GCD 定时器基于分派队列，而不是像 N STimer 那样基于运行循环，这意味着把它们用在多个线程应用中要容易得多。GCD 定时器用块而不是选择器，所以不需要独立的方法处理定时器，这样也更容易避免重复 GCD 定时器的循环保留。
下面的这个 Demo 使用 GCD 实现了一个简单的定时器。
主要做到了一下几点：
Simple GCD-based timer based on NSTimer. It starts immediately and stops when released. This avoids many of the typical problems with NSTimer:
RNTimer runs in all modes (unlike NSTimer)
RNTimer runs when there is no runloop (unlike NSTimer)
Repeating RNTimers can easily avoid retain loops (unlike NSTimer)
RNTimer.h
// RNTimer
#import <Foundation/Foundation.h>
/** Simple GCD-based timer based on NSTimer.
Starts immediately and stops when deallocated. This avoids many of the typical problems with NSTimer:
* RNTimer runs in all modes (unlike NSTimer)
* RNTimer runs when there is no runloop (unlike NSTimer)
* Repeating RNTimers can easily avoid retain loops (unlike NSTimer)
*/
@interface RNTimer : NSObject
/**---------------------------------------------------------------------------------------
@name Creating a Timer
-----------------------------------------------------------------------------------------
*/
/** Creates and returns a new repeating RNTimer object and starts running it
After `seconds` seconds have elapsed, the timer fires, executing the block.
You will generally need to use a weakSelf pointer to avoid a retain loop.
The timer is attached to the main GCD queue.
@param seconds The number of seconds between firings of the timer. Must be greater than 0.
@param block Block to execute. Must be non-nil
@return A new RNTimer object, configured according to the specified parameters.
*/
+ (RNTimer *)repeatingTimerWithTimeInterval:(NSTimeInterval)seconds block:(dispatch_block_t)block;
/**---------------------------------------------------------------------------------------
@name Firing a Timer
-----------------------------------------------------------------------------------------
*/
/** Causes the block to be executed.
This does not modify the timer. It will still fire on schedule.
*/
- (void)fire;
/**---------------------------------------------------------------------------------------
@name Stopping a Timer
-----------------------------------------------------------------------------------------
*/
/** Stops the receiver from ever firing again
Once invalidated, a timer cannot be reused.
*/
- (void)invalidate;
@end
RNTimer.m
#import "RNTimer.h"
@interface RNTimer ()
@property (nonatomic, readwrite, copy) dispatch_block_t block;
@property (nonatomic, readwrite, assign) dispatch_source_t source;
@end
@implementation RNTimer
@synthesize block = _block;
@synthesize source = _source;
+ (RNTimer *)repeatingTimerWithTimeInterval:(NSTimeInterval)seconds
block:(void (^)(void))block {
NSParameterAssert(seconds);
NSParameterAssert(block);
RNTimer *timer = [[self alloc] init];
timer.block = block;
timer.source = dispatch_source_create(DISPATCH_SOURCE_TYPE_TIMER,
0, 0,
dispatch_get_main_queue());
uint64_t nsec = (uint64_t)(seconds * NSEC_PER_SEC);
dispatch_source_set_timer(timer.source,
dispatch_time(DISPATCH_TIME_NOW, nsec),
nsec, 0);
dispatch_source_set_event_handler(timer.source, block);
dispatch_resume(timer.source);
return timer;
}
- (void)invalidate {
if (self.source) {
dispatch_source_cancel(self.source);
dispatch_release(self.source);
self.source = nil;
}
self.block = nil;
}
- (void)dealloc {
[self invalidate];
}
- (void)fire {
self.block();
}
@end