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

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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：

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#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：

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#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:[documentDirectorystringByAppendingPathComponent:@"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：

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#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：

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#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.imageViewwaitUntilDone: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，



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#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，





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#import "ProductDetailViewController.h"
//这里引入在第二步中，我们创建的对象
#import "ResourceContainer.h"

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@implementation DetailViewController

@synthesize imageURL = _imageURL;

@synthesize originalIndexArray = _originalIndexArray;
@synthesize originalOperationDic = _originalOperationDic;
@synthesize requestImageQueue = _requestImageQueue;


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- (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