IOS中UITableView异步加载图片的实现

最近做一个项目，需要用到UITableView异步加载图片的例子，看到网上有一个EGOImageView的很好的例子。

但是由于，EGOImageView的实现比较复杂，于是自己就动手做了一个AsynImageView，同样可以实现EGOImageView的效果。

而且自己写的代码比较清晰，容易理解，同样可以实现指定placehoderImage以及指定imageURL，来进行图片的异步加载。

同时，如果图片已经请求过，则不会再重复请求网络，会直接读取本地缓存文件。

效果如下：

具体实现思路如下：

 AsynImageView.h的文件内容：

#import <UIKit/UIKit.h>

@interface AsynImageView : UIImageView
{
    NSURLConnection *connection;
    NSMutableData *loadData;
}
//图片对应的缓存在沙河中的路径
@property (nonatomic, retain) NSString *fileName;

//指定默认未加载时，显示的默认图片
@property (nonatomic, retain) UIImage *placeholderImage;
//请求网络图片的URL
@property (nonatomic, retain) NSString *imageURL;

@end

AsynImageView.m中的文件内容：

#import "AsynImageView.h"
#import <QuartzCore/QuartzCore.h>

@implementation AsynImageView

@synthesize imageURL = _imageURL;
@synthesize placeholderImage = _placeholderImage;

@synthesize fileName = _fileName;

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        // Initialization code

        self.layer.borderColor = [[UIColor whiteColor] CGColor];
        self.layer.borderWidth = 2.0;
        self.backgroundColor = [UIColor grayColor];

    }
    return self;
}

//重写placeholderImage的Setter方法
-(void)setPlaceholderImage:(UIImage *)placeholderImage
{
    if(placeholderImage != _placeholderImage)
    {
        [_placeholderImage release];

        _placeholderImage = placeholderImage;
        self.image = _placeholderImage;    //指定默认图片
    }
}

//重写imageURL的Setter方法
-(void)setImageURL:(NSString *)imageURL
{
    if(imageURL != _imageURL)
    {
        self.image = _placeholderImage;    //指定默认图片
       [_imageURL release];
        _imageURL = [imageURL retain];
    }

    if(self.imageURL)
    {
        //确定图片的缓存地址
        NSArray *path=NSSearchPathForDirectoriesInDomains(NSDocumentationDirectory, NSUserDomainMask, YES);
        NSString *docDir=[path objectAtIndex:0];
        NSString *tmpPath=[docDir stringByAppendingPathComponent:@"AsynImage"];

        NSFileManager *fm = [NSFileManager defaultManager];
        if(![fm fileExistsAtPath:tmpPath])
        {
            [fm createDirectoryAtPath:tmpPath withIntermediateDirectories:YES attributes:nil error:nil];
        }
        NSArray *lineArray = [self.imageURL componentsSeparatedByString:@"/"];
        self.fileName = [NSString stringWithFormat:@"%@/%@", tmpPath, [lineArray objectAtIndex:[lineArray count] - 1]];

        //判断图片是否已经下载过，如果已经下载到本地缓存，则不用重新下载。如果没有，请求网络进行下载。
        if(![[NSFileManager defaultManager] fileExistsAtPath:_fileName])
        {
            //下载图片，保存到本地缓存中
            [self loadImage];
        }
        else
        {
            //本地缓存中已经存在，直接指定请求的网络图片
            self.image = [UIImage imageWithContentsOfFile:_fileName];
        }
    }
}

//网络请求图片，缓存到本地沙河中
-(void)loadImage
{
    //对路径进行编码
    @try {
        //请求图片的下载路径
        //定义一个缓存cache
        NSURLCache *urlCache = [NSURLCache sharedURLCache];
        /*设置缓存大小为1M*/
        [urlCache setMemoryCapacity:1*124*1024];

        //设子请求超时时间为30s
        NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:[NSURL URLWithString:self.imageURL] cachePolicy:NSURLRequestReloadIgnoringCacheData timeoutInterval:60.0];

        //从请求中获取缓存输出
        NSCachedURLResponse *response = [urlCache cachedResponseForRequest:request];
        if(response != nil)
        {
            //            NSLog(@"如果又缓存输出，从缓存中获取数据");
            [request setCachePolicy:NSURLRequestReturnCacheDataDontLoad];
        }

        /*创建NSURLConnection*/
        if(!connection)
            connection = [[NSURLConnection alloc] initWithRequest:request delegate:self startImmediately:YES];
        //开启一个runloop，使它始终处于运行状态

        UIApplication *app = [UIApplication sharedApplication];
        app.networkActivityIndicatorVisible = YES;

        [[NSRunLoop currentRunLoop] runMode:NSDefaultRunLoopMode beforeDate:[NSDate distantFuture]];

    }
    @catch (NSException *exception) {
        //        NSLog(@"没有相关资源或者网络异常");
    }
    @finally {
        ;//.....
    }
}

#pragma mark - NSURLConnection Delegate Methods
//请求成功，且接收数据(每接收一次调用一次函数)
-(void)connection:(NSURLConnection *)connection didReceiveData:(NSData *)data
{
    if(loadData==nil)
    {
        loadData=[[NSMutableData alloc]initWithCapacity:2048];
    }
    [loadData appendData:data];
}

-(void)connection:(NSURLConnection *)connection didReceiveResponse:(NSURLResponse *)response
{

}

-(NSCachedURLResponse *)connection:(NSURLConnection *)connection willCacheResponse:(NSCachedURLResponse *)cachedResponse
{
    return cachedResponse;
    //    NSLog(@"将缓存输出");
}

-(NSURLRequest *)connection:(NSURLConnection *)connection willSendRequest:(NSURLRequest *)request redirectResponse:(NSURLResponse *)response
{
    //    NSLog(@"即将发送请求");
    return request;
}
//下载完成，将文件保存到沙河里面
-(void)connectionDidFinishLoading:(NSURLConnection *)theConnection
{
    UIApplication *app = [UIApplication sharedApplication];
    app.networkActivityIndicatorVisible = NO;

    //图片已经成功下载到本地缓存，指定图片
    if([loadData writeToFile:_fileName atomically:YES])
    {
        self.image = [UIImage imageWithContentsOfFile:_fileName];
    }

    connection = nil;
    loadData = nil;

}
//网络连接错误或者请求成功但是加载数据异常
-(void)connection:(NSURLConnection *)theConnection didFailWithError:(NSError *)error
{
    UIApplication *app = [UIApplication sharedApplication];
    app.networkActivityIndicatorVisible = NO;

    //如果发生错误，则重新加载
    connection = nil;
    loadData = nil;
    [self loadImage];
}

-(void)dealloc
{
    [_fileName release];
    [loadData release];
    [connection release];

    [_placeholderImage release];
    [_imageURL release];

    [super dealloc];
}

@end

上面的AsynImageView的.h和.m文件，就是所要实现的核心代码。如果想要调用AsynImageView，则只需执行如下代码即可：（需导入#import"AsynImageView.h"）

asynImgView = [[AsynImageView alloc] initWithFrame:CGRectMake(0, 5, 200, 100)];
        asynImgView.placeholderImage = [UIImage imageNamed:@"place.png"];
        asynImgView.imageURL = [NSString stringWithFormat:@"http://images.17173.com/2012/news/2012/10/10/lj1010sb10ds.jpg"];
        [self.view addSubView:asynImgView];

下面是我实现的UITableView异步加载图片的程序链接，就是上面的效果图的程序完整代码，大家可以参考一下：

http://download.csdn.net/detail/enuola/5112070

如有不恰当的地方，还望指点。

另外，图片的缓存可以定期进行清理，在此处没有写出清理代码，可以自行添加。