| Filename extension | .appcache |
|---|---|
| Internet media type | text/cache-manifest |
| Developed by | World Wide Web Consortium |
| Standard(s) | HTML5 |
| Open format? | Yes |
| Website | www.whatwg.org/specs/web-apps/current-work/multipage/offline.html |
The cache manifest in HTML5 is a software storage feature which provides the ability to access a web application even without a network connection.
Contents |
Web applications consist of web pages that need to be downloaded from a network. For this to happen there must be a network connection. However there are many instances when users cannot connect to a network due to reasons beyond their control. HTML5 provides the ability to access the web application even without a network connection using the cache manifest.
Web applications consists of a list of web addresses. These can be HTML, CSS, JavaScript, images or any other source that is required for a web application to be rendered. These addresses or URLs can be copied onto a manifest file, which can be updated regularly by the author of the web application, indicating any new web addresses that are added or deleted. When connecting to a network for the first time a web browser will read the HTML5 manifest file, download the resources given and store them locally. Then, in the absence of a network connection, the web browser will shift to the local copies instead and render the web application offline.
In order for the offline applications to work, a cache manifest file must be created by the web developer. If the web application exceeds more than one page then each page must have a manifest attribute that points to the cache manifest. The cache manifest file is a text file located in another part of the server. It must be served with the following content type: [1]
text/cache-manifest
The following line must be added to the html element in order for the cache manifest file to work.[1]
<!DOCTYPE HTML> <html manifest="/cache.appcache"> <body> … </body> </html>
Consider the HTML file given below. The <html> element indicates the test.appcache file. This manifest will contain the list of resources (i.e. test.js, test.css) needed for this web page to work offline.
<!—- test.html --> <!DOCTYPE HTML> <html manifest="test.appcache"> <head> <title>Test</title> <script src="test.js"></script> <link rel="stylesheet" href="test.css"> </head> <body> Testing the manifest file. </body> </html>
Given below is a list of some rules and syntax required when writing the manifest file.[2]
CACHE MANIFEST # the above is a required line # this is a comment # spaces are ignored # blank lines are ignored
Given below is an example of a cache manifest file.
Example 1:
CACHE MANIFEST /test.css /test.js /test.png
This manifest file lists three resources: a CSS file, a JavaScript file and a PNG image. When the above file is loaded, the browser will download the test.css, test.js and test.png files from the root directory in the web server.[1] As a result, whenever your network is not connected, the resources will be available to you offline.
Cache manifests can also use absolute paths or even absolute URLs as shown below.[2][3][4]
Example 2:
CACHE MANIFEST /main/features.js /main/settings/index.css http://files/images/scene.jpg http://files/images/world.jpg
The cache manifest file consists of three section headers.[1]
note: Example 1 and Example 2 above, do not indicate any section header and are therefore considered an explicit section by default.
Example 3:
CACHE MANIFEST NETWORK: /checking.cgi CACHE: /test.css /test.js /test.png
This example consists of headers. The line, NETWORK: is the start of the “online whitelist” section. The resources listed under this section are never cached and are not available offline.[1]As a result an error will occur when an attempt is made offline to load the resource.
There is a shift to the explicit section by the header CACHE: and the resources (the CSS stylesheet , JavaScript and the image file) can be downloaded and used offline.
The fallback section in a cache manifest file can be used to substitute online resources that cannot be cached or were not cached successfully.[1]
Example 4:
CACHE MANIFEST FALLBACK: / /offline.html NETWORK: …
In Example 4, the fallback section consist of a single line. i.e. / /offline.html. The single character (/) before ‘offline’ will match any URL pattern on your site.[1]If the browser does not find the page in the appcache, the application will display the page /offline.html.
Events are under the ApplicationCache JavaScript object.
If the browser visits a web page, has NOT seen the web page before and as a result does not recognize the manifest file, the following events will ensue.[2]
Checking Event - occurs when the browser visits a web page and reads the manifest attribute on the <html> element.Downloading Event - if the browser has never come across this manifest file before, it will download all the resources given in the manifest file.Progress Event - contains information of how many files have been downloaded and how many files are left to be downloaded.Cached Event - occurs once all the files have been downloaded and the offline web application is equipped to be used offline.If the browser has visited the web page before and recognizes the manifest file the following events will ensue.[2]
Noupdate Event - this will take place if the cache manifest has not changed.Downloading Event - if the cache manifest has changed the resources the file will be downloaded again.Progress Event - this contains information of how many files have been downloaded and how many files are left to be downloaded.Updateready Event - after the re-downloading is complete, this event is triggered, indicating that the new offline version is ready to be used.If an error occurs at any instance in the above events, the browser will trigger an error event and stop the process. Given below are a few errors that can occur when re-downloading resources.[3]