Tài liệu Thủ thuật lập trình cho iPhone part 6 docx

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Bài 7: SQLite trong iPhone

SQLite là một CSDL thu gọn được hỗ trợ trong iPhone và Android để lưu
trữ lượng dữ liệu nhỏ.SQLite không hỗ trợ thiết kế mô hình quan hệ như là
SQL server mà chỉ có thực thi các câu lệnh SQL. Trong firefox có một
plugin giúp cho chúng ta có thể dễ dàng tạo và quản lý CSDL SQLite.
Các lệnh trong SQLite cũng không được đầy đủ và chính xác như trong
SQL server nếu ko muốn nói rằng có một lệnh bị lỗi.
SQLite ra đời nhằm mục đích phục vụ cho các thiết bị có cấu hình thấp
hơn so vói máy tính như iPhone và Android để quản lý dữ liệu.

If you have been following my tutorials, you know that we have been
working primarily with UITableViews. This is mostly because SO many
applications can be developed using this simple control. This final
UITableView tutorial will be taking all of the skills learned from previous
tutorials, putting them all together, and adding SQLite to create a prioritized
To-Do list. I will also be showing you how to add multiple columns to your
table cells and we will be exploring some of the other controls that the
iPhone has to offer. What good would the tutorials be if we didn‟t use them
to create something useful.
I will move a little faster in this tutorial while still explaining the new stuff in
detail. I will assume that you have completed the fruits tutorial and it‟s
prerequisites.
This tutorial will be a multipart series as it will be a little longer than my
previous ones. In this first tutorial, you will learn:
 Create a NavigationBased Application
 Create a Database

 Add the Database to Your Project
 Add the SQLite3 Framework
 Create a Todo Class Object
 Initialize the Database
So let‟s get started…

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Open up X-Code and Select File->New Project… Select Navigation-
Based Application and click Choose…

Name your project todo. Now let‟s create the todo database that we will be
using. Open up the Terminal application on your Mac. This is located in
Applications > Utilities.
If you have installed XCode, you should have mysqlite3 already on your
computer. To check this, type:
sqlite3 into the Terminal and sqlite3 should start. Type .quit to exit. If sqlite3
is not installed, install all of the XTools from your Mac Installation Disk.
Now that the terminal is open let‟s create the database. This is done with
the command:
sqlite3 todo.sqlite
SQLite3 will now start and load the todo.sqlite database. By default the
database is empty and contains no tables. If you need a refresher on the
basics of SQL databases Google It. Since our application is fairly simple,

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we only need to create one table. We will create a table called todo by

typing the following statement:
CREATE TABLE todo(pk INTEGER PRIMARY KEY, text VARCHAR(25),
priority INTEGER, complete BOOLEAN);
One thing to note here is the pk field. It is the primary key of the table. This
adds functionality such that every time a row is added to the database, it
auto-increments this field. This will be a unique identifier to identify each
row. All of the other fields should be fairly self explanitory.
Now that our table has been created, let‟s add some data. We will
eventually be adding todo items within our app, but for now we will add
some defaults. Type the following commands below.
INSERT INTO todo(text,priority,complete) VALUES('Take out the
trash',3,0);
INSERT INTO todo(text,priority,complete) VALUES('Do Computer Science
homework',1,0);
INSERT INTO todo(text,priority,complete) VALUES('Learn Objective
C',1,0);
INSERT INTO todo(text,priority,complete) VALUES('DIGG this tutorial',2,0);
You can add as many todo items as you would like. For this tutorial, make
sure you enter a priority between 1 and 3 (You‟ll see why later). Now our
database has been created and populated let‟s exit out of SQLite3. Do this
by typing .quit. Your terminal window should look something like this.


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Now go back to XCode. Do a Control-Click (right click) on the folder named
Resources. Click Add -> Existing Files… and browse to your todo.sqlite
file and click Add. It will then prompt you with a screen like this.

Make sure you check the box that says Copy items into destination
group’s folder (if needed). You should now see the todo.sqlite file inside
of the resource folder.

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Now that we have added the database, we need to load the Objective C
libraries so we can use it. Do a control-click (right click) on the
Frameworks folder. Click Add -> Existing Frameworks. Now this part is a
little strange. It has been my experience that these libraries are not in the
same place on all machines. So in the search bar type in libsqlite3. The
file we are looking for is called libsqlite3.0.dylib. This may pull up multiple
files as OSX has it‟s own versions of this file. Just click on the largest of the
files that show up and click Add. As you can see, mine is about 1.7 MB.

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Now it should add the framework and your directory will look something like
this:

We need to create an object to hold our todo information. We will
eventually be making an array of these objects to populate a UITableView.
Go ahead and click File -> New File… Select NSObject Subclass and
click Next.

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Name this object todo.m and check the box that says Also create
“Todo.h” and click Finish.

Open up todo.h and add the following code.

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We see some new things here…First, there is a variable of type sqlite3
called database. This will be a reference to the applications database and
will allow the todo object to communicate with it. Make sure you add a
#import<sqlite3.h> in your imports.
Next, we see a primary key. Notice that in the property declaration it has
the keywords assign and readonly. This tells the compiler that this
variable, once assiged, can not be changed again. This is good since each
todo will be uniquely identified by this variable.
Also, I have declared a method called initWithPrimaryKey. This will be the
contstructor for this object. It takes an integer to assign as the primary key
and an sqlite3 object to use as the database reference.
Let‟s implement this method…Open up todo.m and add the following code.
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There are quite a few new things that need to be explained here. I will just
go through it line by line.
static sqlite3_stmt *init_statement = nil
This will hold our initialize statement when retrieving todo data from the
database. This statement is static, meaning it is independent of any
instance. In other words, there will be only one of them no matter how
many todo objects we create. This statement will get compiled and allow us
to do some neat things. I‟ll explain more in a bit.
The next lines makes sure that the super class (NSObject) initilizes
properly before we initilize a todo object. We then set the local primary key
and database objects to the parameters passed to the initWithPrimaryKey
method.
Now some interesting stuff happens. The next lines checks if our
init_statment is null. This will happen only once per launch of the
application. If it is null, we create a new string containing an SQL
statement. If you are familiar with SQL at all, this should look pretty familiar
with one exception. What is a question mark doing in there? Well, I will tell
you. After the SQL statement gets compiled, we can bind a value to it that

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will eventually replace the question mark. So this allows us to have 1
generic SQL statement, but bind different values to it to retrieve different
results. So the next line, you guessed it, prepares the statement and stores
it in our init_statement. The if statement just checks to see if this finished

correctly and prints an error if there was a problem.
Moving on… The line sqlite3_bind_int simply replaces that question mark
with the primary key of the current todo object, so what we end up with is
statements like this:
SELECT text FROM todo WHERE pk = 1;
SELECT text FROM todo WHERE pk = 2;
SELECT text FROM todo WHERE pk = 3;
SELECT text FROM todo WHERE pk = n;
After that, the sqlite3_step(init_statement) method is called. This method
executes the SQL statement on the database. It is contained inside of an if
statement to make sure it executed properly. Now we can finally access the
todo data. We see this line:
self.text = [NSString stringWithUTF8String:(char*)
sqlite3_column_text(init_statement,0)];
Wow, that‟s a mouthful… Let‟s analyze it. The sqlite3_column_text method
tells SQL that we want to retrieve a string object from the database. It has 2
parameters. The first, is just a reference to the SQL statement that was
used. The second is the column number that we wish to get text from. So in
this case, we only have one column (SELECT text FROM…) so there is
only 1 index and that‟s the 0th index. Next, the (char *) is just a cast to a
string (might not be needed, but good practice). And finally, we build an
NSString object with the data returned so that we can assign self.text to it.
This is quite a bit to explain in just text. If I have lost you, feel free to ask
me questions in the comments.
We are done with the todo object for now…
Go ahead and open up todoAppDelegate.h and add the following code.

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This should look a little familiar with the exception of a few lines. Notice that
I have created an NSMutableArray of todo objects. This will be (like the fruit
example) an array to hold our todo items. We will eventually use this array
to populate a UITableView. The only new lines here are the import of
sqlite3.h and the sqlite3 *database line. Now let‟s open up
todoAppDelegate.m and add some code.

One new thing we see here is a private interface. We declared it here
because it‟s specific to this object so it does not need to be declared in the
.h file. The 2 functions we will implement are
createEditableCopyOfDatabaseIfNeeded and initializeDatabase. Much of
the code for these has already been written for us inside of Apple‟s
SQLBooks tutorial. I will going through this code and explaining it the best
that I can. Add the following code.

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What this method is essentially doing is copying the database from your
project folder to the documents folder on your iPhone. This will only happen
once as it first checks if the database already exists in the documents
folder. I‟m not going to go through this line by line as it is fairly self
explanitory. Apple does a great job of naming functions and variables so
that we can understand what is going on. If I get enough requests in the
comments, I‟ll do a line-by-line writup of this function.
The next function we will implement is initializeDatabase. Add the
following code:

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That‟s a lot of text! Don‟t worry it‟s mostly comments. Let‟s analyze this
code…Some of it is very similar to the fruits example.
The first line creates and initializes a NSMutableArray. We then go on to
set this array to our object‟s todos array and release the temporary object.
The next 3 lines locate the database we created inside of the documents
folder. Following that, the sqlite3_open line open‟s the database so we
can access its data. If the database opens correctly, we then proceed to
retrieve todo items. The first line:
const char *sql = "SELECT pk FROM todo";
is an SQL statement that we will use to get all of the primary keys from the
database. We then prepare the statement (as we did inside the todo.m file)
only this time there is no “?” in the statement. That is because there is not
condition for retrieving the primary keys. We are simply saying “give me all
of the primary keys in the database”.
Now we see a while loop that is stepping through the SQL results. Every
time we call sqlite3_step, the next result gets retrieved. The line:
int primaryKey = sqlite3_column_int(statement,0);
retrieves the primary key from each result. This is very similar to retrieving
the text in the todo.m class only we use the sqlite3_column_int method
instead of the sqlite3_column_text method. This is done for obvious
reasons.
After we have the primary key, we create a new Todo object and call the
initWithPrimaryKey constructor that we created. The primary key gets
passed as well as a reference to the database. This allows the Todo object
to essentially “look itself up” in the database. Finally, we add the newly
created Todo object to our array of todos.

The last statement sqlite3_finalize clears the statement from memory and
does some other cleanup.
The last part of this tutorial is calling these functions to create and initialize
the database. So add the following code to applicationDidFinishLaunching:

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We are simply calling these functions. You can now click Build and Go but
your application won‟t display anything! You might be quite frustrated that
you completed this portion of the tutorial and have yet to see anything.
Well, stay tuned! I will have the next portion of this tutorial up soon.
For you ambitious programmers you could move on. If you notice, at this
point we are in a similar situation as the fruit tutorial. We have an Array of
objects that will eventually populate a UITableView.
This tutorial will be a 4 part series and I will show you how to use a few
more controls. We will be adding, editing, and deleting todo items. If you
have any questions, please leave them in the comments. Also, if you get
lost you can download the sample code here