthread: withobject: waituntildone:

// Waituntildone: No

Dispatch_async (queue, ^ {

[Myobject dosomething: Foo withdata: bar];


// Waituntildone: Yes

Dispatch_sync (queue, ^ {

[Myobject dosomething: Foo withdata: bar];


GCD code, equivalent to javasmselector: withobject: afterdelay:Dispatch_time_t delay; delay = dispatch_time (dispatch_time_now, 50000/* 50 μs */); dispatch_after (delay, queue, ^ {[myobject dosomething: Foo withdata: bar];})The GCD code is equivalent to javasmselectorinbackground: withobject:Dispatch_queue_t queue = dispatch_get_global_queue (0, 0); dispatch_async (queue, ^ {[myobject dosomething: Foo withdata: bar];});
Global queues1. enqueue/dequeue is FIFO

2. Concurrent execution-non-FIFO completion order

3. dispatch_get_global_queue (priority, 0)

4. Global queues map GCD activity to real threads 5. Priority bands-dispatch_queue_priority_high-dispatch_queue_priority_default-dispatch_queue_priority_low
The following three sections describe a lot of content, but I have not understood it yet. If you are interested, watch the original video. -Dispatch sources
-Source cancellation-target queues
GCD object

GCD method Overview

Manage object Lifecycle

1. Ensure objects captured by blocks are valid when blocks are executed

-Objects must be retained and released around asynchronous operations

2. Objective-C objects captured by blocks are auto-retained and auto-released

3. Other Objects captured by blocks must be retained by your code

-Cfretain ()/cfrelease ()

-Dispatch_retain ()/dispatch_release ()

Pending and restoring

1. Suspend and resume only affects queues and sources you create

-Sources are created suspended

2. suspension is asynchronous

-Takes Effect Between Blocks

3. Your queues can predictably suspend objects that target them

Program Context

1. Applications can attach custom data to GCD objects

-Dispatch_set_context ()/dispatch_get_context ()

2. Optional finalizer callback

-Dispatch_set_finalizer_f ()

-Allows attached context to be freed with object

-Called on the target queue of the object

Wwdc2010 session211 simplifying iPhone app development with Grand Central Dispatch

Daniel Steffen-core OS

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