Open source and UI consistency
Android is a revolutionary open source platform in many ways and as a developer there are many degrees of freedom in creating new and innovative services and applications. A great deal of effort has gone into making the G1 Android UI as consistent as possible. However, maintaining a high level of consistency in the interface and user experience is tricky when working in an open source environment. Developers are (and should be) free to choose how applications should look and behave. Keeping a tight relationship between the design of individual applications and the look and feel of the underlying platform is important from a product perspective. A user interface, which is consistent and predictable, results in a user who won't feel lost in their own phone.
Android will have the luxury to be incrementally developed, often branching out in different directions by different developers, and will undoubtedly become better and better over time. But designers still must ensure consistency with the overall UI paradigm, so as to reduce fragmentation of form and function and provide a solid user experience.
Supporting multiple interaction modes
Android is designed to support a wide range of device configurations right out of the box. For instance, it can be run on a large screen touch-only device or on a small screen device with 4-way navigation keys. Great effort has been put into making one, scalable interaction paradigm that works in a large number of different contexts.
This means a few paradigm differences compared with other platforms. For instance, there is a highlight in indirect manipulation contexts, like when a user uses a D-Pad, which disappears during touch interaction. There is no real need for highlighting an icon or list item when using a touch screen--your finger is the highlight.
Another difference is that actions are divided into item-specific and screen-specific ones. Item-specific actions are things such as "Send via MMS" for an image, and only available for individual items. Screen-specific actions are those things you would like to perform on all items in a screen, such as "Sort by time" for all images. The item-specific actions are available via long press on individual items, screen-specific actions via the menu key. Since there are cases when items have no highlight, it's not recommended to place contextual, item-specific actions under the menu key. To put this in PC terms--how can you right-click an object without seeing where the cursor is located?
In general, designing an application for Android means designing without knowing what the configuration for the device will be or even knowing which device. Thus, an application needs to support user behavior, assumptions and interactions for both touch and non-touch devices.
Notifications management
Android has a unique notification system that is non-intrusive, yet universally accessible and very extensible in its nature. The pull down window shade is essentially an extension of the status bar: It can be reached from within all applications and provides additional actions and information for new event notifications.
The notification system is very flexible and scalable--any 3rd party developer can expose new event notifications here. If someone creates a new Twitter application, a new twit from someone can become a new event notification. However, it's important to consider the ecology of applications and events and use the notification system sparingly in order to make best possible use of the user's limited attention span.
Support seamless interaction
A cool thing about Android is its built-in intent handling functionality: an application can ask to use functionality in other applications: A Web browser can ask for suitable viewers of PDF-files, and the applications on the device that can handle this can seamlessly contribute with functionality.
This is a great, extensible system from a framework perspective, but also very powerful from a user perspective. Intent handling makes it possible for developers to support seamless interaction, making a user task span over multiple applications. It's a human task-centered approach, rather than a systemic application-centered one. So despite its name, Android is a very human platform.
About the author
Daniel Johansson is an Interaction Designer at The Astonishing Tribe (TAT). TAT is the Swedish based company that provides user interface technology that has added the "WOW" affect to the user experience in more than 240 million mobile phones shipped worldwide TAT. TAT is a member of the OHA and was invited by Google to design the user interface for Android. Dan was on the TAT design team for the Google Android UI.
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