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By default, any object you store will be encoded and decoded using NSKeyedArchiver/NSKeyedUnarchiver. You can customize this by providing encoder and decoder blocks, like this:
ldb.encoder = ^ NSData * (LevelDBKey *key, id object) { // return some data, given an object } ldb.decoder = ^ id (LevelDBKey *key, NSData * data) { // return an object, given some data }
NSMutableDictionary-like API
ldb[@"string_test"] = @"laval"; // same as: [ldb setObject:@"laval"forKey:@"string_test"];
All available methods can be found in its header file (documented).
Enumeration
[ldb enumerateKeysAndObjectsUsingBlock:^(LevelDBKey *key, id value, BOOL *stop) { // This step is necessary since the key could be a string or raw data (use NSDataFromLevelDBKey in that case) NSString *keyString = NSStringFromLevelDBKey(key); // Assumes UTF-8 encoding // Do something clever }];
// Enumerate with options [ldb enumerateKeysAndObjectsBackward:TRUE lazily:TRUE// Block below will have a block(void) instead of id argument for value startingAtKey:someKey // Start iteration there (NSString or NSData) filteredByPredicate:predicate // Only iterate over values matching NSPredicate andPrefix:prefix // Only iterate over keys prefixed with something usingBlock:^(LevelDBKey *key, void(^valueGetter)(void), BOOL *stop) {
// If we had wanted the value directly instead of a valueGetter block, we would've set the // above 'lazily' argument to FALSE id value = valueGetter(); }]
More iteration methods are available, just have a look at the header section
Snapshots, NSDictionary-like API (immutable)
A snapshot is a readonly interface to the database, permanently reflecting the state of
the database when it was created, even if the database changes afterwards.
LDBSnapshot *snap = [ldb newSnapshot]; // You get ownership of this variable, so in non-ARC projects, // you'll need to release/autorelease it eventually [ldb removeObjectForKey:@"string_test"];
// The result of these calls will reflect the state of ldb when the snapshot was taken NSLog(@"String Value: %@", [snap objectForKey:@"string_test"]); NSLog(@"Dictionary Value: %@", [ldb objectForKey:@"dict_test"]);
All available methods can be found in its header file
Write batches, atomic sets of updates
Write batches are a mutable proxy to a LevelDB database, accumulating updates
without applying them, until you do using -[LDBWritebatch apply]
// Those changes aren't yet applied to ldb // To apply them in batch, [wb apply];
All available methods can be found in its header file
LevelDB options
// The following values are the default LevelDBOptions options = [LevelDB makeOptions]; options.createIfMissing = true; options.errorIfExists = false; options.paranoidCheck = false; options.compression = true; options.filterPolicy = 0; // Size in bits per key, allocated for a bloom filter, used in testing presence of key options.cacheSize = 0; // Size in bytes, allocated for a LRU cache used for speeding up lookups
// Then, you can provide it when initializing a db instance. LevelDB *ldb = [LevelDB databaseInLibraryWithName:@"test.ldb"andOptions:options];
Per-request options
db.safe = true; // Make sure to data was actually written to disk before returning from write operations. [ldb setObject:@"laval"forKey:@"string_test"]; [ldb setObject:[NSDictionarydictionaryWithObjectsAndKeys:@"val1", @"key1", @"val2", @"key2", nil] forKey:@"dict_test"]; db.safe = false; // Switch back to default
db.useCache = false; // Do not use DB cache when reading data (default to true);
Concurrency
As Google's documentation states, updates and reads from a leveldb instance do not require external synchronization
to be thread-safe. Write batches do, and we've taken care of it, by isolating every LDBWritebatch it inside a serial dispatch
queue, and making every request dispatch synchronously to it. So use it from wherever you want, it'll just work.
However, if you are using something like JSONKit for encoding data to JSON in the database, and you are clever enough to
preallocate a JSONDecoder instance for all data decoding, beware that this particular object is not thread-safe, and you will
need to take care of it manually.
Testing
If you want to run the tests, you will need Xcode 5, as the test suite uses the new XCTest.
Clone this repository and, once in it,
./setup-test.sh cd Tests && open Objective-LevelDB.xcworkspace
Currently, all tests were setup to work with the iOS test suite.