Memory Cache Mechanism for iOS development

The purpose of using the cache is to allow the application to respond to user input more quickly and to run the program efficiently. Sometimes we need to cache the data obtained by the remote web server to reduce multiple requests to the same url.

We can use the NSURLCache class in the sdk for memory caching. NSURLRequest requires a cache parameter to describe how the requested url caches data. Let's take a look at its CachePolicy type.

1. NSURLRequestUseProtocolCachePolicy NSURLRequest default cache policy, which is defined using the Protocol.

2. NSURLRequestReloadIgnoringCacheData ignores the cache and downloads it directly from the original address.

3. NSURLRequestReturnCacheDataElseLoad can be downloaded from the original address only when no data exists in the cache.

4. NSURLRequestReturnCacheDataDontLoad only uses the cache data. If the cache does not exist, the request fails. It is used when no offline network connection mode is established;

5. NSURLRequestReloadIgnoringLocalAndRemoteCacheData: ignores local and remote cache data and downloads it directly from the original address, similar to NSURLRequestReloadIgnoringCacheData.

6. NSURLRequestReloadRevalidatingCacheData: verify whether the local data is the same as the remote data. If the local data is different, download the remote data. Otherwise, use the local data.

NSURLCache also provides many methods to facilitate the caching of applications. The following example shows how to reduce multiple requests to the same url. See the following code:

 
 

  
  
-(IBAction) buttonPress :( id) sender
  
  
{
  
  
NSString * paramURLAsString = @ "http://www.baidu.com /";
  
  
If ([paramURLAsString length] = 0 ){
  
  
NSLog (@ "Nil or empty URL is given ");
  
  
Return;
  
  
}
  
  
NSURLCache * urlCache = [NSURLCache sharedURLCache];
  
  
/* Set the cache size to 1 MB */
  
  
[UrlCache setMemoryCapacity: 1*1024*1024];
  
  
// Create an nsurl
  
  
NSURL * url = [NSURL URLWithString: paramURLAsString];
  
  
// Create a request
  
  
NSMutableURLRequest * request =
  
  
[NSMutableURLRequest
  
  
RequestWithURL: url
  
  
CachePolicy: NSURLRequestUseProtocolCachePolicy
  
  
TimeoutInterval: 60366f];
  
  
// Obtain the cache output from the request
  
  
NSCachedURLResponse * response =
  
  
[UrlCache cachedResponseForRequest: request];
  
  
// Determine whether a cache exists
  
  
If (response! = Nil ){
  
  
NSLog (@ "get data from cache if cache output exists ");
  
  
[Request setCachePolicy: NSURLRequestReturnCacheDataDontLoad];
  
  
}
  
  
Self. connection = nil;
  
  
/* Create NSURLConnection */
  
  
NSURLConnection * newConnection =
  
  
[[NSURLConnection alloc] initWithRequest: request
  
  
Delegate: self
  
  
StartImmediately: YES];
  
  
Self. connection = newConnection;
  
  
[NewConnection release];
  
  
}
 
 

In this example, the request url is www.baidu.com. If the url is cached, we can directly obtain data from the cache. Otherwise, we can retrieve data from www.baidu.com. We set the cache size to 1 MB.

Use the following code to print out the request process:

 
 

  
  
-(Void) connection :( NSURLConnection *) connection
  
  
DidReceiveResponse :( NSURLResponse *) response {
  
  
NSLog (@ "receive output ");
  
  
}
  
  
-(NSURLRequest *) connection :( NSURLConnection *) connection
  
  
WillSendRequest :( NSURLRequest *) request
  
  
RedirectResponse :( NSURLResponse *) redirectResponse {
  
  
NSLog (@ "request to be sent ");
  
  
Return (request );
  
  
}
  
  
-(Void) connection :( NSURLConnection *) connection
  
  
DidReceiveData :( NSData *) data {
  
  
NSLog (@ "Accept data ");
  
  
NSLog (@ "data length = % lu", (unsigned long) [data length]);
  
  
}
  
  
-(NSCachedURLResponse *) connection :( NSURLConnection *) connection
  
  
WillCacheResponse :( NSCachedURLResponse *) cachedResponse {
  
  
NSLog (@ "");
  
  
Return (cachedResponse );
  
  
}
  
  
-(Void) connectionDidFinishLoading :( NSURLConnection *) connection {
  
  
NSLog (@ "request completed ");
  
  
}
  
  
-(Void) connection :( NSURLConnection *) connection
  
  
DidFailWithError :( NSError *) error {
  
  
NSLog (@ "request failed ");
  
  
}
 
 

When we click the button on the interface for the first time, the output is as follows:

 
 

  
  
18:50:24. 910 Caching [3971: 207] The request will be sent soon
  
  
18:50:28. 557 Caching [3971: 207] will receive the output
  
  
18:50:31. 677 Caching [3971: 207] accept data
  
  
18:50:31. 681 Caching [3971: 207] Data Length = 4414
  
  
18:50:31. 682 Caching [3971: 207] accept data
  
  
18:50:31. 682 Caching [3971: 207] Data Length = 2996
  
  
18:50:38. 107 Caching [3971: 207]
  
  
18:50:38. 109 Caching [3971: 207] request completed
 
 

After reading the button on the page for the second time, the print result is as follows:

 
 

  
  
18:52:18. 894 Caching [3971: 207] The request will be sent soon
  
  
18:52:18. 895 Caching [3971: 207] will receive the output
  
  
18:52:18. 895 Caching [3971: 207] accept data
  
  
18:52:18. 896 Caching [3971: 207] Data Length = 7410
  
  
18:52:18. 896 Caching [3971: 207] request completed
 
 

We can see that there is no "outputs the cache" item. The requested data is the accumulation of the first request, that is, the second request is to obtain data from the memory.