IOS开发网络加载图片缓存策略之──ASIDownloadCache缓存策略

在我们实际工程中，很多情况需要从网络上加载图片，然后将图片在imageview中显示出来，但每次都要从网络上请求，会严重影响用户体验，为了不是每次显示都需要从网上下载数据，希望将图片放到本地缓存，因此我们需要一个好的的缓存策略，今天我将我在项目工程中的实际经验分享给大家，我这里主要介绍一下强大的ASIHTTPRequest的缓存策略，以及使用方法：

下面是具体步骤：

一、设置缓存策略

首先在SplitDemoAppDelegate委托代理中，实现如下代码：

在SplitDemoAppDelegate.h文件中，代码如下：

#import <UIKit/UIKit.h>

@class ASIDownloadCache;

@interface SplitDemoAppDelegate : NSObject <UIApplicationDelegate,UITabBarControllerDelegate> {

UIWindow *_window;

ASIDownloadCache *_downloadCache; //下载缓存策略

}

@property (nonatomic, retain) ASIDownloadCache *downloadCache;

@end

在SplitDemoAppDelegate.m文件中，代码如下：

#import "SplitDemoAppDelegate.h"

@implementation SplitDemoAppDelegate

@synthesize window=_window;

@synthesize downloadCache = _downloadCache;

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions

{

//初始化ASIDownloadCache缓存对象

ASIDownloadCache *cache = [[ASIDownloadCache alloc] init];

self.downloadCache = cache;

[cache release];

//路径

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask, YES);

NSString *documentDirectory = [paths objectAtIndex:0];

//设置缓存存放路径

[self.downloadCache setStoragePath:[documentDirectory stringByAppendingPathComponent:@"resource"]];

//设置缓存策略

[self.downloadCache setDefaultCachePolicy:ASIOnlyLoadIfNotCachedCachePolicy];

// Override point for customization after application launch.

[self.window makeKeyAndVisible];

return YES;

}

- (void)dealloc

{

[_window release];

[_downloadCache release];

[super dealloc];

}

@end

二、创建缓存线程

这一步是创建一个NSOperation类，实现缓存的方法，代码如下：

ResourceContainer.h文件实现：

#import <Foundation/Foundation.h>

#import "ASIHTTPRequest.h"

#import "SplitDemoAppDelegate.h"

@interface ResourceContainer : NSOperation {

NSURL *_resourceURL; //资源请求url

NSObject *_hostObject;

SEL _resourceDidReceive; //资源接手响应方法

SplitDemoAppDelegate *_appDelegate; //应用委托对象

ASIHTTPRequest *_httpRequest;

UIImageView *_imageView;

}

@property (nonatomic, retain) NSURL *resourceURL;

@property (nonatomic, retain) NSObject *hostObject;

@property (nonatomic, assign) SEL resourceDidReceive;

@property (nonatomic, assign) SplitDemoAppDelegate *appDelegate;

@property (nonatomic, retain) ASIHTTPRequest *httpRequest;

@property (nonatomic, retain) UIImageView *imageView;

//http请求回调方法

-(void)didStartHttpRequest:(ASIHTTPRequest *)request;

-(void)didFinishHttpRequest:(ASIHTTPRequest *)request;

-(void)didFailedHttpRequest:(ASIHTTPRequest *)request;

//取消资源请求

-(void)cancelReourceGet;

//资源接收回调方法

-(void)resourceDidReceive:(NSData *)resource;

@end

ResourceContainer.m文件实现：

#import "ResourceContainer.h"

#import "HttpConstant.h"

#import "ASIDownloadCache.h"

@implementation ResourceContainer

@synthesize resourceURL = _resourceURL;

@synthesize hostObject = _hostObject;

@synthesize resourceDidReceive = _resourceDidReceive;

@synthesize appDelegate = _appDelegate;

@synthesize httpRequest = _httpRequest;

@synthesize imageView = _imageView;

-(id)init{

if(self == [super init]){

self.appDelegate = (SplitDemoAppDelegate *)[[UIApplication sharedApplication] delegate];

}

return self;

}

-(void)main{

if(self.hostObject == nil)

return;

if(self.resourceURL == nil){

[self resourceDidReceive:nil];

return;

}

ASIHTTPRequest *request = [ASIHTTPRequest requestWithURL:self.resourceURL];

self.httpRequest = request;

[self.httpRequest setDownloadCache:self.appDelegate.downloadCache];

[self.httpRequest setDelegate:self];

[self.httpRequest setDidStartSelector:@selector(didStartHttpRequest:)];

[self.httpRequest setDidFinishSelector:@selector(didFinishHttpRequest:)];

[self.httpRequest setDidFailSelector:@selector(didFailedHttpRequest:)];

//发异步请求

[self.httpRequest startAsynchronous];

}

- (void)dealloc {

[_resourceURL release];

[_hostObject release];

[_httpRequest release];

[_imageView release];

[super dealloc];

}

//开始请求

-(void)didStartHttpRequest:(ASIHTTPRequest *)request{

[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:YES];

}

//请求成功返回处理结果

-(void)didFinishHttpRequest:(ASIHTTPRequest *)request{

[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];

if([request responseStatusCode] == 200 || [request responseStatusCode] == 304){

//判断是否来自缓存

if([request didUseCachedResponse]){

NSLog(@"=========资源请求：%@ 来自缓存============",[self.resourceURL absoluteURL]);

}else{

NSLog(@"=========资源请求：图片不来自缓存============");

}

[self resourceDidReceive:[request responseData]];

}else {

[self resourceDidReceive:nil];

}

}

//失败请求返回处理结果

-(void)didFailedHttpRequest:(ASIHTTPRequest *)request{

[[UIApplication sharedApplication] setNetworkActivityIndicatorVisible:NO];

[self resourceDidReceive:nil];

}

//取消资源请求

-(void)cancelReourceGet{

[self.httpRequest cancel];

}

//资源接收处理方法

-(void)resourceDidReceive:(NSData *)resource{

if([self.hostObject respondsToSelector:self.resourceDidReceive]){

if(resource != nil && self.imageView != nil){

self.imageView.image = [UIImage imageWithData:resource];

}

[self.hostObject performSelectorOnMainThread:self.resourceDidReceive withObject:self.imageView waitUntilDone:NO];

}

}

@end

到第二步，我们的缓存策略的设置，以及资源请求和接收数据方法已经构建完毕，下面介绍一下如何使用我们上面创建的NSOperation类

三、图片请求（利用上面创建的类）

这里以我的工程为例进行分析：

在DetailViewController.h声明文件中，

#import <UIKit/UIKit.h>

@interface DetailViewController :UIViewController {

NSURL *_imageURL; //图片url

NSMutableArray *_originalIndexArray; //保存请求图片的号

NSMutableDictionary *_originalOperationDic; //保存图片请求队列

NSOperationQueue *_requestImageQueue; //图片请求队列

}

@property (nonatomic, retain) NSURL *imageURL;

@property (nonatomic, retain) NSMutableArray *originalIndexArray;

@property (nonatomic, retain) NSMutableDictionary *originalOperationDic;

@property (nonatomic, retain) NSOperationQueue * requestImageQueue;

//显示图片信息

-(void)displayProductImage;

//根据图片序号显示请求图片资源

-(void)displayImageByIndex:(NSInteger)index ByImageURL:(NSURL *)url;

//处理图片请求返回信息

-(void)imageDidReceive:(UIImageView *)imageView;

@end

在DetailViewController.m实现文件中，

#import "ProductDetailViewController.h"

//这里引入在第二步中，我们创建的对象

#import "ResourceContainer.h"

@implementation DetailViewController

@synthesize imageURL = _imageURL;

@synthesize originalIndexArray = _originalIndexArray;

@synthesize originalOperationDic = _originalOperationDic;

@synthesize requestImageQueue = _requestImageQueue;

- (void)viewDidLoad

{

[super viewDidLoad];

NSOperationQueue *tempQueue = [[NSOperationQueue alloc] init];

self.requsetImageQueue = tempQueue;

[tempQueue release];

NSMutableArray *array = [[NSMutableArray alloc] init];

self.originalIndexArray = array;

[array release];

NSMutableDictionary *dic = [[NSMutableDictionary alloc] init];

self.originalOperationDic = dic;

[dic release];

}

//显示图片信息

-(void)displayProductImage

{

NSURL *url = [NSURL URLWithString:@"http://xxx.xxx.xxx.xxx"];

//这个是从器返回有图片数目，self.xxxx根据具体的场合

int imageCount = [self.xxxx.imageNum intValue];

for (int i=0; i<imageCount; i++) {

NSString *str1 = @"这里是拼图片请求url，根据实际需求";

self.imageURL = [url URLByAppendingPathComponent:str1];

//根据图片号请求资源

[self displayImageByIndex:i ByImageURL:self.productImageURL];

}

}

//根据图片序号显示请求图片资源

-(void) displayImageByIndex:(NSInteger)index ByImageURL:(NSURL *)url

{

NSString *indexForString = [NSString stringWithFormat:@"%d",index];

//若数组中已经存在该图片编号，说明图片加载完毕，直接返回

if ([self.originalIndexArray containsObject:indexForString]) {

return;

}

//创建UIImageView对象

UIImageView *imageView = [[UIImageView alloc] init];

imageView.tag = index;

//创建资源请求对象

ResourceContainer *imageOperation = [[ResourceContainer alloc] init];

imageOperation.resourceURL = url;

imageOperation.hostObject = self;

//设置收到图片信息处理理方法

imageOperation.resourceDidReceive = @selector(imageDidReceive:);

imageOperation.imageView = imageView;

[imageView release];

//将图片请求对象加入图片请求队列中

[self.requsetImageQueue addOperation:imageOperation];

[self.originalOperationDic setObject:imageOperation forKey:indexForString];

[imageOperation release];

}

//处理图片请求返回信息

-(void)imageDidReceive:(UIImageView *)imageView

{

if (imageView == nil||imageView.image == nil) {

imageView.image = [UIImage imageNamed:@"no-pic-300-250.png"];

}

//将图片信息加载到前台，self.openFlowView是我用的coverFlow，coverFlow的使用方法网上很多，自己找吧

[self.openFlowView setImage:imageView.image forIndex:imageView.tag];

[self.originalIndexArray addObject:[NSString stringWithFormat:@"%d",imageView.tag]];

[self.originalOperationDic removeObjectForKey:[NSString stringWithFormat:@"%d",imageView.tag]];

}

- (void)dealloc

{

[_requestImageQueue release];

[_originalIndexArray release];

[_originalOperationDic release];

[_imageURL release];

[super dealloc];

}

@end

经过上述步骤，我们实现了加载网络图片时缓存功能，增强了用户体验效果。代码中可能会有诸多问题，希望网友指教，有更好的缓存方法，也希望一起交流！