Tài liệu Thủ thuật lập trình cho iPhone part 6 docx

Now let‟s create the todo database that we will be using.. Collection By traibingo 67 we only need to create one table.. We will create a table called todo by typing the following state

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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…

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

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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

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

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

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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Collection By traibingo 73

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

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

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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

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:

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