TOutputCache enables caching a portion of a Web page, also known as partial caching. The content being cached are HTML page source coming from static texts on a PRADO template or rendered by one or several controls on the template. When the cached content is used, controls generating the content are no longer created for the page hierarchy and thus significant savings in page processing time can be achieved. The side-effect, as you might already find out, is that the content displayed may be stale if the cached version is shown to the users.
To use TOutputCache, simply enclose the content to be cached within the TOutputCache component tag on a template (either page or non-page control template), e.g.,
content to be cached
where content to be cached can be static text and/or template tags. If the latter, the rendering results of the template tags will be cached. You can place one or several TOutputCache on a single template and they can be nested.
TOutputCache stores cached content via PRADO cache modules (e.g. TDbCache) and thus requires at least one cache module loaded when the application runs.
The validity of the cached content is determined based on two factors: the Duration and the cache dependency. The former specifies the number of seconds that the data can remain valid in cache (defaults to 60s), while the latter specifies conditions that the cached data depends on. If a dependency changes (e.g. relevant data in DB are updated), the cached data will be invalidated and discarded.
There are two ways to specify cache dependency. One may write event handlers to respond to the OnCheckDependency event and set the event parameter's IsValid property to indicate whether the cached data remains valid or not. One can also extend TOutputCache and override its getCacheDependency() method.
The content fetched from cache may be variated with respect to some parameters. TOutputCache supports variation with respect to request parameters, which is specified by VaryByParam property. If a specified request parameter is different, a different version of cached content is used. This is extremely useful if a page's content may be variated according to some GET parameters. The content being cached may also be variated with user sessions if VaryBySession is set true. To variate the cached content by other factors, override calculateCacheKey() method.
Output caches can be nested. An outer cache takes precedence over an inner cache in determining the validity of cached contents. This means, if the content cached by the inner cache expires or is invalidated, while that by the outer cache not, the outer cached content will be used.
By default, TOutputCache is effective only for non-postback page requests and when a cache module is enabled. Do not attempt to address child controls of TOutputCache when the cached content is currently being used. Use ContentCached property to determine whether the content is cached or not.