Storage

# Warm up:

Recall our [Hope/Expectation](https://edstem.org/us/courses/21053/lessons/31147/slides/186236)
from the first day of class.

Now fill out this [survey](https://edstem.org/us/courses/21053/lessons/31165/slides/180492)...

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# Storage

Lauren Bricker

CSE 340 Spring 2022

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[//]: # (Outline Slide)
# Today's goals

- Administrivia
  - Layout part 3-4 & reflection due Thursday
  - Practice quiz 3 out, due Sunday 10pm
- Finish [Properties of People - Vision](people-vision.html#53)
- Storage
  - Bundles
  - Shared Preferences
- [Android Security](security.html)

---
# So you got lots of apps..

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![:img Android Activity Lifecycle which shows when the operating system starts; pauses and kills android processes, 80%, width](img/storage/activity_lifecycle.png)
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They all want to use _tons_ of memory...
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...but they don't **need** that memory all the time.
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# So you got lots of apps..

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![:img Android Activity Lifecycle which shows when the operating system starts; pauses and kills android processes, 80%, width](img/storage/activity_lifecycle.png)

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What happens when the user closes an application? Does it completely go away?
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It depends...
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...was it completely unloaded or did it essentially hibernate?
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However....
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# So you got lots of apps..

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![:img Android Activity Lifecycle which shows when the operating system starts; pauses and kills android processes, 80%, width](img/storage/activity_lifecycle.png)

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**Android**: You get no/minimal memory when you're not actively being used.
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**Apps**: But wait! You took away my memory, all my variables are _GONE_ 😲.
How do I remember things when I *am* being used if you take away my memory?
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**Android**: Store it!
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# Storage options

There are two main types different storage options to save data -

- Short term volatile memory
  - Data that goes away when the phone is turned off
- Long term memory
  - Data that can be recovered when the phone is turned off and back on

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# Short Term Storage option

[Bundle](https://developer.android.com/reference/android/os/Bundle)

- Usually stores a small amount of data
- Saved RIGHT before the memory is cleared.
- Typically used when you want to safely transfer data between activities or between an activity and a fragment.
- The `Bundle` is sent back when the activity starts running again.
- A `Bundle` is destroyed if the user **force quits** the app/phone is turned off.

.footnote[More on [Activity state and ejection from memory](https://developer.android.com/guide/components/activities/activity-lifecycle#asem)]
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# What is in the Bundle?

You, the application developer, decides what needs to be stored

Values are mapped to unique KEYS (for set/retrieve)

The `Bundle` is like a `Map<String, Object>` that only supports `Serializable`
  objects (like primitives and `String`)

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# Bundles: Code Example

```java
public abstract class MainActivity extends AppCompatActivity {

@Override
  public void onSaveInstanceState(Bundle outState) {
      super.onSaveInstanceState(outState);

outState.putString("OUR_KEY", "bundles? bundles!!!");
  }

@Override
  protected void onRestoreInstanceState(Bundle savedInstanceState) {
      super.onRestoreInstanceState(savedInstanceState);

String whatWeSaved = savedInstanceState.getString("OUR_KEY");
      // whatWeSaved would contain "bundles? bundles!!!"
  }
}
```
---
# Example from Layout

In `TabActivity.java`:

```java
protected void onSaveInstanceState(Bundle outState) {
    super.onSaveInstanceState(outState);
    outState.putInt(CURRENT_TAB_ID_KEY, mTabId);
}
protected int getSavedTabId(Bundle state, @IdRes int defaultVal) {
  if (state == null) { return defaultVal; }
  return state.getInt(CURRENT_TAB_ID_KEY, defaultVal);
}
```

We will see this more in Accessibility

---

# Bundles & Code

- Bundle management is occuring at the `Activity` layer, not to be confused with any individual `View`.
- When the user closes the app, right before it closes and its memory is cleared, Android invokes
`onSaveInstanceState(Bundle outState)` on the activity, providing a reference to the activity for the app to save values into.
- When the user opens the app again, Android invokes `onRestoreInstanceState(Bundle savedInstanceState)`
returning the same bundle from earlier.

_(Read more [here](https://developer.android.com/topic/libraries/architecture/saving-states))_

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# Bundles & Code

- Bundle data is transient - it will go away if the app is completely unloaded from memory.
  - Typically small amounts of data.
  - Used to share information between activities.

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# How can we store things for longer??!?!?

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# Long Term Storage

Longer term, persistent storage stay around even if the phone is turned off.

Shared Preferences
- Store private primitive data in key-value pairs.
Internal Storage
- Store private data on the device memory.
External Storage
- Store public data on the shared external storage.
SQLite Databases
- Store structured data in a private database.
Network Connection
- Store data on the web with your own network server.

.footnote[From [Android documentation](https://android-doc.github.io/guide/topics/data/data-storage.html)]
---

# Long Term Storage

Note that the app *could* write/read its state to longer term storage whenever it is being closed/opened ...

--
... but that is time consuming and would delay the OS launching new things...

---
# Shared Preferences

Stores data in persistent internal memory on the phone.

Like `Bundle`, also stored in Key Value pairs

[SharedPreferences](https://developer.android.com/reference/android/content/SharedPreferences) documentation

---
# Shared Preferences

To see what is in your shared preferences

1. start the Device File Explorer
(View->Tool windows->Device File Explorer)
2. Use the toggles to open up the following folders

````
data
  data
    <packagename> for your app, such as cse340.askforhelp
      shared_prefs
        <packagename>.PREFERENCES.xml
```
---
# Shared Preferences

Creating the Shared Preferences File

```java
protected SharedPreferences getPrefs() {
  if ( mSharedPreferences == null ) {
      try {
          Context context = getApplicationContext();
          mSharedPreferences = context.getSharedPreferences(context.getPackageName() + ".PREFERENCES",
                  Context.MODE_PRIVATE);
      } catch (Exception e) {
          //failed to edit shared preferences file
          showToast(R.string.shared_pref_error);
      }
  }
  return mSharedPreferences;
}
```

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# Shared Preferences

Saving to/restoring from the Shared Preferences file.

Example (if we chose to store in SharedPreferences)

```java
// setting data
SharedPreferences.Editor editor = getPrefs().edit();
editor.putInt(CURRENT_TAB_ID_KEY, mTabId);
editor.apply();  // have to force the write.

// getting data, the last parameter is the default.
mTabId = getPrefs().getInt(CURRENT_TAB_ID_KEY, 1);

```

We will see this in Accessibility

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# Shared Preferences