Data storage and exchange in Android

Transcription

1 Mobile App Development

2 1 Overview 2 3 SQLite Overview Implementation 4 Overview Methods to implement URI like SQL 5 Internal storage External storage

3 Overview 1 Overview 2 3 SQLite Overview Implementation 4 Overview Methods to implement URI like SQL 5 Internal storage External storage

4 Options we have Outline Overview SharedPreferences Implements named maps of name/value pairs that can be persisted across sessions and shared among application components running within the same application sandbox.

5 Options we have Outline Overview SharedPreferences Implements named maps of name/value pairs that can be persisted across sessions and shared among application components running within the same application sandbox. SQLite Is a SQL implementation.

6 Options we have Outline Overview SharedPreferences Implements named maps of name/value pairs that can be persisted across sessions and shared among application components running within the same application sandbox. Content provider An interface used between applications. The server application that hosts the data manages it through basic create, read, update, and delete (CRUD) operations. SQLite Is a SQL implementation.

7 Options we have Outline Overview SharedPreferences Implements named maps of name/value pairs that can be persisted across sessions and shared among application components running within the same application sandbox. SQLite Is a SQL implementation. Content provider An interface used between applications. The server application that hosts the data manages it through basic create, read, update, and delete (CRUD) operations. Files Local files stored in shared or app specific space

8 1 Overview 2 3 SQLite Overview Implementation 4 Overview Methods to implement URI like SQL 5 Internal storage External storage

9 How it works? are stored within the application s sandbox, so they can be shared between application s components but aren t available to other applications.

10 How it works? are stored within the application s sandbox, so they can be shared between application s components but aren t available to other applications. How to use it? To modify a Shared Preference, use the SharedPreferences.Editor class. Get the Editor object by calling edit on the object you want to change. Use editor.putxxxx([name], [val]) to add preferences and editor.getxxxx([name]) to retrieve.

11 User Preferences Outline Use built-in PreferencesActivity Creating your own Activity to control user preferences is considered bad practice. Android Studio provides you with a standard settings screen using the Settings Activity.

12 User Preferences Outline Use built-in PreferencesActivity Creating your own Activity to control user preferences is considered bad practice. Android Studio provides you with a standard settings screen using the Settings Activity. Question Why is building custom settings activity a bad practice?

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14 SQLite Overview Implementation 1 Overview 2 3 SQLite Overview Implementation 4 Overview Methods to implement URI like SQL 5 Internal storage External storage

15 SQLite Outline SQLite Overview Implementation Android uses the SQLite database engine: self-contained, transactional, requires no separate server process, actively developed by a large community

16 SQLite Overview Implementation What is SQLite? SQLite is an international, open source project implementing a transactional, self-contained relational database. It is not a Google project, but Google has contributed to it.

17 SQLite Overview Implementation How it is stored? With SQLite, the database is a simple single disk file. All of the data structures making up a relational databasetables, views, indexes, etc.are within this file.

18 Before we start Outline SQLite Overview Implementation Good practices It is a good practice to separate database specific code from the rest of your application and encapsulate it in a single class. Other classes should use standard Java classes or Cursors and be unaware of how the data is actually stored.

19 SQLite Overview Implementation

20 Schema and Contract Outline SQLite Overview Implementation Schema A formal declaration of how the database is organized. The schema is reflected in the SQL statements that you use to create your database. Contract A companion class, which explicitly specifies the layout of your schema in a systematic and self-documenting way.

21 SQLite Overview Implementation Why do we need a Contract class? A contract class is a container for constants that define names for URIs, tables, and columns. The contract class allows you to use the same constants across all the other classes in the same package. This lets you change a column name in one place and have it propagate throughout your code. A good way to organize a contract class is to put definitions that are global to your whole database in the root level of the class. Then create an inner class for each table that enumerates its columns.

22 Note:BaseColumns Outline SQLite Overview Implementation Note By implementing the BaseColumns interface, your inner class can inherit a primary key field called _ID that some Android classes such as cursor adaptors will expect it to have. It s not required, but this can help your database work harmoniously with the Android framework.

23 Example Outline SQLite Overview Implementation 1 p u b l i c f i n a l c l a s s F e e d R e a d e r C o n t r a c t { 2 // To p r e v e n t someone from a c c i d e n t a l l y i n s t a n t i a t i n g t h e c o n t r a c t c l a s s, 3 // g i v e i t an empty c o n s t r u c t o r. 4 p u b l i c F e e d R e a d e r C o n t r a c t ( ) {} 5 6 / I n n e r c l a s s t h a t d e f i n e s t h e t a b l e c o n t e n t s / 7 p u b l i c s t a t i c a b s t r a c t c l a s s FeedEntry implements BaseColumns { 8 p u b l i c s t a t i c f i n a l S t r i n g TABLE NAME = e n t r y ; 9 p u b l i c s t a t i c f i n a l S t r i n g COLUMN NAME ENTRY ID = e n t r y i d ; 10 p u b l i c s t a t i c f i n a l S t r i n g COLUMN NAME TITLE = t i t l e ; 11 p u b l i c s t a t i c f i n a l S t r i n g COLUMN NAME SUBTITLE = s u b t i t l e ; } 14 }

24 Database interface Outline SQLite Overview Implementation We will implement a class MyDatabaseManager serving as an interface to the database. It extends the abstract SQLiteOpenHelper class, and therefore must override the oncreate and onupgrade methods.

25 SQLite Overview Implementation oncreate Automatically called when the application starts for the first time; its job is to create the database onupgrade Used to update a database on the phone when new version of the application is shipped

26 General elements of the class SQLite Overview Implementation Constants DATABASE NAME holds the filename of the database DATABASE VERSION defines the database version understood by the software that defines the constant. If the version of the database on the machine is less than DATABASE VERSION, the application should run onupgrade

27 General elements of the class SQLite Overview Implementation Constructor call constructor from super store a context object (usually an activity that opens the database)

28 oncreate Outline SQLite Overview Implementation You can use two approaches to the implementation that are : Embedded SQL SQL commands are embedded into Java. The code is in a single file Separated SQL SQL command are stored into XML file strings.xml and recalled when needed. Code distributed among two (potentially more) files

29 Reading Data from the Database SQLite Overview Implementation Create an SQL statement that describes the data that you need to retrieve. Execute that statement against the database. Map the resulting SQL data into data structures that the language you re working in can understand.

30 SQLite Overview Implementation Better approach Android offers slightly better approach custom cursors. Coursor It is a class encapsulating a query results. Query returns a simple cursor, while custom cursor provides (in addition) methods dealing with specific structure of the data

31 SQLite Overview Implementation

32 Overview Methods to implement URI like SQL 1 Overview 2 3 SQLite Overview Implementation 4 Overview Methods to implement URI like SQL 5 Internal storage External storage

33 Content Provider Overview Overview Methods to implement URI like SQL What is Content Provider It is a mechanism enabling data sharing across applications. In general reading from other application is not allowed. Both reader and writer (producer/consumer) uses an API to provide or access data

34 Nine steps to success Outline Overview Methods to implement URI like SQL 1 Extend the ContentProvider class. 2 Define the CONTENT URI for your content provider. 3 Create the data storage for your content. 4 Create the column names for communication with clients. 5 Define the process by which binary data is returned to the client. 6 Declare public static Strings that clients use to specify columns. 7 Implement the CRUD methods of a Cursor to return to the client. 8 Update the AndroidManifest.xml file to declare your < provider >. 9 Define MIME types for any new data types.

35 Overview Methods to implement URI like SQL oncreate This method is called during the content provider s startup. Any code you want to run just once, such as making a database connection, should reside in this method. gettype This method, when given a URI, returns the MIME type of the data that this content provider provides at that URI. The URI comes from the client application interested in accessing the data.

36 Overview Methods to implement URI like SQL insert/create This method is called when the client code wishes to insert data into the database your content provider is serving. Normally, the implementation for this method will either directly or indirectly result in a database insert operation. query/read This method is called whenever a client wishes to read data from the content provider s database. It is normally called through ContentProvider s managedquery method. Normally, here you retrieve data using a SQL SELECT statement and return a cursor containing the requested data.

37 Overview Methods to implement URI like SQL update This method is called when a client wishes to update one or more rows in the ContentProvider s database. It translates to a SQL UPDATE statement. delete This method is called when a client wishes to delete one or more rows in the ContentProvider s database. It translates to a SQL DELETE statement.

38 URI Outline Overview Methods to implement URI like SQL where: content://authority/path/id content:// is the standard required prefix. authority is the name of the provider. Using your fully qualified package name is recommended to prevent name collisions. path is a virtual directory within the provider that identifies the kind of data being requested. id is the primary key of a specific record being requested. To request all records of a particular type, omit this and the trailing slash.

39 Overview Methods to implement URI like SQL Default content provided by Android content://browser content://contacts content://media content://settings

40 Overview Methods to implement URI like SQL

41 Internal storage External storage 1 Overview 2 3 SQLite Overview Implementation 4 Overview Methods to implement URI like SQL 5 Internal storage External storage

42 Storage types Outline Internal storage External storage Internal storage It s always available. No special permission required Files saved here are accessible by only your app by default. When the user uninstalls your app, the system removes all your app s files from internal storage. External storage It s not always available (e.g. removed USB storage or SD card) It s world-readable, so files saved here may be read outside of your control. When the user uninstalls your app, the system removes your app s files only from getexternalfilesdir().

43 Which one should I choose? Internal storage External storage Internal storage Internal storage is best when you want to be sure that neither the user nor other apps can access your files. External storage External storage is the best place for files that don t require access restrictions and for files that you want to share with other apps or allow the user to access with a computer.

44 Storage and App location Internal storage External storage App installation Although apps are installed onto the internal storage by default, you can specify the android:installlocation attribute in your manifest so your app may be installed on external storage. Users appreciate this option when the APK size is very large and they have an external storage space that s larger than the internal storage.

45 Required permissions for external Internal storage External storage 1 <uses p e r m i s s i o n a n d r o i d : name= a n d r o i d. p e r m i s s i o n. READ EXTERNAL STORAGE /> 1 <uses p e r m i s s i o n a n d r o i d : name= a n d r o i d. p e r m i s s i o n. WRITE EXTERNAL STORAGE /> Note You need only one of the above because WRITE_EXTERNAL_STORAGE implicitly gives you a READ permission.

46 Internal Storage Outline Internal storage External storage Your app s internal storage directory is specified by your app s package name in a special location of the Android file system. Another app can read your internal files if you set the file mode to be readable but Other apps cannot browse your internal directories and do not have read or write access unless you explicitly set the files to be readable or writable. If you use MODE_PRIVATE for your files on the internal storage, they are never accessible to other apps.

47 Create a file Outline Internal storage External storage 1 F i l e f i l e = new F i l e ( c o n t e x t. g e t F i l e s D i r ( ), f i l e n a m e ) ; Figure: Create empty file

48 Create a file Outline Internal storage External storage 1 S t r i n g f i l e n a m e = m y f i l e ; 2 S t r i n g s t r i n g = H e l l o w o r l d! ; 3 F i l e O u t p u t S t r e a m outputstream ; 4 t r y { 5 outputstream = o p e n F i l e O u t p u t ( f i l e n a m e, Context. MODE PRIVATE) ; 6 outputstream. w r i t e ( s t r i n g. g e t B y t e s ( ) ) ; 7 outputstream. c l o s e ( ) ; 8 } c a t c h ( E x c e p t i o n e ) { 9 e. p r i n t S t a c k T r a c e ( ) ; 10 } Figure: Alternatively, you can write to a file in your internal directory using a stream.

49 Create a file Outline Internal storage External storage 1 p u b l i c F i l e gettempfile ( Context c o n t e x t, S t r i n g u r l ) { 2 F i l e f i l e ; 3 t r y { 4 S t r i n g f i l e N a m e = U r i. p a r s e ( u r l ). getlastpathsegment ( ) ; 5 f i l e = F i l e. c r e a t e T e m p F i l e ( filename, n u l l, c o n t e x t. g e t C a c h e D i r ( ) ) ; 6 c a t c h ( I O E x c e p t i o n e ) { 7 // E r r o r w h i l e c r e a t i n g f i l e 8 } 9 r e t u r n f i l e ; 10 } Figure: Create a temporary file e.g, to cache some data

50 External storage Outline Internal storage External storage Because the external storage may be unavailable such as when the user has mounted the storage to a PC or has removed the SD card that provides the external storage you should always verify that the volume is available before accessing it. You can query the external storage state

51 Storage state Outline Internal storage External storage Checks if external storage is available to at least read 1 p u b l i c b o o l e a n i s E x t e r n a l S t o r a g e R e a d a b l e ( ) { 2 S t r i n g s t a t e = Environment. g e t E x t e r n a l S t o r a g e S t a t e ( ) ; 3 i f ( Environment.MEDIA MOUNTED. e q u a l s ( s t a t e ) 4 Environment. MEDIA MOUNTED READ ONLY. e q u a l s ( s t a t e ) ) { 5 r e t u r n t r u e ; 6 } 7 r e t u r n f a l s e ; 8 }

52 Storage state Outline Internal storage External storage Checks if external storage is available for read and write 1 p u b l i c b o o l e a n i s E x t e r n a l S t o r a g e W r i t a b l e ( ) { 2 S t r i n g s t a t e = Environment. g e t E x t e r n a l S t o r a g e S t a t e ( ) ; 3 i f ( Environment.MEDIA MOUNTED. e q u a l s ( s t a t e ) ) { 4 r e t u r n t r u e ; 5 } 6 r e t u r n f a l s e ; 7 }

53 Internal storage External storage File management in exernal storage Public files Files that should be freely available to other apps and to the user. When the user uninstalls your app, these files should remain available to the user. photos downloaded files Private files Files that rightfully belong to your app and should be deleted with the app. These files are accessible by the user and other apps but they are files that don t provide value to the user outside your app. additional resources temporary media files

54 Query free space Outline Internal storage External storage Query If you know ahead of time how much data you re saving, you can find out whether sufficient space is available without causing an IOException by calling getfreespace() or gettotalspace() Warning The system does not guarantee that you can write as many bytes as are indicated by getfreespace().

55 Internal storage External storage