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Learning Android

by Marko Gargenta

I, Michael Parker, own this book and took these notes to further my own learning. If you enjoy these notes, please purchase the book!

Chapter 3: Quick Start

  • pg 20: The mainfest file explains what the application consits of, what its main building blocks are, what permissions it requires, and so on.
  • pg 21: The layout XML is responsible for the layout of widgets, while the strings XML is responsible for their textual content.

Chapter 4: Main Building Blocks

  • pg 29: An activity transitioning from a starting state to a running state is an expensive operation.
  • pg 30: An activity that is visible but not in focus is paused. Once not visible, it is in a stopped state.
  • pg 30: A destroyed activity is no longer in memory. There's no guarantee that it will be stopped before being destroyed, so do all important work en route to the paused state.
  • pg 31: An explicit intent specifies the receiving component, while an implicit intent specifies only the type of receiver.
  • pg 32: Services are either starting, running, or destroyed, and this life cycle is controlled by the developer and not so much by the system.
  • pg 34: Broadcast receivers are a system-wide publish-subscribe system. Receivers are not actively running in memory, but do get to run some code when triggered.

Chapter 6: Android User Interface

  • pg 48: Use XML to declare everything about the UI that is static, and then a programmatic approach to define what happens when the user interacts with its widgets.
  • pg 49: Layouts allocate space for their children, which can be views or other layouts in turn.
  • pg 50: If you nest multiple LinearLayout instances, consider RelativeLayout. Heavy nesting has negative effects on time to start the activity, CPU, and battery consumption.
  • pg 50: AbsoluteLayout positions its children at absolute coordinates on the screen. Although simple, it breaks when screen size, density, or orientation changes.
  • pg 54: Specifying fill_parent for layout_height or layout_width uses all available space, while wrap_content only uses as much space as is necessary.
  • pg 55: The layout_gravity specifies how a widget is positioned within its layout, while gravity specifies how the widget's content is positioned within the widget itself.
  • pg 57: The setContentView inflates from XML, or parses the XML and creates a corresponding Java object for each element.
  • pg 62: Beware that if your project doesn't have R.java generated, then Organize Imports will import the android.R class.
  • pg 64: A terminal running adb logcat at all times is faster for debugging than switching to the DDMS perspective in Eclipse.
  • pg 67: The AsyncTask class is used to help handle long operations that need to report to the UI thread when progress is made or finally completed.
  • pg 75: Do not use extensions when referring to other file resources; Android figures out the best file format automatically.
  • pg 77: Android optionally expands on the RGB color set with the alpha channel, so you can express values for each channel in hexadecimal as #ARGB or #AARRGGBB.
  • pg 79: Alternative resources, like strings in different languages or images with different pixel density, work by specifying the qualifiers in the names of their resource folders.
  • pg 81: The Hierarchy Viewer tool attaches to any device or emulator and introspect the structure of the current view. For performance, aim for flat, nested layouts.

Chapter 7: Preferences, the Filesystem, the Options Menu, and Intents

  • pg 87: A subclass of PreferenceActivity uses addPreferencesFromResource() instead of setContentView() to set its content from an XML file containing preferences.
  • pg 88: Any building block like an activity, service, broadcast receiver, or content provider must be defined in the AndroidManifest.xml file.
  • pg 90: The "title condensed" attribute of a menu item is shown instead of the title attribute if space is limited.
  • pg 91: The onCreateOptionsMenu() is only called once to inflate the menu, and doesn't get called again until the activity is destroyed.
  • pg 96: The /sdcard partition is a poorly structured, free-for-all partition that is a suitable place to store large files such as music, photos, or videos.
  • pg 97: The data subfolder of the /data partition contains subfolders corresponding to each application, each named by the package used to sign the corresponding application.

Chapter 8: Services

  • pg 101: An unbound service runs independently of activities. A bound service provides more specific APIs through Android Interface Definition Language, or AIDL.
  • pg 102: As long as any part of your app is running, the Application object will be created, and so is a good place for common state.
  • pg 104: You must add an android.name attribute to the application element in AndroidManifest.xml to specify your subclass of Application.
  • pg 107: The onStartCommand method is called whenever the service receives a startService intent, and unlike onCreate and onDestroy, can be called repeatedly.

Chapter 9: The Database

  • pg 120: The single-file nature of SQLite makes security straightforward, as it boils down to filesystem security.
  • pg 122: Class SQLiteDatabase supports prepared statements for INSERT, UPDATE, DELETE, and SELECT; all other SQL statements must be executed directly.
  • pg 124: The versioning provided by SQLiteOpenHelper simplifies recognizing when the schema has changed and tables must be altered.
  • pg 127: The database is stored in the databases subdirectory in your application directory along the /data/data path.
  • pg 129: On the command line, sqlite3 will not complain if the file you refer to does not exist, and will simply create a new database.

Chapter 10: Lists and Adapters

  • pg 138: A ScrollView contains only one direct child, and should have its width and height specified as fill_parent.
  • pg 141: The startManagingCursor() method of Activity manages the cursor's life cycle the same way it manages its own.
  • pg 143: The ListActivity is convenient where the built-in ListView is the only widget in the activity.
  • pg 148: The method DateUtils.getRelativeTimeSpan provides a human-readable relative time since a given timestamp.
  • pg 150: The ViewBinder assigned to an adapter specifies what data should be bound in the default manner, and what data requires a custom bind.
  • pg 151: The category android.intent.category.LAUNCHER must be added to an activity's <intent_filter> for the application to be shown in the launcher drawer.
  • pg 156: The onMenuOpened() callback allows you to customize menu items before the menu is displayed.

Chapter 11: Broadcast Receivers

  • pg 163: If you don't specify a needed permission, you won't be notified when the event occurs, which could be a hard bug to find.
  • pg 167: If broadcasts are sent with an intent that no code is listening for, the broadcasts are simply ignored.
  • pg 169: The BOOT_COMPLETED intent requires the RECEIVE_BOOT_COMPLETED permission, but CONNECTIVITY_CHANGE requires ACCESS_NETWORK_STATE.
  • pg 172: A receiver can accept broadcasts only from senders with permission to send, while a sender can broadcast only to receivers with a permission to receive.

Chapter 12: Content Providers

  • pg 176: The URI for a content provider has an authority named after the class, one or more segments to specify the data type, and an optional identifier.
  • pg 179: For updating, deleting, and querying data, the selection argument and some others are only used if no identifier was specified in the URI.
  • pg 180: The query method should not close the database, as that will destroy the returned cursor.
  • pg 184: The onUpdate method of a widget should update all widgets specified by the appWidgetIds parameter.
  • pg 185: The include element in a layout allows reusing one layout inside another so that code is not duplicated.

Chapter 13: System Services

  • pg 190: Requesting updates from a system service may drain battery, so register for updates in onResume and unregister in onPause.
  • pg 194: The onDraw method of a view draws the view on the given canvas.
  • pg 199: Android divides location permissions into abstract fine location and coarse location permissions, such as GPS and wireless networks, respectively.
  • pg 206: An IntentService is a subclass of Service but runs on its own worker thread, and the work to perform is put in its onHandleIntent method.
  • pg 209: A PendingIntent allows you to start an activity, start a service, or send a broadcast at a future time.
  • pg 211: The ELAPSED_REALTIME parameter will keep the AlarmManager from waking up the phone just to run a scheduled alarm.
  • pg 214: The getString method of Context accepts a variable number of arguments at the end and applies them to the string from strings.xml using String.format.