多线程-NSThread-demo

实现功能: 从网络下载图片,显示在界面上

注意点:
1.Xcode 7开始默认使用HTTPS协议,可能需要修改plist文件
2.所有耗时操作都在子线程进行,防止卡死UI
3.几乎所有UIKit提供的类都是线程不安全的,所有更新UI的操作都在主线程上执行

示例代码:

#import "ViewController.h"

@interface ViewController ()
@property (nonatomic,strong) UIImageView *imageView;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    
    self.imageView = [[UIImageView alloc] initWithFrame:[UIScreen mainScreen].bounds];
    self.imageView.backgroundColor = [UIColor redColor];
    [self.view addSubview:self.imageView];
    
    // 异步下载图片
    [NSThread detachNewThreadSelector:@selector(downloadImage) toTarget:self withObject:nil];
    
}

- (void)downloadImage{
    
    // 根据URL下载图片
    NSURL *url = [NSURL URLWithString:@"http://b.hiphotos.bdimg.com/wisegame/pic/item/d37eca8065380cd7107d123aa044ad3458828182.jpg"];
    NSData *data = [NSData dataWithContentsOfURL:url];
    UIImage *image = [UIImage imageWithData:data];
    
    // 回到主线程刷新UI
    [self performSelectorOnMainThread:@selector(updateUIWithImage:) withObject:image waitUntilDone:NO];

}

- (void)updateUIWithImage:(UIImage *)image{
    self.imageView.image = image;
}

@end

demo.png

这里回到主线程使用了:

- (void)performSelectorOnMainThread:(SEL)aSelector withObject:(nullable id)arg waitUntilDone:(BOOL)wait;

waitUntilDone: 代表是否等待当前代码执行完毕再执行后面的代码
如果设置YES,代表执行完后在执行performSelectorOnMainThread后面的代码
如果设置NO,代表不用等待performSelectorOnMainThread执行完就可以执行后面的代码

例如下面的代码,我讲waitUntilDone设置为YES,在回到主线程刷新UI时延迟5s执行,最终的效果就是,等刷新UI操作完成后,才会执行后面的打印

- (void)downloadImage{
    
    // 根据URL下载图片
    NSURL *url = [NSURL URLWithString:@"http://b.hiphotos.bdimg.com/wisegame/pic/item/d37eca8065380cd7107d123aa044ad3458828182.jpg"];
    NSData *data = [NSData dataWithContentsOfURL:url];
    UIImage *image = [UIImage imageWithData:data];
    
    // 回到主线程刷新UI waitUntilDone: 代表等待当前代码执行完毕再执行后面的代码
    [self performSelectorOnMainThread:@selector(updateUIWithImage:) withObject:image waitUntilDone:YES];
    
    NSLog(@"回到主线程刷新UI操作完毕");

}

- (void)updateUIWithImage:(UIImage *)image{
    
    [NSThread sleepForTimeInterval:5];
    self.imageView.image = image;
}

同理,如果设置为NO,在刷新UI前就可以执行打印这一句代码了