iPhone开发多线程开发之NSOperation&NSOperationQueue——异步下载图片 .
实现的功能：1）演示多线程NSOperation&NSOperationQueue开发；2）子线程中执行下载图片工作，图片下载完成前显示等待框和下载进度条；

关键词：多线程 NSOperation NSOperationQueue 等待框


效果图如下：
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1、新建视图控制器ViewController（不带xib），作为根视图控制器，通过ViewController的-(void)loadView方法构建UI。




2、新建继承自NSOperation且实现协议NSURLConnectionDelegate的类DownLoadImageTask，DownLoadImageTask.h如下：

#import "ViewController.h"#import "DownLoadImageTask.h"@implementation ViewController@synthesize queue;@synthesize appImgView;-(void)loadView{    //初始化视图    [self initViews];        //显示等待框    [self showWaiting];    NSString *url = @"http://hiphotos.baidu.com/newwen666666/pic/item/01ec7750863e49600cf3e3cc.jpg";    int index = 1;        DownLoadImageTask *task = [[DownLoadImageTask alloc]initWithURLString:url];    task.downloadImageDelegate = self;    task.operationId = index++;        queue = [[NSOperationQueue alloc]init];    [queue addOperation:task];}//初始化视图组件-(void)initViews{    CGRect frame = [UIScreen mainScreen].applicationFrame;    UIView *appView = [[UIView alloc]initWithFrame:frame];    self.view = appView;    [self.view setBackgroundColor:[UIColor colorWithWhite:1.0 alpha:1.0]];        frame = CGRectMake(0, 0, frame.size.width, frame.size.height);    appImgView = [[UIImageView alloc]initWithFrame:frame];        [self.view addSubview:appImgView];}//展示等待框-(void)showWaiting{    CGRect frame = CGRectMake(0, -20, 320, 480);    int x = frame.size.width;      int progressWidth = 150;    int progressHeight = 32;    frame = CGRectMake((x-progressWidth)/2, 100, progressWidth, progressHeight);    UIProgressView *progress = [[UIProgressView alloc]initWithProgressViewStyle:UIProgressViewStyleBar];    progress.frame = frame;    progress.progress = 0.0;    progress.backgroundColor = [UIColor whiteColor];        UILabel *showValue = [[UILabel alloc]init];      frame = showValue.frame;      frame.origin.x = CGRectGetMaxX(progress.frame)+10;      frame.origin.y = CGRectGetMinY(progress.frame);      frame.size.width = 45;      frame.size.height = 15;      showValue.frame = frame;      showValue.backgroundColor = [UIColor redColor];      showValue.text = @"0.0";              int progressIndWidth = 32;    int progressIndHeight = 32;    frame = CGRectMake((x-progressIndWidth)/2, 100+32, progressIndWidth, progressIndHeight);    UIActivityIndicatorView *progressInd = [[UIActivityIndicatorView alloc]initWithFrame:frame];    [progressInd startAnimating];    progressInd.activityIndicatorViewStyle = UIActivityIndicatorViewStyleWhiteLarge;        frame = CGRectMake((x-70)/2, 100+32+32, 80, 20);    UILabel *waitinglabel = [[UILabel alloc]initWithFrame:frame];    waitinglabel.text = @"正在下载应用程序图片...";    waitinglabel.textColor = [UIColor redColor];    waitinglabel.font = [UIFont systemFontOfSize:15];    waitinglabel.backgroundColor = [UIColor clearColor];        frame = CGRectMake(0, -20, 320, 480);    UIView *theView = [[UIView alloc]initWithFrame:frame];    theView.backgroundColor = [UIColor blackColor];    theView.alpha = 0.7;        [progress setTag:100];    [theView addSubview:progress];    [showValue setTag:101];    [theView addSubview:showValue];        [theView addSubview:progressInd];    [theView addSubview:waitinglabel];        [theView setTag:110];    [self.view addSubview:theView];}//隐藏等待框-(void)hiddenWaiting{    [[self.view viewWithTag:110]removeFromSuperview];}- (void)viewDidUnload{    [super viewDidUnload];    // Release any retained subviews of the main view.}- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation{    return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown);}#pragma mark DownLoadImageDelegate methods//展示下载完毕的图片-(void)imageDownLoadFinished:(UIImage *)img{    //退出等待框    [self hiddenWaiting];    [appImgView setImage:img];}//更新进度条的值-(void)updateDownProgress:(double) value{    UIProgressView *progresss = (UIProgressView *)[self.view viewWithTag:100];    UILabel *showValue = (UILabel*)[self.view viewWithTag:101];    progresss.progress = value;    showValue.text = [NSString stringWithFormat:@"%.1f%",(double)(value*100)];}@end