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  3. [Android Dev] 2.2 Preference

[Android Dev] 2.2 Preference

android Series Part 6 of «Associate Android Developer Fast Track»
目录

Save settings and configurations.

Data Persistance

5 different ways of data persistance:

  • onSavedInstanceState(): store state of views in k-v pairs (Bundles), used when screen rotates / app killed by system, temperary.
  • SharedPreferences: save k-v pairs to a file, can save primitive types.
  • SQLite database: complicated data types
  • Internal / External Storage: save large files to local phone, ex. podcast app / camera app.
  • Server: ex. leaderboard for a game (Firebase)

PreferenceFragments

PreferenceFragment is a class to handle the storing of user preferences.

Fragment

Fragment is a class that represents a modular and reusable piece of an Activity. ex. a booklist-bookdetail app, for tablets want both screens (list/detail) on the same screen ⇒ put the 2 screens into Fragments, and re-use them.

PreferenceFragment

populated with preferences defined in an XML, and (automatically) update k-v pairs in a sharedpref file.

in the pref XML file: define what are in the preferences, keys(names) and default values, and summary of a pref item.

Adding a settings screen from a pref-xml file Recipe

1. create a settings activity

  • add a menu xml, in the xml, add a menu item
  • show menu in mian activity
  • when "settings" item is clicked: open settings activity

2. create a preferences xml

  • setup gradle dependency (build.gradle):

compile 'com.android.support:preference-v7:25.1.0'

  • create a xml file in res/xml folder, add CheckBoxPreference.

    <?xml version="1.0" encoding="utf-8"?>

3. create a class that extends PreferenceFragment

// TODO (2) Create a class called SettingsFragment that extends PreferenceFragmentCompat
public class SettingsFragment extends PreferenceFragmentCompat{
// TODO (5) In SettingsFragment's onCreatePreferences method add the preference file using the
// addPreferencesFromResource method
    @Override
    public void onCreatePreferences(Bundle savedInstanceState, String rootKey) {
        addPreferencesFromResource(R.xml.pref_visualizer);
    }
}

4. change root element of settings activity to a fragment

<?xml version="1.0" encoding="utf-8"?>
<fragment xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_settings"
    android:name="android.example.com.visualizerpreferences.SettingsFragment"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
</fragment>

5. add a preferenceTheme

in res/values/styles.xml:

<item name="preferenceTheme">@style/PreferenceThemeOverlay</item>

6. read from SharedPreferences and update

in onStart or onResume of main activity: 

// TODO (2) Get a reference to the default shared preferences from the PreferenceManager class
SharedPreferences pref = PreferenceManager.getDefaultSharedPreferences(this);
// TODO (3) Get the value of the show_bass checkbox preference and use it to call setShowBass

mVisualizerView.setShowBass(pref.getBoolean("show_bass", true));

best practice: using resources instead of using constant iterals everywhere.

i.e. extract strings/bools into strings.xml or bools.xml: 

<string name="pref_show_bass_title">Show Bass</string>
<string name="pref_show_bass_key" translatable="false">show_bass</string>
<string name="pref_show_bass_summaryon">Shown</string>
<string name="pref_show_bass_summaryoff">Hidden</string>
<bool name="pref_show_bass_default">true</bool>

in other xml files: 

<CheckBoxPreference
        android:defaultValue="@bool/pref_show_bass_default"
        android:key="@string/pref_show_bass_key"
        android:summaryOff="@string/pref_show_bass_summaryoff"
        android:summaryOn="@string/pref_show_bass_summaryon"
        android:title="@string/perf_show_bass_title" />

in java files: 

mVisualizerView.setShowBass(perf.getBoolean(getString(R.string.pref_show_bass_key),
                    getResources().getBoolean(R.bool.pref_show_bass_default)));

PreferenceChangeListener

instead of updating preferences in onStart/onResume of main activity, use onPreferenceChangeListener, this is called whenever a sharedpref is changed.

Reciept

step 1: determine the activity who's UI needs to be changed when pref changes — in our case, is the main activity

step 2: let this activity implement OnSharedPreferenceChangeListener and override the onSharedPreferenceChanged method

@Override
public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
    Preference pref = findPreference(key);
    if (!(pref instanceof CheckBoxPreference)) {
        String value = sharedPreferences.getString(key, "");
        setPreferenceSummary(pref, value);
    }
}

step 3: link this Listener to the SharedPreference object that it listens to, using registerOnSharedPreferenceChangeListener in onCreate(): 

SharedPreferences sharedPreferences = PreferenceManager.getDefaultSharedPreferences(this);
sharedPreferences.registerOnSharedPreferenceChangeListener(this);

step 4: unregister the Listener when activity is shut down in onDestroy(): 

@Override
protected void onDestroy() {
    super.onDestroy();
    PreferenceManager.getDefaultSharedPreferences(this)
            .unregisterOnSharedPreferenceChangeListener(this);
}

List Preference

Compared to CheckBoxPreference, ListPreference offers an array of possible choices.

ListPreference Recipe

define options labels/values in res/values/strings.xml:

<string name="pref_color_key">pref_color</string>
<string name="pref_color_title">Pick a color</string>
<string name="pref_color_red_label">Red</string>
<string name="pref_color_blue_label">Blue</string>
<string name="pref_color_green_label">Green</string>

<string name="pref_color_red_value" translatable="false">color_red</string>
<string name="pref_color_blue_value" translatable="false">color_blue</string>
<string name="pref_color_green_value" translatable="false">color_green</string>

(note: xx_value is for internal usage, xx_lablel is what is shown on screen to users)

create res/values/arrays.xml: 

<?xml version="1.0" encoding="utf-8"?>
<!-- TODO (3) Add a res->values->arrays.xml file which contains two arrays, one for
        labels and one for values. The arrays should contain strings found in this file-->
<resources>
    <array name="pref_color_option_labels">
        <item>@string/pref_color_red_label</item>
        <item>@string/pref_color_blue_label</item>
        <item>@string/pref_color_green_label</item>
    </array>
    <array name="pref_color_option_values">
        <item>@string/pref_color_red_value</item>
        <item>@string/pref_color_blue_value</item>
        <item>@string/pref_color_green_value</item>
    </array>
</resources>

in preference xml: add a ListPreference element

<ListPreference
        android:defaultValue="@string/pref_color_red_value"
        android:entries="@array/pref_color_option_labels"
        android:entryValues="@array/pref_color_option_values"
        android:key="@string/pref_color_key"
        android:title="@string/pref_color_title"/>

finally in main activity: 

String default_color_value = getString(R.string.pref_color_red_value);
mVisualizerView.setColor(sharedPreferences.getString(getString(R.string.pref_color_key),
                default_color_value) );

add summary for list preference

we have to this programatically in the java code.

Here is the recipe:

step 1: let the SettingsFragment class implement OnSharedPreferenceChangeListener:

// TODO (1) Implement OnSharedPreferenceChangeListener
public class SettingsFragment extends PreferenceFragmentCompat implements OnSharedPreferenceChangeListener

step 2:set preference's summary by setSummary():

// TODO (2) Create a setPreferenceSummary which takes a Preference and String value as parameters.
// This method should check if the preference is a ListPreference and, if so, find the label
// associated with the value. You can do this by using the findIndexOfValue and getEntries methods
// of Preference.
private void setPreferenceSummary(Preference sharedPref, String value) {
    if (sharedPref instanceof ListPreference) {
        ListPreference listPref = (ListPreference) sharedPref;
        int idx = listPref.findIndexOfValue(value);
        listPref.setSummary(listPref.getEntries()[idx]);
    }
}

step 3: set pref summary in onCreatePreferences: 

@Override
public void onCreatePreferences(Bundle bundle, String s) {

    // Add visualizer preferences, defined in the XML file in res->xml->pref_visualizer
    addPreferencesFromResource(R.xml.pref_visualizer);

    // TODO (3) Get the preference screen, get the number of preferences and iterate through
    // all of the preferences if it is not a checkbox preference, call the setSummary method
    // passing in a preference and the value of the preference
    PreferenceScreen prefScreen = getPreferenceScreen();
    int nPrefs = prefScreen.getPreferenceCount();
    for(int i=0; i<nPrefs; i++){
        Preference p = prefScreen.getPreference(i);
        if(p instanceof CheckBoxPreference) continue;
        else {
            String value = prefScreen.getSharedPreferences().getString(p.getKey(), "");
            setPreferenceSummary(p, value);
        }
    }
}

step 4: implement onSharedPreferenceChanged listener: 

// TODO (4) Override onSharedPreferenceChanged and, if it is not a checkbox preference,
// call setPreferenceSummary on the changed preference
@Override
public void onSharedPreferenceChanged(SharedPreferences sharedPreferences, String key) {
    if (sharedPreferences instanceof CheckBoxPreference) return;
    else{
        Preference pref = findPreference(key);
        String value = sharedPreferences.getString(key, "");
        setPreferenceSummary(pref, value);
    }

}

step 5: finally register/unregister the listener in oncreate/ondestroy: 

// TODO (5) Register and unregister the OnSharedPreferenceChange listener (this class) in
// onCreate and onDestroy respectively.
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    getPreferenceScreen().getSharedPreferences()
            .registerOnSharedPreferenceChangeListener(this);
}

@Override
public void onDestroy() {
    super.onDestroy();
    getPreferenceScreen().getSharedPreferences()
            .unregisterOnSharedPreferenceChangeListener(this);
}
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