使用NSoperation 实现异步加载图片

采用UITableView显示从网络上下载的图片，因为网络图片下载比较耗费时间，一般采用边显示文字，内容，后台下载图片，下载完成后刷新TableViewCell ，本文将演示如何通过自定的UITableViewCell，显示图片。

1。定义ImageTableViewCell 

@interface ImageTableViewCell : UITableViewCell {
    UILabel *txtLabel;
    UIImageView *imageView;
}
@property(nonatomic, retain)IBOutlet UILabel *txtLabel;
@property(nonatomic, retain)IBOutlet UIImageView *imageView;

- (void)setCell:(UIImage *)image text:(NSString *)text;

@end 

方法添加如下 

 - (void)setCell:(UIImage *)image text:(NSString *)text

{
    if (image != nil)
    {
        self.imageView.image = image;
    }
    
    self.txtLabel.text = text;

}

 具体实现代码，请在源代码里查看

2。定义ImageDownloader ，这个类继承NSOperation，因为需要并发，所以需要实现下面4个方法 

//是否允许并发，

-(BOOL)isConcurrent 

- (BOOL)isExecuting

//是否已经完成，这个必须要重载，不然放在放在NSOperationQueue里的NSOpertaion不能正常释放。

- (BOOL)isFinished

//具体下载的方法在这里执行。

- (void)start 

而对应于非并发的情况下，只需要重载main方法就好了。

#import <Foundation/Foundation.h>

@protocol imageDownloaderDelegate;

@interface ImageDownloader : NSOperation 
{
    NSURLRequest* _request;
    
    NSURLConnection* _connection;
    
    NSMutableData* _data;
    
    BOOL _isFinished; 
    
    id<imageDownloaderDelegate> delegate;
    
    NSObject *delPara;
}

- (id)initWithURLString:(NSString *)url;

@property (readonly) NSData *data;
@property(nonatomic, assign) id<imageDownloaderDelegate> delegate;
@property(nonatomic, retain) NSObject *delPara;

@end

@protocol imageDownloaderDelegate

@optional

//图片下载完成的委托
- (void)imageDidFinished:(UIImage *)image para:(NSObject *)obj;

@end

 实现文件如下

 #import "ImageDownloader.h"

@implementation ImageDownloader

@synthesize data=_data;
@synthesize delegate;
@synthesize delPara;

- (void)dealloc 
{
    
    [_request release]; 
    _request=nil;
    
    [_data release];
    _data=nil;
    
    [_connection release];
    _connection=nil;
    
    [delPara release];
    
    [super dealloc];
    
}
- (id)initWithURLString:(NSString *)url 
{
    
    self = [self init];
    if (self) {

        _request = [[NSURLRequest alloc] initWithURL:[NSURL URLWithString:url]];
        
        _data = [[NSMutableData data] retain];
        
    }
    
    return self;
    
}



// 开始处理-本类的主方法

- (void)start {
    
    if (![self isCancelled]) {
        
        [NSThread sleepForTimeInterval:3];
        // 以异步方式处理事件，并设置代理
        
        _connection=[[NSURLConnection connectionWithRequest:_request delegate:self]retain];
        
        while(_connection != nil) {
            
            [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];   
            
        }
        
    }
    
}

#pragma mark NSURLConnection delegate Method

// 接收到数据（增量）时

- (void)connection:(NSURLConnection*)connection

    didReceiveData:(NSData*)data 
{
    // 添加数据
    [_data appendData:data];
    
}

- (void)connectionDidFinishLoading:(NSURLConnection*)connection {
    
    [_connection release],
    _connection=nil;
    NSLog(@"%s", __func__);
    
    UIImage *img = [[[UIImage alloc] initWithData:self.data] autorelease];
    
    if (self.delegate != nil)
    {
        [delegate imageDidFinished:img para:self.delPara];
    }
    
    
}

-(void)connection: (NSURLConnection *) connection didFailWithError: (NSError *) error
{
    [_connection release],
    _connection=nil; 
}

-(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;  
}


然后再UITableViewController里每个Cell创建的时候，如果没有图片的话，就通过ImageDownloader去下载，下载完了刷新这个Cell就可以了。

#import <UIKit/UIKit.h>
#import "ImageDownloader.h"
@interface RootViewController : UITableViewController <imageDownloaderDelegate>
{
    NSOperationQueue *queue;
    NSMutableArray *siteArray;
    NSMutableArray *imageArray;
    NSMutableDictionary *dic;
}
@property(nonatomic, retain)NSOperationQueue *queue;
@property(nonatomic, retain)NSMutableArray *siteArray;
@property(nonatomic, retain)NSMutableArray *imageArray;

@end

 实现文件

//
//  RootViewController.m
//  NSOperationTest
//
//

#import "RootViewController.h"
#import "ImageDownloader.h"
#import "ImageTableViewCell.h"

@implementation RootViewController
@synthesize queue;
@synthesize siteArray;
@synthesize imageArray;

- (void)dealloc
{
    [self.queue release];
    [self.siteArray release];
    [self.imageArray release];
    [super dealloc];
}

- (void)viewDidLoad
{
    [super viewDidLoad];
    self.queue = [[[NSOperationQueue alloc] init] autorelease];
    self.siteArray = [[[NSMutableArray alloc] init] autorelease];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972321636938600.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953930325377400.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953847613105000.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972570125915500.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131954190510086200.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972321636938600.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953930325377400.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953847613105000.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972570125915500.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131954190510086200.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972321636938600.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953930325377400.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953847613105000.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972570125915500.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131954190510086200.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972321636938600.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953930325377400.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953847613105000.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972570125915500.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131954190510086200.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972321636938600.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953930325377400.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953847613105000.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972570125915500.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131954190510086200.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972321636938600.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953930325377400.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131953847613105000.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/27/131972570125915500.jpg"];
    [self.siteArray addObject:@"http://s1.lashouimg.com/zt/201110/25/131954190510086200.jpg"];
    
    self.imageArray = [[[NSMutableArray alloc] init] autorelease];
    for ( int i = 0; i  < [self.siteArray count]; i++) 
    {
        [self.imageArray addObject: [NSNull null]];
    }
    dic = [NSDictionary dictionary];
    
}

// Customize the number of sections in the table view.
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    return [siteArray count];
}

// Customize the appearance of table view cells.
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";
    
    ImageTableViewCell *cell = (ImageTableViewCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];
    if (cell == nil) 
    {
        NSArray *bundle = [[NSBundle mainBundle] loadNibNamed:@"ImageTableViewCell" owner:self options:nil];
        cell = (ImageTableViewCell *)[bundle objectAtIndex:0];
    }

    NSString *text = [NSString stringWithFormat:@"%d", indexPath.row];

    
    NSNull *null = (NSNull *)[self.imageArray objectAtIndex:indexPath.row];
    
    if (![null isKindOfClass:[NSNull class]])
    {
        [cell setCell:[imageArray objectAtIndex:indexPath.row] text:text];
  
    }
    else
    {
        [cell setCell:nil text:text];
        NSString *url = [self.siteArray objectAtIndex:indexPath.row];
        ImageDownloader *downloader = [[[ImageDownloader alloc] initWithURLString:url] autorelease];
        downloader.delegate = self;
        downloader.delPara = indexPath;
        [self.queue addOperation:downloader];
    }

    // Configure the cell.
    return cell;
}

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath
{
    return 112;
}




- (void)imageDidFinished:(UIImage *)image para:(NSObject *)obj
{
    
    NSIndexPath *indexPath = (NSIndexPath *)obj;
    [self.imageArray replaceObjectAtIndex:indexPath.row withObject:image];
    ImageTableViewCell *cell = (ImageTableViewCell *)[self.tableView cellForRowAtIndexPath:indexPath];
    NSString *text = [NSString stringWithFormat:@"%d", indexPath.row];
    [cell setCell:image text:text];
    [cell setNeedsDisplay];

    
}

@end 

本文是通过委托实现的刷新Cell，你也可以通过一个Observer，观察图片是否下载完成，完成后就刷新图片所再Cell。通过Observer如何实现请参考这个网址http://blog.csdn.net/kmyhy/article/details/6050345 

本文地址：http://www.cnblogs.com/likwo/archive/2011/10/30/2229582.html

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