5cosc005w mobile application development lecture 8: …
TRANSCRIPT
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5COSC005W MOBILE APPLICATIONDEVELOPMENT
Lecture 8: Data Storage - Part II
Dr Dimitris C. Dracopoulos
Module Web page:
https://dracopd.users.ecs.westminster.ac.uk/DOCUM/courses/5cosc005w/5cosc005w.html
This material is based on Android Developer Fundamentals 2 by Google used under
a Creative Commons Attribution 4.0 International license.
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Data Storage
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Storage Options
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Storing data
● SQLite Databases—Structured data in a private database
● Shared Preferences—Private primitive data in key-value pairs
● Internal Storage—Private data on device memory
● External Storage—Public data on device or external storage
● Content Providers—Store privately and make available publicly
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Storing data beyond Android
● Network Connection—On the web with your own server
● Cloud Backup—Back up app and user data in the cloud
● Firebase Realtime Database—Store and sync data with NoSQL cloud database across clients in realtime
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
SQLite Database
● Ideal for repeating or structured data, such as contacts ● Android provides SQL-like database● Covered in previous week
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Files
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Android File System
● External storage -- Public directories
● Internal storage -- Private directories for just your app
Apps can browse the directory structure
Structure and operations similar to Linux and java.io
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Internal storage
● Always available
● Uses device's filesystem
● Only your app can access files, unless explicitly set to be readable or writable
● On app uninstall, system removes all app's files from internal storage
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
External storage
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● Not always available, can be removed
● Uses device's file system or physically external storage like SD card
● World-readable, so any app can read
● On uninstall, system does not remove files private to app
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
When to use internal/external storage
Internal is best when● you want to be sure that neither the user nor other apps
can access your files
External is best for files that● don't require access restrictions and for ● you want to share with other apps● you allow the user to access with a computer
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Save user's file in shared storage
● Save new files that the user acquires through your app to a public directory where other apps can access them and the user can easily copy them from the device
● Save external files in public directories
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Internal Storage
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Internal Storage
● Uses private directories just for your app
● App always has permission to read/write
● Permanent storage directory—getFilesDir()
● Temporary storage directory—getCacheDir()
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Creating a file
File file = new File(
context.getFilesDir(), filename);
Use standard java.io file operators or streams to interact with files
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
External Storage
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
External Storage● On device or SD card
● Set permissions in Android Manifest○ Write permission includes read permission
<uses-permission
android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
<uses-permission
android:name="android.permission.READ_EXTERNAL_STORAGE" />
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Always check availability of storage
public boolean isExternalStorageWritable() {
String state = Environment.getExternalStorageState();
if (Environment.MEDIA_MOUNTED.equals(state)) {
return true;
}
return false;
}
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Example external public directories
● DIRECTORY_ALARMS and DIRECTORY_RINGTONESFor audio files to use as alarms and ringtones
● DIRECTORY_DOCUMENTSFor documents that have been created by the user
● DIRECTORY_DOWNLOADSFor files that have been downloaded by the user
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developer.android.com/reference/android/os/Environment.html
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Accessing public external directories
1. Get a path getExternalStoragePublicDirectory()
2. Create file
File path = Environment.getExternalStoragePublicDirectory(
Environment.DIRECTORY_PICTURES);
File file = new File(path, "DemoPicture.jpg");
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
How much storage left?
● If there is not enough space, throws IOException
● If you know the size of the file, check against space○ getFreeSpace()○ getTotalSpace().
● If you do not know how much space is needed○ try/catch IOException
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Delete files no longer needed
● External storage
myFile.delete();
● Internal storage
myContext.deleteFile(fileName);
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Do not delete the user's files!
When the user uninstalls your app, your app's private storage directory and all its contents are deleted
Do not use private storage for content that belongs to the user!
For example
● Photos captured or edited with your app● Music the user has purchased with your app
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Shared Preferences
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Shared Preferences
● Read and write small amounts of primitive data as key/value pairs to a file on the device storage
● Covered in later here
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Other Storage Options
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Use Firebase to store and share data
Store and sync data with the Firebase cloud database
Data is synced across all clients, and remains available when your app goes offline
firebase.google.com/docs/database/
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Firebase Realtime Database
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● Connected apps share data
● Hosted in the cloud
● Data is stored as JSON
● Data is synchronized in realtime to every connected client
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Data storageAndroid Developer Fundamentals V2This work is licensed under a Creative Commons Attribution 4.0 International License.
Network Connection
● You can use the network (when it's available) to store and retrieve data on your own web-based services
● Use classes in the following packages○ java.net.*○ android.net.*
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
Shared Preferences
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
What is Shared Preferences?
● Read and write small amounts of primitive data as key/value pairs to a file on the device storage
● SharedPreference class provides APIs for reading, writing, and managing this data
● Save data in onPause()restore in onCreate()
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
Shared Preferences and Saved Instance State
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● Small number of key/value pairs
● Data is private to the application
Shared Preferences AND Saved Instance State
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
Shared Preferences vs. Saved Instance State
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● Persist data across user sessions, even if app is killed and restarted, or device is rebooted
● Data that should be remembered across sessions, such as a user's preferred settings or their game score
● Common use is to store user preferences
● Preserves state data across activity instances in same user session
● Data that should not be remembered across sessions, such as the currently selected tab or current state of activity.
● Common use is to recreate state after the device has been rotated
Shared Preferences vs. Saved Instance State
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
Creating Shared Preferences
● Need only one Shared Preferences file per app
● Name it with package name of your app—unique and easy to associate with app
● MODE argument for getSharedPreferences() is for backwards compatibility—use only MODE_PRIVATE to be secure
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
getSharedPreferences()
private String sharedPrefFile =
"com.example.android.hellosharedprefs";
mPreferences =
getSharedPreferences(sharedPrefFile,
MODE_PRIVATE);
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
Saving Shared Preferences
● SharedPreferences.Editor interface
● Takes care of all file operations
● put methods overwrite if key exists
● apply() saves asynchronously and safely
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
SharedPreferences.Editor
@Override
protected void onPause() {
super.onPause();
SharedPreferences.Editor preferencesEditor =
mPreferences.edit();
preferencesEditor.putInt("count", mCount);
preferencesEditor.putInt("color", mCurrentColor);
preferencesEditor.apply();
}
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
Restoring Shared Preferences
● Restore in onCreate() in Activity
● Get methods take two arguments—the key, and the default value if the key cannot be found
● Use default argument so you do not have to test whether the preference exists in the file
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
Getting data in onCreate()mPreferences = getSharedPreferences(sharedPrefFile, MODE_PRIVATE);
mCount = mPreferences.getInt("count", 1);
mShowCount.setText(String.format("%s", mCount));
mCurrentColor = mPreferences.getInt("color", mCurrentColor);
mShowCount.setBackgroundColor(mCurrentColor);
mNewText = mPreferences.getString("text", "");
// ...
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
Clearing
● Call clear() on the SharedPreferences.Editor and apply changes
● You can combine calls to put and clear. However, when you apply(), clear() is always done first, regardless of order!
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
clear()
SharedPreferences.Editor preferencesEditor =
mPreferences.edit();
preferencesEditor.clear();
preferencesEditor.apply();
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
Listening to Changes
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Shared PreferencesAndroid Developer Fundamentals V2
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Listening to changes● Implement interface
SharedPreference.OnSharedPreferenceChangeListener
● Register listener with registerOnSharedPreferenceChangeListener()
● Register and unregister listener in onResume() and onPause()
● Implement on onSharedPreferenceChanged() callback
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Shared PreferencesAndroid Developer Fundamentals V2
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Interface and callbackpublic class SettingsActivity extends AppCompatActivity
implements OnSharedPreferenceChangeListener { ...
public void onSharedPreferenceChanged(
SharedPreferences sharedPreferences, String key) {
if (key.equals(MY_KEY)) {
// Do something
}
}
}
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
Creating and registering listener
SharedPreferences.OnSharedPreferenceChangeListener listener =
new SharedPreferences.OnSharedPreferenceChangeListener() {
public void onSharedPreferenceChanged(
SharedPreferences prefs, String key) {
// Implement listener here
}
};
prefs.registerOnSharedPreferenceChangeListener(listener);
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Shared PreferencesAndroid Developer Fundamentals V2
This work is licensed under a Creative Commons Attribution 4.0 International License.
You need a STRONG reference to the listener
● When registering the listener the preference manager does not store a strong reference to the listener
● You must store a strong reference to the listener, or it will be susceptible to garbage collection
● Keep a reference to the listener in the instance data of an object that will exist as long as you need the listener
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