Android Development Notes

July 10, 2019 1425 words 7 minutes

Contents

Table of Contents
Basics
App Resources
Permissions
Gradle
Layout
Activities
Fragments
Notifications
Android Studio Tips
Miscellaneous
Resources

Basics

Layout is typically defined in XML, whereas activities are in Java class.
The Android operating system is a multi-user Linux system in which each app is a different user.

App Resources

String array: android:entries="@array/foo".

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<string-array name="foo">
    <item>bar1</item>
    <item>bar2</item>
</string-array>

Creating alias resources:

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<?xml version="1.0" encoding="utf-8"?>
<resources>
    <drawable name="icon">@drawable/icon_ca</drawable>
</resources>

Styles and Themes
Referencing style attributes: android:textColor="?android:textColorSecondary
Support different pixel densities
Declare restricted screen support

Permissions

Gradle

compileSdkVersion vs targetSdkVersion vs minSdkVersion:
- compileSdkVersion: The app can use all the API features from this version and lower.
- targetSdkVersion: The version we’ve tested our app.
- minSdkVersion: The minimum version the app can support.

Layout

<EditText> is an editable text field:
- android:hint="@string/hint": normally “Enter a message”, telling the user what to type;
- android:ems="10": 10-M space.
android:layout_weight="number" makes a view stretch.
- If a has the weight of 1 and b has 2, a’ll have 1/3 and b’ll have 2/3 of the screen.
- We usually have android:layout_height="0dp" (for vertical layout) above layout_weight.
android:gravity="top" moves the CONTENT of a view to the top of the view, whereas android:layout_gravity="end" moves the placement of the VIEW itself to the end.
<FrameLayout></FrameLayout> allows views to overlap. Ues it when we need to replace the fragments and add the changes to the back stack.
Surround LinearLayout/FrameLayout with <ScrollView></ScrollView> to get a vertical scrollbar. (HorizontalScrollView for the horizontal scrollbar.)
A CoordinatorLayout allows the behavior of one view to affect the behavior of another.

AdapterView:

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ArrayAdapter<String> adapter = new ArrayAdapter<String>(this,
    android.R.layout.simple_list_item_1, myStringArray);
ListView listView = (ListView) findViewById(R.id.listview);
listView.setAdapter(adapter)
// Create a message handling object as an anonymous class.
private OnItemClickListener messageClickedHandler = new OnItemClickListener() {
    public void onItemClick(AdapterView parent, View v, int position, long id) {
    // Do something in response to the click
    }
};

listView.setOnItemClickListener(messageClickedHandler);

Create a List with RecyclerView
Re-using layouts with include: <merge></merge> <include layout="@layout/foo"/>

Spinners:

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Spinner s1 = (Spinner) findViewById(R.id.spinner1);
ArrayAdapter adapter = ArrayAdapter.createFromResource(
this, R.array.colors, android.R.layout.simple_spinner_item);
adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
s1.setAdapter(adapter);
s1.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
    @Override
    public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
        // TODO
    }

    @Override
    public void onNothingSelected(AdapterView<?> parent) {
       // sometimes you need nothing here
    }
});

Buttons Set onClick programmatically:

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Button button = (Button) findViewById(R.id.button_send);
button.setOnClickListener(new View.OnClickListener() {
    public void onClick(View v) {
        // Do something in response to button click
    }
});

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findViewById(R.id.button_send).setOnClickListener {
    foo(it)
}
// or
findViewById(R.id.button_send).setOnClickListener { view ->
    //...
}

Checkboxes
Radio Buttons

Switch Buttons

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android:checked="true"
android:textOff="OFF"
android:textOn="ON

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Switch sw = (Switch) findViewById(R.id.switch1);
sw.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
    public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
        if (isChecked) {
            // The toggle is enabled
        } else {
            // The toggle is disabled
        }
    }
});

Pickers (Time/Date Pickers)

Centralized onClick events:

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public class ActivityA extends Activity implements View.OnClickListener {
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        findViewById(R.id.first).setOnClickListener(this);
        findViewById(R.id.second).setOnClickListener(this);
        findViewById(R.id.third).setOnClickListener(this);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()) {
            case R.id.first:
                // Manage click.
                break;

            case R.id.second:
                // Manage click.
                break;

            case R.id.third:
                // Manage click.
                break;
        }
    }
}

App bar:

Set up the app bar (Use Toolbar instead of the native ActionBar) and Add an up action

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android:theme="@style/Theme.AppCompat.Light.NoActionBar"

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<android.support.v7.widget.Toolbar
    android:id="@+id/my_toolbar"
    android:layout_width="match_parent"
    android:layout_height="?attr/actionBarSize"
    android:background="?attr/colorPrimary"
    android:elevation="4dp"
    android:theme="@style/ThemeOverlay.AppCompat.ActionBar"
    app:popupTheme="@style/ThemeOverlay.AppCompat.Light"/>

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    <!-- A child of the main activity -->
<activity
   android:name="com.example.myfirstapp.MyChildActivity"
   android:label="@string/title_activity_child"
   android:parentActivityName="com.example.myfirstapp.MainActivity" >

<!-- Parent activity meta-data to support 4.0 and lower -->
    <meta-data
        android:name="android.support.PARENT_ACTIVITY"
        android:value="com.example.myfirstapp.MainActivity" />
</activity>

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// OnCreate
Toolbar myToolbar = (Toolbar) findViewById(R.id.my_toolbar);
setSupportActionBar(myToolbar);
    // Get a support ActionBar corresponding to this toolbar
ActionBar ab = getSupportActionBar();

// Enable the Up button
ab.setDisplayHomeAsUpEnabled(true);

Add and handle actions

Toasts: Toast toast = Toast.makeText(this, "hello world", Toast.LENGTH_SHORT); toast.show();

Activities

All activities have to extend the Activity class or its subclass.
R is a special Java class that enables you to retrieve references to resources. TextView foo = findViewById(R.id.foo)

Use Intent to call another activity (can be from other apps):

Explicit intent:

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public void onSendMessage(View view) {
    Intent intent = new Intent(this, FooActivity.class); // explicit intent
    intent.putExtra("message", value); // message is the name of the extra
    startActivity(intent);
}

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...
Intent intent = getIntent();
String string = intent.getStringExtra("message");

Implicit intent: (with actions to allow users to choose which app to run)

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Intent intent = new Intent(Intent.ACTION_SEND); // ACTION_DIAL/ACTION_WEB_SEARCH
intent.setType("text/plain");
intent.putExtra(Intent.EXTRA_TEXT, messageText);
String chooserTitle = getString(R.string.chooser);
// Ensure that the user always get the chance to choose an activity
Intent chosenIntent = Intent.createChooser(intent, chooserTitle);

// Start an activity if it's safe
if (intent.resolveActivity(getPackageManager()) != null) {
    startActivity(chosenIntent);
}

Common Intents

Save the instance state:

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// Either in onCreate savedInstanceState != null or use the method below
public void onRestoreInstanceState(Bundle savedInstanceState) {
    super.onRestoreInstanceState(savedInstanceState);
    seconds = savedInstanceState.getInt("seconds");
    running = savedInstanceState.getBoolean("running");
}

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    super.onSaveInstanceState(savedInstanceState);
    savedInstanceState.putInt("seconds", seconds);
    savedInstanceState.putBoolean("running", running);
}

Activity lifecycle:
- When we implement them (onCreate(), onStart(), onResume(), onPause(), onStop(), onRestart(), onDestroy()), we must call the super class methods.
- onResume() is called when the activity is started OR resumed; onPause() is called when the activity is paused OR stopped.
Defining launch modes (singleTop, singleTask, etc.)

Fragments

Fragment lifecycle:

Example:

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public static class ExampleFragment extends Fragment {
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.example_fragment, container, false);
        // In this case, this is false because the system is already inserting the inflated layout into the container
    }
}

Add the fragment programmatically:

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FragmentManager fragmentManager = getSupportFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
ExampleFragment fragment = new ExampleFragment();
fragmentTransaction.add(R.id.fragment_container, fragment);
fragmentTransaction.commit();

Fragment transactions:

Example:

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// Create new fragment and transaction
Fragment newFragment = new ExampleFragment();
FragmentTransaction transaction = getSupportFragmentManager().beginTransaction();

// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(R.id.fragment_container, newFragment);
transaction.addToBackStack(null);

// Commit the transaction
transaction.commit();

For dynamic fragments, use FrameLayout instead of Fragment and use FragmentTransaction.

Notifications

Create a Notification

Android Studio Tips

Improve code inspection with annotations (@Nullable, @NonNull…)
Find sample code
Tools attributes reference (tools:text="foo", tools:itemCount="3"...)
Debug your app ( private static final String TAG = "MyActivity"; Log.d(TAG, "foo");)
Write and View Logs with Logcat (e (error), w (warning), i (information), d (debug), v (verbose))
Debug Your layout with Layout Inspector
Manage your app’s UI resources with Resource Manager
Create app icons with Image Asset Studio
Create resizable bitmaps (9-Patch files)
Add Android App Links

Miscellaneous

Android builds the back stack to keep track of the activity/fragment transactions.

Create Deep Links to App Content:

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<intent-filter>
  ...
  <data android:scheme="https" android:host="www.example.com" />
  <data android:scheme="app" android:host="open.my.app" />
</intent-filter>

Verify Android App Links
Adding Swipe-to-Refresh To Your App (Add SwipeRefreshLayout as the parent of a single ListView or GridView.) (android.support.v4.widget.SwipeRefreshLayout)

Responding to Refresh Request

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mySwipeRefreshLayout.setOnRefreshListener(
    new SwipeRefreshLayout.OnRefreshListener() {
        @Override
        public void onRefresh() {
            Log.i(LOG_TAG, "onRefresh called from SwipeRefreshLayout");

            // This method performs the actual data-refresh operation.
            // The method calls setRefreshing(false) when it's finished.
            myUpdateOperation();
        }
    }
);

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// For refreshing in the app bar
@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {

        // Check if user triggered a refresh:
        case R.id.menu_refresh:
            Log.i(LOG_TAG, "Refresh menu item selected");

            // Signal SwipeRefreshLayout to start the progress indicator
            mySwipeRefreshLayout.setRefreshing(true);

            // Start the refresh background task.
            // This method calls setRefreshing(false) when it's finished.
            myUpdateOperation();

            return true;
    }

    // User didn't trigger a refresh, let the superclass handle this action
    return super.onOptionsItemSelected(item);
}