一 概述
使用dispatch_once函数能保证某段代码在程序运行过程中只被执行1次
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| static dispatch_oncce_t onceToken;
dispatch_once(&onceToken,^{
//只执行1次的代码(这里面默认是线程安全的)
});
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二 示例
2.1 示例一(变量控制)
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| #import "ViewController.h"
@interface ViewController ()
@property(nonatomic,assign) BOOL hasExecuted;
@end
@implementation ViewController
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
if(self.hasExecuted)return;
NSLog(@"下载图片---------");
self.hasExecuted=YES;
}
@end
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2.2 示例2(下载工具类)
下载工具类(ILImageDownloader)
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| #import "ILImageDownloader.h"
@interface ILImageDownloader()
@property(nonatomic,assign) BOOL hasExecuted;
@end
@implementation ILImageDownloader
-(void)download
{
if (self.hasExecuted) return;
NSLog(@"下载图片-----");
self.hasExecuted=YES;
}
@end
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ViewController
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| #import "ViewController.h"
#import "ILImageDownloader.h"
@interface ViewController ()
@property(nonatomic,strong) ILImageDownloader *downloader;
@end
@implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
self.downloader=[[ILImageDownloader alloc]init];
}
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
[self.downloader download];
}
@end
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2.3 示例三(dispatch_once)
下载工具类(ILImageDownloader)
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| #import "ILImageDownloader.h"
@implementation ILImageDownloader
-(void)download
{
NSLog(@"下载图片-----");
}
@end
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ViewController
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#import "ILImageDownloader.h"
@interface ViewController ()
@property(nonatomic,strong) ILImageDownloader *downloader;
@end
@implementation ViewController
-(void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event
{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
ILImageDownloader *downloader=[[ILImageDownloader alloc]init];
[downloader download];
});
}
@end
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