-GCD的使用-提高性能

FMDB v2.4

This is an Objective-C wrapper around SQLite: http://sqlite.org/

The FMDB Mailing List:

http://groups.google.com/group/fmdb

Read the SQLite FAQ:

http://www.sqlite.org/faq.html

Since FMDB is built on top of SQLite, you're going to want to read this page top to bottom at least once. And while you're there, make sure to bookmark the SQLite Documentation page:http://www.sqlite.org/docs.html

Contributing

Do you have an awesome idea that deserves to be in FMDB? You might consider pinging ccgus first to make sure he hasn't already ruled it out for some reason. Otherwise pull requests are great, and make sure you stick to the local coding conventions. However, please be patient and if you haven't heard anything from ccgus for a week or more, you might want to send a note asking what's up.

CocoaPods

 

FMDB can be installed using CocoaPods.

pod 'FMDB'# pod 'FMDB/FTS'   # FMDB with FTS# pod 'FMDB/standalone'   # FMDB with latest SQLite amalgamation source# pod 'FMDB/standalone/FTS'   # FMDB with latest SQLite amalgamation source and FTS# pod 'FMDB/SQLCipher'   # FMDB with SQLCipher

If using FMDB with SQLCipher you must use the FMDB/SQLCipher subspec. The FMDB/SQLCipher subspec declares SQLCipher as a dependency, allowing FMDB to be compiled with the -DSQLITE_HAS_CODEC flag.

FMDB Class Reference:

http://ccgus.github.io/fmdb/html/index.html

Automatic Reference Counting (ARC) or Manual Memory Management?

You can use either style in your Cocoa project. FMDB Will figure out which you are using at compile time and do the right thing.

Usage

There are three main classes in FMDB:

1. FMDatabase - Represents a single SQLite database. Used for executing SQL statements.
2. FMResultSet - Represents the results of executing a query on an FMDatabase.
3. FMDatabaseQueue - If you're wanting to perform queries and updates on multiple threads, you'll want to use this class. It's described in the "Thread Safety" section below.

Database Creation

An FMDatabase is created with a path to a SQLite database file. This path can be one of these three:

1. A file system path. The file does not have to exist on disk. If it does not exist, it is created for you.
2. An empty string (@""). An empty database is created at a temporary location. This database is deleted with the FMDatabase connection is closed.
3. NULL. An in-memory database is created. This database will be destroyed with the FMDatabaseconnection is closed.

(For more information on temporary and in-memory databases, read the sqlite documentation on the subject: http://www.sqlite.org/inmemorydb.html)

FMDatabase *db = [FMDatabase databaseWithPath:@"/tmp/tmp.db"];

Opening

Before you can interact with the database, it must be opened. Opening fails if there are insufficient resources or permissions to open and/or create the database.

if (![db open]) {    [db release];    return;}

Executing Updates

Any sort of SQL statement which is not a SELECT statement qualifies as an update. This includesCREATE, UPDATE, INSERT, ALTER, COMMIT, BEGIN, DETACH, DELETE, DROP, END, EXPLAIN,VACUUM, and REPLACE statements (plus many more). Basically, if your SQL statement does not begin with SELECT, it is an update statement.

Executing updates returns a single value, a BOOL. A return value of YES means the update was successfully executed, and a return value of NO means that some error was encountered. You may invoke the -lastErrorMessage and -lastErrorCode methods to retrieve more information.

Executing Queries

A SELECT statement is a query and is executed via one of the -executeQuery... methods.

Executing queries returns an FMResultSet object if successful, and nil upon failure. You should use the -lastErrorMessage and -lastErrorCode methods to determine why a query failed.

In order to iterate through the results of your query, you use a while() loop. You also need to "step" from one record to the other. With FMDB, the easiest way to do that is like this:

FMResultSet *s = [db executeQuery:@"SELECT * FROM myTable"];while ([s next]) {    //retrieve values for each record}

You must always invoke -[FMResultSet next] before attempting to access the values returned in a query, even if you're only expecting one:

FMResultSet *s = [db executeQuery:@"SELECT COUNT(*) FROM myTable"];if ([s next]) {    int totalCount = [s intForColumnIndex:0];}

FMResultSet has many methods to retrieve data in an appropriate format:

• intForColumn:
• longForColumn:
• longLongIntForColumn:
• boolForColumn:
• doubleForColumn:
• stringForColumn:
• dateForColumn:
• dataForColumn:
• dataNoCopyForColumn:
• UTF8StringForColumnName:
• objectForColumnName:

Each of these methods also has a {type}ForColumnIndex: variant that is used to retrieve the data based on the position of the column in the results, as opposed to the column's name.

Typically, there's no need to -close an FMResultSet yourself, since that happens when either the result set is deallocated, or the parent database is closed.

Closing

When you have finished executing queries and updates on the database, you should -close theFMDatabase connection so that SQLite will relinquish any resources it has acquired during the course of its operation.

[db close];

Transactions

FMDatabase can begin and commit a transaction by invoking one of the appropriate methods or executing a begin/end transaction statement.

Multiple Statements and Batch Stuff

You can use FMDatabase's executeStatements:withResultBlock: to do multiple statements in a string:

NSString *sql = @"create table bulktest1 (id integer primary key autoincrement, x text);"                 "create table bulktest2 (id integer primary key autoincrement, y text);"                 "create table bulktest3 (id integer primary key autoincrement, z text);"                 "insert into bulktest1 (x) values ('XXX');"                 "insert into bulktest2 (y) values ('YYY');"                 "insert into bulktest3 (z) values ('ZZZ');";success = [db executeStatements:sql];sql = @"select count(*) as count from bulktest1;"       "select count(*) as count from bulktest2;"       "select count(*) as count from bulktest3;";success = [self.db executeStatements:sql withResultBlock:^int(NSDictionary *dictionary) {    NSInteger count = [dictionary[@"count"] integerValue];    XCTAssertEqual(count, 1, @"expected one record for dictionary %@", dictionary);    return 0;}];

Data Sanitization

When providing a SQL statement to FMDB, you should not attempt to "sanitize" any values before insertion. Instead, you should use the standard SQLite binding syntax:

INSERT INTO myTable VALUES (?, ?, ?)

The ? character is recognized by SQLite as a placeholder for a value to be inserted. The execution methods all accept a variable number of arguments (or a representation of those arguments, such as an NSArray, NSDictionary, or a va_list), which are properly escaped for you.

Alternatively, you may use named parameters syntax:

INSERT INTO myTable VALUES (:id, :name, :value)

The parameters must start with a colon. SQLite itself supports other characters, but internally the Dictionary keys are prefixed with a colon, do not include the colon in your dictionary keys.

NSDictionary *argsDict = [NSDictionary dictionaryWithObjectsAndKeys:@"My Name", @"name", nil];[db executeUpdate:@"INSERT INTO myTable (name) VALUES (:name)" withParameterDictionary:argsDict];

Thus, you SHOULD NOT do this (or anything like this):

[db executeUpdate:[NSString stringWithFormat:@"INSERT INTO myTable VALUES (%@)", @"this has \" lots of ' bizarre \" quotes '"]];

Instead, you SHOULD do:

[db executeUpdate:@"INSERT INTO myTable VALUES (?)", @"this has \" lots of ' bizarre \" quotes '"];

All arguments provided to the -executeUpdate: method (or any of the variants that accept ava_list as a parameter) must be objects. The following will not work (and will result in a crash):

[db executeUpdate:@"INSERT INTO myTable VALUES (?)", 42];

The proper way to insert a number is to box it in an NSNumber object:

[db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:42]];

Alternatively, you can use the -execute*WithFormat: variant to use NSString-style substitution:

[db executeUpdateWithFormat:@"INSERT INTO myTable VALUES (%d)", 42];

Internally, the -execute*WithFormat: methods are properly boxing things for you. The following percent modifiers are recognized: %@, %c, %s, %d, %D, %i, %u, %U, %hi, %hu, %qi, %qu,%f, %g, %ld, %lu, %lld, and %llu. Using a modifier other than those will have unpredictable results. If, for some reason, you need the % character to appear in your SQL statement, you should use %%.

Using FMDatabaseQueue and Thread Safety.

Using a single instance of FMDatabase from multiple threads at once is a bad idea. It has always been OK to make a FMDatabase object per thread. Just don't share a single instance across threads, and definitely not across multiple threads at the same time. Bad things will eventually happen and you'll eventually get something to crash, or maybe get an exception, or maybe meteorites will fall out of the sky and hit your Mac Pro. This would suck.

So don't instantiate a single FMDatabase object and use it across multiple threads.

Instead, use FMDatabaseQueue. It's your friend and it's here to help. Here's how to use it:

First, make your queue.

FMDatabaseQueue *queue = [FMDatabaseQueue databaseQueueWithPath:aPath];

Then use it like so:

[queue inDatabase:^(FMDatabase *db) {    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:1]];    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:2]];    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:3]];    FMResultSet *rs = [db executeQuery:@"select * from foo"];    while ([rs next]) {        …    }}];

An easy way to wrap things up in a transaction can be done like this:

[queue inTransaction:^(FMDatabase *db, BOOL *rollback) {    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:1]];    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:2]];    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:3]];    if (whoopsSomethingWrongHappened) {        *rollback = YES;        return;    }    // etc…    [db executeUpdate:@"INSERT INTO myTable VALUES (?)", [NSNumber numberWithInt:4]];}];

FMDatabaseQueue will run the blocks on a serialized queue (hence the name of the class). So if you call FMDatabaseQueue's methods from multiple threads at the same time, they will be executed in the order they are received. This way queries and updates won't step on each other's toes, and every one is happy.

Note: The calls to FMDatabaseQueue's methods are blocking. So even though you are passing along blocks, they will not be run on another thread.

Making custom sqlite functions, based on blocks.

You can do this! For an example, look for "makeFunctionNamed:" in main.m

Swift

You can use FMDB in Swift projects too.

To do this, you must:

1. Copy the FMDB .m and .h files into your project.

2. If prompted to create a "bridging header", you should do so. If not prompted and if you don't already have a bridging header, add one.

   For more information on bridging headers, see Swift and Objective-C in the Same Project.

3. In your briding header, add a line that says:

   #import "FMDB.h"

4. Optionally, copy the FMDatabaseVariadic.swift from the "src/extra/Swift Extensions" folder into your project. This allows you to use executeUpdate and executeQuery with variadic parameters, rather than the withArgumentsInArray rendition.

If you do the above, you can then write Swift code that uses FMDatabase. For example:

let documentsFolder = NSSearchPathForDirectoriesInDomains(.DocumentDirectory, .UserDomainMask, true)[0] as Stringlet path = documentsFolder.stringByAppendingPathComponent("test.sqlite")let database = FMDatabase(path: path)if !database.open() {    println("Unable to open database")    return}if !database.executeUpdate("create table test(x text, y text, z text)", withArgumentsInArray: nil) {    println("create table failed: \(database.lastErrorMessage())")}if !database.executeUpdate("insert into test (x, y, z) values (?, ?, ?)", withArgumentsInArray: ["a", "b", "c"]) {    println("insert 1 table failed: \(database.lastErrorMessage())")}if !database.executeUpdate("insert into test (x, y, z) values (?, ?, ?)", withArgumentsInArray: ["e", "f", "g"]) {    println("insert 2 table failed: \(database.lastErrorMessage())")}if let rs = database.executeQuery("select x, y, z from test", withArgumentsInArray: nil) {    while rs.next() {        let x = rs.stringForColumn("x")        let y = rs.stringForColumn("y")        let z = rs.stringForColumn("z")        println("x = \(x); y = \(y); z = \(z)")    }} else {    println("select failed: \(database.lastErrorMessage())")}database.close()

History

The history and changes are availbe on its GitHub page and are summarized in the "CHANGES_AND_TODO_LIST.txt" file.

Contributors

The contributors to FMDB are contained in the "Contributors.txt" file.

Additional projects using FMDB, which might be interesting to the discerning developer.

• FMDBMigrationManager, A SQLite schema migration management system for FMDB:https://github.com/layerhq/FMDBMigrationManager
• FCModel, An alternative to Core Data for people who like having direct SQL access:https://github.com/marcoarment/FCModel

Quick notes on FMDB's coding style

Spaces, not tabs. Square brackets, not dot notation. Look at what FMDB already does with curly brackets and such, and stick to that style.

Reporting bugs

Reduce your bug down to the smallest amount of code possible. You want to make it super easy for the developers to see and reproduce your bug. If it helps, pretend that the person who can fix your bug is active on shipping 3 major products, works on a handful of open source projects, has a newborn baby, and is generally very very busy.

And we've even added a template function to main.m (FMDBReportABugFunction) in the FMDB distribution to help you out:

• Open up fmdb project in Xcode.
• Open up main.m and modify the FMDBReportABugFunction to reproduce your bug.
  • Setup your table(s) in the code.
  • Make your query or update(s).
  • Add some assertions which demonstrate the bug.

Then you can bring it up on the FMDB mailing list by showing your nice and compact FMDBReportABugFunction, or you can report the bug via the github FMDB bug reporter.

Optional:

Figure out where the bug is, fix it, and send a patch in or bring that up on the mailing list. Make sure all the other tests run after your modifications.