IOS 开发缓存机制----内存缓存机制

使用缓存的目的是为了使用的应用程序能更快速的响应用户输入，是程序高效的运行。有时候我们需要将远程web服务器获取的数据缓存起来，减少对同一个url多次请求。

内存缓存我们可以使用sdk中的NSURLCache类。NSURLRequest需要一个缓存参数来说明它请求的url何如缓存数据的，我们先看下它的CachePolicy类型。

 

1、NSURLRequestUseProtocolCachePolicy NSURLRequest默认的cache policy，使用Protocol协议定义。

2、NSURLRequestReloadIgnoringCacheData 忽略缓存直接从原始地址下载。

3、NSURLRequestReturnCacheDataElseLoad 只有在cache中不存在data时才从原始地址下载。

4、NSURLRequestReturnCacheDataDontLoad 只使用cache数据，如果不存在cache，请求失败；用于没有建立网络连接离线模式；

5、NSURLRequestReloadIgnoringLocalAndRemoteCacheData：忽略本地和远程的缓存数据，直接从原始地址下载，与NSURLRequestReloadIgnoringCacheData类似。

6、NSURLRequestReloadRevalidatingCacheData:验证本地数据与远程数据是否相同，如果不同则下载远程数据，否则使用本地数据。

 

NSURLCache还提供了很多方法，来方便我们实现应用程序的缓存机制。

//
//  RootViewController.h
//  cacheDemo
//  (内存缓存demo-声明)
//  Created by iMilo TEANA on 12-2-1.
//  Copyright (c) 2012年 TEANA. All rights reserved.
//
 
#import <UIKit/UIKit.h>
 
@interface RootViewController : UIViewController
<NSURLConnectionDelegate>
 
- (IBAction)cachePress:(UIButton *)button;
 
@end
 
//
//  RootViewController.m
//  cacheDemo
//  (内存缓存demo-定义)
//  Created by iMilo TEANA on 12-2-1.
//  Copyright (c) 2012年 TEANA. All rights reserved.
//
 
#import "RootViewController.h"
 
@interface RootViewController()
 
@property(nonatomic, retain) NSURLConnection *connection;
 
@end
 
@implementation RootViewController
 
@synthesize connection = _connection;
 
#pragma mark -
#pragma mark - super code
 
- (void)dealloc
{
    [_connection release];
    [super dealloc];
}
 
- (void)viewDidLoad
{
    [super viewDidLoad];
    [[self view] setBackgroundColor:[UIColor redColor]];
}
 
- (void)viewDidUnload
{
    [super viewDidUnload];
}
 
- (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation
{
    return (interfaceOrientation == UIInterfaceOrientationPortrait);
}
 
#pragma mark -
#pragma mark - user code
 
- (IBAction)cachePress:(UIButton *)button
{
    //URL
    NSString *URLStr = @"http://www.baidu.com";
    if ([URLStr length] == 0)
    {
        NSLog(@"Nil or empty URL is given");
        return;
    }
    //设置缓存
    NSURLCache *urlCache = [NSURLCache sharedURLCache];
    //设置缓存大小未1M
    [urlCache setMemoryCapacity:1*1024*1024];
    //创建URL
    NSURL *URL = [NSURL URLWithString:URLStr];
    //创建请求
    NSMutableURLRequest *request =
    [NSMutableURLRequest requestWithURL:URL
                            cachePolicy:NSURLRequestUseProtocolCachePolicy //缓存策略(默认)
                        timeoutInterval:60.0f];//
    //从请求中获取缓存输出
    NSCachedURLResponse *response = [urlCache cachedResponseForRequest:request];
    //判断是否有缓存
    if (response != nil)
    {
        NSLog(@"如果有缓存，从缓存中获取数据");
        [request setCachePolicy:NSURLRequestReturnCacheDataDontLoad];
    }
    //将链接置为空
    _connection = nil;
    //重新创建链接
    NSURLConnection *connection =
    [[NSURLConnection alloc] initWithRequest:request
                                    delegate:self
                            startImmediately:YES];
    [self setConnection:connection];
    [connection release];
    connection = nil;
}
 
#pragma mark -
#pragma mark - delegate code
 
- (void)connection:(NSURLConnection *)connection
 idReceiveResponse:(NSURLResponse *)response
{
    NSLog(@"将要接收数据");
}
 
- (NSURLRequest *)connection:(NSURLConnection *)connection
             willSendRequest:(NSURLRequest *)request
            redirectResponse:(NSURLResponse *)redirectResponse
{
    NSLog(@"即将发送请求");
    return request;
}
 
- (void)connection:(NSURLConnection *)connection
    didReceiveData:(NSData *)data
{
    NSLog(@"接收数据，数据长度为:%lu",(unsigned long)[data length]);
}
 
- (NSCachedURLResponse *)connection:(NSURLConnection *)connection
                  willCacheResponse:(NSCachedURLResponse *)cachedResponse
{
    NSLog(@"将缓存输出");
    return(cachedResponse);
}
 
- (void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    NSLog(@"请求完成");  
}
 
- (void)connection:(NSURLConnection *)connection
  didFailWithError:(NSError *)error
{
    NSLog(@"请求失败");
}
 
@end