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 
@protocol DownLoadImageDelegate;
@interface DownLoadImageTask : NSOperation{
 int operationId;
 long long totalLength;
 BOOL done;
}
@property int operationId;
@property(nonatomic,assign) iddownloadImageDelegate;
@property(nonatomic,retain) NSMutableData *buffer;
@property(nonatomic,retain) NSURLRequest *request;
@property(nonatomic,retain) NSURLConnection *connection;
- (id)initWithURLString:(NSString *)url;
@end

@protocol DownLoadImageDelegate
//图片下载完成的委托
-(void)imageDownLoadFinished:(UIImage *)img;
//更新图片下载进度条的值
-(void)updateDownProgress:(double) value;
@end

DownLoadImageTask.m如下：

#import "DownLoadImageTask.h"
@implementation DownLoadImageTask
@synthesize operationId;
@synthesize downloadImageDelegate;
@synthesize buffer;
@synthesize request;
@synthesize connection;
- (id)initWithURLString:(NSString *)url{
 NSLog(@"url=%@",url);
 self = [super init];
 if(self){
 request = [NSURLRequest requestWithURL:[NSURL URLWithString:url]];
 buffer = [NSMutableData data];
 }
 return self;
}
//主要处理方法
-(void)start{ //或者main
 NSLog(@"DownLoadImageTask-start");
 
 if(![self isCancelled]){
 //暂停一下
 [NSThread sleepForTimeInterval:1];
 //设置connection及其代理
 connection = [[NSURLConnection alloc]initWithRequest:request delegate:self];
 if(connection!=nil){
 while(!done){
 [[NSRunLoop currentRunLoop]runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];
 }
 }
 }
}
-(void)httpConnectEndWithError{
 //[self hiddenWaiting];
 NSLog(@"httpConnectEndWithError");
}
-(void)dealloc{
 buffer = nil;
 connection = nil;
 request = nil;
 downloadImageDelegate = nil;
}
#pragma NSURLConnection delegate methods
//不执行缓存
-(NSCachedURLResponse *)connection:(NSURLConnection *)connection willCacheResponse:(NSCachedURLResponse *)cachedResponse{
 return nil;
}
//连接发生错误
-(void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error{
 [self performSelectorOnMainThread:@selector(httpConnectEndWithError) withObject:self waitUntilDone:NO];
 [buffer setLength:0];
}
//收到响应
- (void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response{
 NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse *)response;
 if(httpResponse && [httpResponse respondsToSelector:@selector(allHeaderFields)]){
 NSDictionary *httpResponseHeaderFields = [httpResponse allHeaderFields];
 totalLength = [[httpResponseHeaderFields objectForKey:@"Content-Length"] longLongValue];
 NSLog(@"totalLength is %lld",totalLength);
 }
}
//接收数据
-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data{
 //NSLog(@"didReceiveData...");
 [buffer appendData:data];
 
 double progressValue = totalLength==0?0:((double)([buffer length])/(double)totalLength);
 //更新进度条值
 [downloadImageDelegate updateDownProgress:progressValue];
}
//下载完毕
-(void)connectionDidFinishLoading:(NSURLConnection *)connection{
 done = YES;
 UIImage *img = [[UIImage alloc] initWithData:buffer];
 [downloadImageDelegate imageDownLoadFinished:img];
}
-(BOOL)isConcurrent {
 //返回yes表示支持异步调用，否则为支持同步调用
 return YES;
 
}
- (BOOL)isExecuting{
 return connection == nil; 
}
- (BOOL)isFinished{
 return connection == nil; 
}
@end

DownLoadImageTask中定义了协议DownLoadImageDelegate，ViewController需要实现协议DownLoadImageDelegate；DownLoadImageTask下载工作完成后，会回调ViewController实现的DownLoadImageDelegate协议中的方法。
2、ViewController.h如下：

#import 
#import "DownLoadImageTask.h"
@interface ViewController:UIViewController{
}
@property(strong,nonatomic)NSOperationQueue *queue;
@property(strong,nonatomic)UIImageView *appImgView;
@end

ViewController.m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

3、效果如下：

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