高精度的Timer（Objetive C）

 

 

－、NSTimer定时精度

　　在实现一个坚实系统剪贴板变化当程序中，发现使用NSTimer精度达不到要求，类似当问题可以在stackoverflow中找到，如下：

 

经过查询文档之后，timer当触发是在runloop当循环中检查是否已经到达触发条件，如果没有到达，就在下一次循环中继续检查。因此NSTimer当精度受制与Runloop的循环时间，并且收到线程调度的影响，文档中说明是50～100ms，对于一般的应用可以满足。

 

二、更精确的Timer

　　在stackoverflow上面搜索如何获得更精确的timer之后，发现下面这种方式能解决一般问题，但是精度还是大于200ms，但是比NSTimer更稳定一些，当程序退到后台之后，仍旧能正常触发。

　　解决办法：

／／建立新线程
[NSThread detachNewThreadSelector:@selector(timer) toTarget:self withObject:nil]; //make a new thread


／／在线程中使用NSdate进行判断
- (void)timer {
    
    @autoreleasepool {
        NSDate *startDate = [NSDate date];
        while (YES) {
            usleep(10000);
            if([[NSDate date] timeIntervalSinceDate:startDate] >= watch_interval)
            {
                startDate = [NSDate date];
                objc_msgSend(self,@selector(watchLoop),nil);
            }
        }
        
    }
}

 　　这个方法仍然有弊端，可能是NSDate到更新间隔，导致200ms到间隔不能缩小。

 

三、更精确到定时器，如果想获得更精确到定时器，可以参考apple到这份文档，文档中不建议使用该定时器，除非特别需要。

High Precision Timers in iOS / OS X

 链接：https://developer.apple.com/library/ios/technotes/tn2169/_index.html

 

四、监视定时器变化到代码

.h 文件

//
//  PastboardWatcher.h
//  PastboardWatcher
//
//  Created by  on 14-3-20.
//  Copyright (c) 2014年 master. All rights reserved.
//

#import <Foundation/Foundation.h>

@protocol PastboardWatcherDelegate;

@interface PastboardWatcher : NSObject

+ (id)shanreInstance;

- (void)registerObsever:(id<PastboardWatcherDelegate>)delegate;

- (void)removeDelegate:(id<PastboardWatcherDelegate>)delegate;



@end



@protocol PastboardWatcherDelegate <NSObject>

- (void)generalPastboardDidChange;


@end

 

.m文件

//
//  PastboardWatcher.m
//  PastboardWatcher
//
//  Created by  on 14-3-20.
//  Copyright (c) 2014年 master. All rights reserved.
//
#import <objc/message.h>
#import "PastboardWatcher.h"


static const NSTimeInterval watch_interval = 0.2;



@interface PastboardWatcher ()

@property (nonatomic, strong) NSMutableArray *obseverArray;
@property (nonatomic, assign) NSInteger      changeCount;
@property (nonatomic, strong) NSRecursiveLock *lock;
@end



@implementation PastboardWatcher

+ (id)shanreInstance
{
    static PastboardWatcher *instance = nil;
    static dispatch_once_t once_token;
    dispatch_once(&once_token,^{
        instance = [[PastboardWatcher alloc] init];
                  });
    return instance;
}



- (id)init
{
    if(self = [super init])
    {
        _obseverArray = [[NSMutableArray alloc] init];
        _changeCount  = [[NSPasteboard generalPasteboard] changeCount];
        _lock         = [[NSRecursiveLock alloc] init];
        [NSThread detachNewThreadSelector:@selector(timer) toTarget:self withObject:nil]; //make a new thread
    }
    return self;
}


- (void)registerObsever:(id<PastboardWatcherDelegate>)delegate
{
    [_lock lock];
    [_obseverArray addObject:delegate];
    [_lock unlock];
}

- (void)removeDelegate:(id<PastboardWatcherDelegate>)delegate
{
    [_lock lock];
    NSInteger findIndex = [_obseverArray indexOfObject:delegate];
    if(findIndex != NSNotFound) [_obseverArray removeObjectAtIndex:findIndex];
    [_lock unlock];
}

- (void)timer {
    
    @autoreleasepool {
        NSDate *startDate = [NSDate date];
        while (YES) {
            usleep(10000);
            if([[NSDate date] timeIntervalSinceDate:startDate] >= watch_interval)
            {
                startDate = [NSDate date];
                objc_msgSend(self,@selector(watchLoop),nil);
            }
        }
        
    }
}


- (void)watchLoop
{
    NSInteger current_changeCount = [NSPasteboard generalPasteboard].changeCount;
    if(current_changeCount != _changeCount)
    {
        _changeCount = current_changeCount;
        objc_msgSend(self, @selector(notifyObseverChange),nil);
    }
}


- (void)notifyObseverChange
{
    [_lock lock];
    NSArray *tempArray = [_obseverArray copy];
    [_lock unlock];
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
        
        for (id<PastboardWatcherDelegate> delegate in tempArray) {
            if([delegate respondsToSelector:@selector(generalPastboardDidChange)])
            {
                objc_msgSend(delegate, @selector(generalPastboardDidChange));
            }
        }
        
    });
}


@end