一、延迟执行
1.performSelector方法
// 3秒后自动回到当前线程调用self的download:方法，并且传递参数：@"http://555.jpg"
[self performSelector:@selector(download:) withObject:@"http://555.jpg" afterDelay:3];
2.dispatch_after函数
// 任务放到哪个队列中执行
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
double delay = 3; // 延迟多少秒
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delay * NSEC_PER_SEC)), queue, ^{
// 3秒后需要执行的任务
});
来看具体代码：
- (void)delay3
{
//3秒后回到主线程执行block中的代码
//dispatch_queue_t queue = dispatch_get_main_queue();
//dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), queue, ^{
//NSLog(@"------task------%@", [NSThread currentThread]);
//});
// 3秒后自动开启新线程 执行block中的代码
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), queue, ^{
NSLog(@"------task------%@", [NSThread currentThread]);
});
}
- (void)delay2
{
// 一旦定制好延迟任务后，不会卡主当前线程
[self performSelector:@selector(download:) withObject:@"http://555.jpg" afterDelay:3];
}
- (void)delay1
{
// 延迟执行不要用sleep，坏处：卡住当前线程
[NSThread sleepForTimeInterval:3];
NSLog(@"-----下载图片-----");
}
二、一次性代码
1.实现一次性代码
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
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
// 这里面的代码，在程序运行过程中，永远只会执行1次
});
代码示例：
ImageDownloader.h文件
@interface ImageDownloader : NSObject
- (void)download;
@end
ImageDownloader.m文件
@interface HMImageDownloader()
@end
@implementation ImageDownloader
- (void)download
{
NSLog(@"下载图片-------");
}
@end
ViewController.m文件
#import "HMImageDownloader.h"
@interface HMViewController ()
@end
@implementation HMViewController
- (void)viewDidLoad
{
[super viewDidLoad];
}
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
NSLog(@"----touchesBegan");
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
NSLog(@"----once");
HMImageDownloader *downloader = [[HMImageDownloader alloc] init];
[downloader download];
});
}
@end
三、队列组
创建一个队列组，开启一个任务下载图片1，再开启一个任务下载图片2，同时执行下载图片1\下载图片2操作，等group中的所有任务都执行完毕, 再回到主线程执行其他操作
代码示例：
// 1.分别下载2张图片：大图片、LOGO
// 2.合并2张图片
// 3.显示到一个imageView身上
@interface HMViewController ()
@property (weak, nonatomic) IBOutlet UIImageView *imageView;
@property (nonatomic, strong) UIImage *image1;
@property (nonatomic, strong) UIImage *image2;
@end
@implementation HMViewController
- (void)viewDidLoad
{
[super viewDidLoad];
}
// 2D绘图 Quartz2D
// 合并图片 -- 水印
- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
// 1.队列组
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
// 2.下载图片1
__block UIImage *image1 = nil;
dispatch_group_async(group, queue, ^{
NSURL *url1 = [NSURL URLWithString:@"http://g.hiphotos.baidu.com/image/pic/item/f2deb48f8c5494ee460de6182ff5e0fe99257e80.jpg"];
NSData *data1 = [NSData dataWithContentsOfURL:url1];
image1 = [UIImage imageWithData:data1];
});
// 3.下载图片2
__block UIImage *image2 = nil;
dispatch_group_async(group, queue, ^{
NSURL *url2 = [NSURL URLWithString:@"http://su.bdimg.com/static/superplus/img/logo_white_ee663702.png"];
NSData *data2 = [NSData dataWithContentsOfURL:url2];
image2 = [UIImage imageWithData:data2];
});
// 4.合并图片 (保证执行完组里面的所有任务之后，再执行notify函数里面的block)
dispatch_group_notify(group, queue, ^{
// 开启一个位图上下文
UIGraphicsBeginImageContextWithOptions(image1.size, NO, 0.0);
// 绘制第1张图片
CGFloat image1W = image1.size.width;
CGFloat image1H = image1.size.height;
[image1 drawInRect:CGRectMake(0, 0, image1W, image1H)];
// 绘制第2张图片
CGFloat image2W = image2.size.width * 0.5;
CGFloat image2H = image2.size.height * 0.5;
CGFloat image2Y = image1H - image2H;
[image2 drawInRect:CGRectMake(0, image2Y, image2W, image2H)];
// 得到上下文中的图片
UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();
// 结束上下文
UIGraphicsEndImageContext();
// 5.回到主线程显示图片
dispatch_async(dispatch_get_main_queue(), ^{
self.imageView.image = fullImage;
});
});
}
- (void)test2
{
// 异步下载
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// 1.下载第1张
NSURL *url1 = [NSURL URLWithString:@"http://g.hiphotos.baidu.com/image/pic/item/f2deb48f8c5494ee460de6182ff5e0fe99257e80.jpg"];
NSData *data1 = [NSData dataWithContentsOfURL:url1];
self.image1 = [UIImage imageWithData:data1];
[self bindImages];
});
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// 2.下载第2张
NSURL *url2 = [NSURL URLWithString:@"http://su.bdimg.com/static/superplus/img/logo_white_ee663702.png"];
NSData *data2 = [NSData dataWithContentsOfURL:url2];
self.image2 = [UIImage imageWithData:data2];
[self bindImages];
});
}
- (void)bindImages
{
if (self.image1 == nil || self.image2 == nil) return;
// 3.合并图片
// 开启一个位图上下文
UIGraphicsBeginImageContextWithOptions(self.image1.size, NO, 0.0);
// 绘制第1张图片
CGFloat image1W = self.image1.size.width;
CGFloat image1H = self.image1.size.height;
[self.image1 drawInRect:CGRectMake(0, 0, image1W, image1H)];
// 绘制第2张图片
CGFloat image2W = self.image2.size.width * 0.5;
CGFloat image2H = self.image2.size.height * 0.5;
CGFloat image2Y = image1H - image2H;
[self.image2 drawInRect:CGRectMake(0, image2Y, image2W, image2H)];
// 得到上下文中的图片
UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();
// 结束上下文
UIGraphicsEndImageContext();
// 4.回到主线程显示图片
dispatch_async(dispatch_get_main_queue(), ^{
self.imageView.image = fullImage;
});
}
- (void)test1
{
// 异步下载
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// 1.下载第1张
NSURL *url1 = [NSURL URLWithString:@"http://g.hiphotos.baidu.com/image/pic/item/f2deb48f8c5494ee460de6182ff5e0fe99257e80.jpg"];
NSData *data1 = [NSData dataWithContentsOfURL:url1];
UIImage *image1 = [UIImage imageWithData:data1];
// 2.下载第2张
NSURL *url2 = [NSURL URLWithString:@"http://su.bdimg.com/static/superplus/img/logo_white_ee663702.png"];
NSData *data2 = [NSData dataWithContentsOfURL:url2];
UIImage *image2 = [UIImage imageWithData:data2];
// 3.合并图片
// 开启一个位图上下文
UIGraphicsBeginImageContextWithOptions(image1.size, NO, 0.0);
// 绘制第1张图片
CGFloat image1W = image1.size.width;
CGFloat image1H = image1.size.height;
[image1 drawInRect:CGRectMake(0, 0, image1W, image1H)];
// 绘制第2张图片
CGFloat image2W = image2.size.width * 0.5;
CGFloat image2H = image2.size.height * 0.5;
CGFloat image2Y = image1H - image2H;
[image2 drawInRect:CGRectMake(0, image2Y, image2W, image2H)];
// 得到上下文中的图片
UIImage *fullImage = UIGraphicsGetImageFromCurrentImageContext();
// 结束上下文
UIGraphicsEndImageContext();
// 4.回到主线程显示图片
dispatch_async(dispatch_get_main_queue(), ^{
self.imageView.image = fullImage;
});
});
}
@end
运行效果：把图标像水印打在图片上
队列组运行效果.png
