Guest blog on Safari books dev blog

I’ve been published!!!

A few months ago I was approached to write a few blog articles for the Safari Books dev blog. Today the first of my articles has gone live : Android 4.1 Jelly Bean Notifications

If you’ve come from there, hello! If you haven’t, go and have a read – let me know your thoughts.

Messages from the cloud with GCM

In May 2010 Google released a monumental update to it’s Android OS with the release of Android 2.2 (Froyo). Alongside the quite ridiculous improvement in performance over the previous version AND a host of amazing extra features (JIT compilation, USB tethering, Wi-Fi hotspot, an improved browser to name but a few) was the announcement of the Cloud to Device Messaging (C2DM) service, or to use it’s more commonly known name – push notifications. Push notifications systems have fast become an industry standard in the world of advanced mobile computing, allowing a central server to push data to individual devices, rather than the power hungry alternative of continually polling a server for updates.

Announced at Google I/O 2012 was the next iteration of C2DM, now called Google Cloud Messaging (GCM). Along with the moniker change, there are a few key changes between C2DM abnd GCM which add functionality and make it easier to use. Of note:

• Multicast messages: you can broadcast messages of up to 1000 registration IDs at the same time from a single request
• Multiple senders: messages to the same app can be sent by multiple parties thanks to a change in the way registration is handled
• Time-to-live (TTL) messages: messages can have a TTL value of between 0 to 4 weeks helping to ensure messages about events that expire don’t reach your users after the fact. The GCM servers will throttle the messages to prevent notification spam. Messages that can’t be instantly delivered will be stored by the Google servers until they are delievered or expire, messages with a TTL of 0 aren’t stored anywhere and also aren’t subject to any throttling.
• Payload: messages can bundle up to 4Kb (up from 1024 bytes with C2DM) of data to be delivered with the message, up to 100 of these messages can be stored on the GCM servers if they can’t instantly be delivered, after which you’ll receive an error code which your server will have to deal with.

Google states that C2DM will be maintained for moment but won’t accept any new users nor grant new quotas. They also point out that ‘C2DM and GCM are not interoperable. For example, you cannot post notifications from GCM to C2DM registration IDs, nor can you use C2DM registration IDs as GCM registration IDs. Your server-side application must keep keep track of whether a registration ID is from C2DM or GCM and use the proper endpoint.’ fortunately they provide easy to follow instructions as to how to migrate your apps from C2DM to GCM and google has even provided sample GCM apps (both client and server side) for you to look at (something which was sorely missing with C2DM)

If you haven’t yet used Google’s cloud messaging service, it’s now easier than ever – full instructions on getting started with GCM are given on the developer website. Just remember that it requires devices running android 2.2 (sdk version 8) or later.

Android styling tips

One area that has always been a bit lacking for Android developers is the online documentation at developer.android.com. It’s getting better with each new release but is still far from perfect – one area which has left many developers lost is styling. There are now plenty of guides on the internet discussing styling and theming but below are the two tips I wish someone had told me when I’d started Android development:

1, ‘?’
The question mark in a style refers to a dynamically defined theme value. In the following example, taken directly from the Android styles.xml file, the value of ‘android:textColor’ refers to the value of the dynamically defined “textColorPrimary” value:

<style name="TextAppearance">
<item name="android:textColor">?textColorPrimary</item>
<item name="android:textColorHighlight">?textColorHighlight</item>
<item name="android:textColorHint">?textColorHint</item>
<item name="android:textColorLink">?textColorLink</item>
<item name="android:textSize">16sp</item>
<item name="android:textStyle">normal</item>
</style>

As can be seen from the following excerpt from the android-16 themes.xml file, the exact value of the ‘textColorPrimary’ within ‘TextAppearance’, and hence the default text colour throughout the app, will vary depending on the theme applied on your application or activity in the AndroidManifest.xml file:

<style name="Theme.Light">
...
<item name="textColorPrimary">@android:color/primary_text_light</item>
...
</style>
<style name="Theme.Holo">
...
<item name="textColorPrimary">@android:color/primary_text_holo_dark</item>
...
</style>

So if we were to define our own theme and wanted to change the default text colour – we can easily override the above value in order to achieve that throughout all parts of our application which used our custom theme:

<style name="CustomTheme">
...
<item name="android:textColorPrimary">@color/custom_primary_colour</item>
...
</style>

2, Read the source code
This is a general tip really, but enough emphasis can’t be placed on how much you will learn by reading the Android source code, which can be downloaded via the SDK Manager. To find the style and theme code, find where you installed the Android SDK and have a look at styles.xml and themes.xml in the <android-sdk>/platforms/android-XX/data/res/values/ directory.

Really! You’ll learn more by looking at, studying, and playing about with the Android style and theme source code than any guide can ever teach you! For example, there’s no comprehensive documentation of what styles are available to change, but have a look at the default theme (search for ‘<style name=”Theme”>’ in the theme.xml file) and you’ll find out how to style every single widget in Android and lots more. This is the best form of guide and documentation rolled in to one – it’s always going to accurately reflect the live code and every single bit of style information is in there. Only the experience of using it will help you get a deeper knowledge of how it works. So…what are you waiting for?