I'm curious about NSURLCache, NSURLRequest has some policy of cache,
like NSURLRequestUseProtocolCachePolicy, NSURLRequestReturnCacheDataElseLoad,
but after read them, either of them are using local cache data, or not using cache,
My question is if I want start a url request, first I wanna load cache and render ui, then continue interacting with server to grab latest data and refresh the ui, which policy is my choice?
If I understand correctly what you want to achieve (get data quickly from the cache to show in the UI even if it is outdated, then get the current data even if it is slow), you'd have to make two requests, using different cache policies. I'd start the second request only when the first one has finished, and examine the result of the first request first, because the data might not have been available in the cache, so the first request might already returned the uncached data that you wanted.
Related
my app is a blog. On each blog post page, I make a request to my server to get the relevant data. I'd also like to keep a view count each time the blog page is viewed.
I can do this on the backend when it receives a request. I'm not too sure how cache works in iOS, but my question is, if this data is cached on subsequent views, will this request still be made to the backend on each subsequent view?
Or should I explicitly make a background request in my ViewController to update the view count tally?
If you are using URLRequest to call your API, then you can set the cachePolicy so that if your backend responds with the correct caching headers then if nothing has changed it won't reload.
You may want to re-architect your API so that you don't have to load the whole blog post every time just to update the view count. Perhaps that should be a separate API call, which has its own caching policy (since it changes more frequently)
Hi i have read about cache policy but still not quite clear. My purpose is to set request cache 3 minutes. Response cache 3 minutes. keep old cache for 1 day. What is the implementation that we should we use? Is there any setting that we can change beside those default bellow? (I use AFNetworking 3 for request and response). Any help is much appreciate. Thanks
NSURLRequestCachePolicy
NSURLRequest has a cachePolicy property, which specifies the caching behavior of the request according to the following constants:
NSURLRequestUseProtocolCachePolicy: Caching logic defined in the protocol implementation is used for a particular URL load request. This is the default policy.
NSURLRequestReloadIgnoringLocalCacheData: Data should be loaded from the originating source. No existing cache data should be used.
NSURLRequestReloadIgnoringLocalAndRemoteCacheData: Not only should the local cache data be ignored, but proxies and other intermediates should be instructed to disregard their caches so far as the protocol allows.
NSURLRequestReturnCacheDataElseLoad: Existing cached data should be used, regardless of its age or expiration date. If there is no existing data in the cache corresponding to the request, the data is loaded from the originating source.
NSURLRequestReturnCacheDataDontLoad: Existing cache data should be used, regardless of its age or expiration date. If there is no existing data in the cache corresponding to the request, no attempt is made to load the data from the originating source, and the load is considered to have failed, (i.e. “offline” mode).
NSURLRequestReloadRevalidatingCacheData: Existing cache data may be used provided the origin source confirms its validity, otherwise the URL is loaded from the origin source.
You should almost invariably use NSURLRequestUseProtocolCachePolicy unless you're trying to kill caching entirely for some reason.
The cache durations should be set by the server as part of its response (in the Cache-Control header). The iOS URL cache obeys the policies specified in that header.
Whenever I have an issue with a website, one of the first suggestions I will hear is “try to clear your browser cache” along with “and delete your cookies“. So what is this browser cache? What does it store and what is it good for?
I have googled. but didn't find the proper answer.I appreciate if anyone help on this.
A browser cache "caches" (as in keeps local copies) of data downloaded from the internet. The next time your browser needs the same data it can get it from the cache (fast) instead of downloading it over the internet (slow)
The problem is that data can be old. For example imagine the browser cached www.nytimes.com today and 24hrs later you visited www.nytimes.com again. If the browser loaded the cached data it would be old news.
So there are headers (metadata) that the servers send to browser telling them how long they should cache something (if at all).
The data the browser generally caches are "requests" which. In other words if your browser asks for "http://foo.com/bar.html" the first time the browser will "request" that "foo.com" send it "bar.html". If the headers from "foo.com" are set a certain way the browser will the save a local copy of "bar.html". If you request the same thing again the browser may load "bar.html" from it's cache. I say "may" because it depends on the headers sent from the server. The server can say how long (say 10 minutes, 10 hours, 10 days, etc..) or it can say "don't cache this at all, always download the newest version".
If you go to your browser's dev tools (chrome shown below) and look at the network tab (not sure what it's called in other browsers). Load the page again and you can see all the requests. You'll also notice which ones were loaded from the cache
If you click on a request you can see the metadata from both the browser (request headers) and the server (response headers)
The reason clearing the cache often fixes things is if for some reason the server (a bug?) said it was ok to cache or used the cached version but the data on the server has actually been updated. The browser, doing what the server told it to do, is using its copy from the cache, not the newer version which is actually needed. There might also from time to time be bugs in the browser itself related to caching.
When everything is working correctly it's great but if one thing or another is mis-configured or sending the wrong headers then the browser can end up loading old data from the cache instead of downloading the newest data. Clearing your cache effectively forces the browser to download the data again.
You can find out the details of what the various headers do here.
Browser caches are not mere rubbish bins but a mechanism to speed up the way we browse the web. Each website we visit has certain common elements like logos, navigation buttons, GIF animation files, script files etc. It doesn’t make sense for the browser to download each element (also commonly called as Temporary Internet files) when we hop from page to the other and back.
The page elements are downloaded when we visit a website and the browser checks its cache folder for copies when we browse the website. If a copy exists, then the browser doesn’t download the same file again, thus significantly speeding up web browsing speeds.
for more info..
http://www.guidingtech.com/8925/what-are-browser-cache-cookies-does-clearing-them-help/
https://en.wikipedia.org/wiki/Cache_(computing)
First result in Google, this is the proper answer, but I will summarize =]
1) What is Browser Cache?
Cache is a component that stores data so future requests for that data can be served faster; the data stored in a cache might be the results of an earlier computation, or the duplicates of data stored elsewhere.
2) What does it store?
Web browsers and web proxy servers employ web caches to store previous responses from web servers, such as web pages and images.
3) What is it good for?
Web caches reduce the amount of information that needs to be transmitted across the network, as information previously stored in the cache can often be re-used. This reduces bandwidth and processing requirements of the web server, and helps to improve responsiveness for users of the web.
I am using NSUrlConnection to grab some data from a web service that returns JSON data when I hit a url but sometimes the page goes down or something is wrong.
Is there a way to use the cached version of the url in my iOS App to grab that data when the webpage returns a 404?
In the browser if I go to cache:mysite.com/test.php I can see the JSON data even when the page is down but when I try to use the same url in my iOS App, I do not get the JSON data back.
Is there maybe an Obj C class I am not aware of or an option for the NSUrlConnection?
NSURLConnection already uses a cache. By default it only caches responses in-memory, you can customize this by setting it to also use on-disk storage:
NSURLCache *result = [[NSURLCache alloc] initWithMemoryCapacity:[(1024*1024*512) diskCapacity:(1024*1024*1024 * 100) diskPath#"Cache.db"]:
[NSURLCache setSharedURLCache:result];
The reason you are seeing the behavior you describe in your question is that the remote web service is telling your client not to cache the response. You can check this using REDbot or a tool like Charles. By default NSURLRequest will use the protocol's caching policy and semantics, which is almost always the correct thing to do. You can specify a different cache policy by changing the cachePolicy property of the NSURLRequest.
After much search, I didn't find a way to get a cached version of a web page in my iOS app but I handled the 404 error by looking for the response status code and using a service like Google Cache or archive.org/web/ to get the cached version of the url if a 404 was found.
Might not be the most delicate way but I could not find another way to get a cached version of the website when making the request.
I have an active resource model in one of my applications, and I need to be able occasionally do a find(:all), and force it to repull the data from the remote service. How can I do this? I saw the connection(refresh=true) piece, but I don't want it to refresh EVERY SINGLE TIME. More like I just want to be able to flush the cache when I want to, or to force a particular transaction to repull from the remote.
You might check out cached_resource. I am not sure how you are caching currently. Cached resource caches the responses to requests made with active resource. At the moment it seems to cache every request that goes through active resource, but allows you to refresh a specific request by doing:
MyActiveResource.all(:reload => true).
As far as I know, ActiveResource doesn't do any caching and will pull from the remote service every time you do find(:all).