nsICache 编辑

The nsICache is a namespace for various cache constants. It does not represent an actual object.

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Inherits from: nsISupports

Constants

Constant	Value	Description
ACCESS_NONE	0	ACCESS GRANTED - No descriptor is provided.
ACCESS_READ	1	ACCESS REQUESTED - I only want to READ, if there isn't an entry just fail. ACCESS GRANTED - You can READ from this descriptor.
ACCESS_WRITE	2	ACCESS REQUESTED - I have something new I want to WRITE into the cache, make me a new entry and doom the old one, if any. ACCESS GRANTED - You must WRITE to this descriptor because the cache entry was just created for you.
ACCESS_READ_WRITE	3	ACCESS REQUESTED - I want to READ, but I'm willing to update an existing entry if necessary, or create a new one if none exists. ACCESS GRANTED - You can READ the descriptor to determine if it's valid, you may WRITE if it needs updating.
STORE_ANYWHERE	0	The storage policy of a cache entry determines the device(s) to which it belongs. See nsICacheSession and nsICacheEntryDescriptor for more details. Allows the cache entry to be stored in any device. The cache service decides which cache device to use based on "some resource management calculation."
STORE_IN_MEMORY	1	The storage policy of a cache entry determines the device(s) to which it belongs. See nsICacheSession and nsICacheEntryDescriptor for more details. Requires the cache entry to reside in non-persistent storage. That is typically in system RAM.
STORE_ON_DISK	2	The storage policy of a cache entry determines the device(s) to which it belongs. See nsICacheSession and nsICacheEntryDescriptor for more details. Requires the cache entry to reside in persistent storage. That is typically on a system's hard disk.
STORE_ON_DISK_AS_FILE	3	The storage policy of a cache entry determines the device(s) to which it belongs. See nsICacheSession and nsICacheEntryDescriptor for more details. Requires the cache entry to reside in persistent storage, and in a separate file.
STORE_OFFLINE	4	The storage policy of a cache entry determines the device(s) to which it belongs. See nsICacheSession and nsICacheEntryDescriptor for more details. Requires the cache entry to reside in persistent, reliable storage for offline use.
NOT_STREAM_BASED	0	All entries for a cache session are stored as streams of data or as objects. This constant specify that cache session is not a stream based entry when calling nsICacheService.createSession() method.
STREAM_BASED	1	All entries for a cache session are stored as streams of data or as objects. This constant specify that cache session is a stream based entry when calling nsICacheService.createSession() method.
NON_BLOCKING	0	The synchronous OpenCacheEntry() may be blocking or non-blocking. If a cache entry is waiting to be validated by another cache descriptor (so no new cache descriptors for that key can be created), OpenCacheEntry() will return NS_ERROR_CACHE_WAIT_FOR_VALIDATION in non-blocking mode. If the cache entry is validated, then a descriptor for that entry will be created and returned. If the cache entry was doomed, then a descriptor will be created for a new cache entry for the key.
BLOCKING	1	The synchronous OpenCacheEntry() may be blocking or non-blocking. In blocking mode, it will wait until the cache entry for the key has been validated or doomed. If the cache entry is validated, then a descriptor for that entry will be created and returned. If the cache entry was doomed, then a descriptor will be created for a new cache entry for the key.

Note: If you think that you might need to modify cached data or meta data, then you must open a cache entry requesting WRITE access. Only one cache entry descriptor, per cache entry, will be granted WRITE access.

Usually, you will request READ_WRITE access in order to first test the meta data and informational fields to determine if a write, that is going to the net, may actually be necessary. If you determine that it is not, then you would mark the cache entry as valid (using MarkValid) and then simply read the data from the cache.

A descriptor granted WRITE access has exclusive access to the cache entry up to the point at which it marks it as valid. Once the cache entry has been "validated", other descriptors with READ access may be opened to the cache entry.

If you make a request for READ_WRITE access to a cache entry, the cache service will downgrade your access to READ if there is already a cache entry descriptor open with WRITE access.

If you make a request for only WRITE access to a cache entry and another descriptor with WRITE access is currently open, then the existing cache entry will be 'doomed', and you will be given a descriptor (with WRITE access only) to a new cache entry.