73 lines
3.6 KiB
HTML
73 lines
3.6 KiB
HTML
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<html>
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<head>
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<script type="text/javascript" src="http://www.corp.google.com/style/prettify.js"></script>
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<script src="http://www.corp.google.com/eng/techpubs/include/navbar.js" type="text/javascript"></script>
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</head>
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<body>
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<p>High-level classes encapsulating the overall Android application model.
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The central class is {@link android.app.Activity}, with other top-level
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application components being defined by {@link android.app.Service} and,
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from the {@link android.content} package, {@link android.content.BroadcastReceiver}
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and {@link android.content.ContentProvider}. It also includes application
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tools, such as dialogs and notifications.</p>
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<p>This package builds on top of the lower-level Android packages
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{@link android.widget}, {@link android.view}, {@link android.content},
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{@link android.text}, {@link android.graphics}, {@link android.os}, and
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{@link android.util}.</p>
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<p>An {@link android.app.Activity Activity} is a specific operation the
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user can perform, generally corresponding
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to one screen in the user interface.
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It is the basic building block of an Android application.
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Examples of activities are "view the
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list of people," "view the details of a person," "edit information about
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a person," "view an image," etc. Switching from one activity to another
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generally implies adding a new entry on the navigation history; that is,
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going "back" means moving to the previous activity you were doing.</p>
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<p>A set of related activities can be grouped together as a "task". Until
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a new task is explicitly specified, all activites you start are considered
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to be part of the current task. While the only way to navigate between
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individual activities is by going "back" in the history stack, the group
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of activities in a task can be moved in relation to other tasks: for example
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to the front or the back of the history stack. This mechanism can be used
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to present to the user a list of things they have been doing, moving
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between them without disrupting previous work.
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</p>
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<p>A complete "application" is a set of activities that allow the user to do a
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cohesive group of operations -- such as working with contacts, working with a
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calendar, messaging, etc. Though there can be a custom application object
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associated with a set of activities, in many cases this is not needed --
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each activity provides a particular path into one of the various kinds of
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functionality inside of the application, serving as its on self-contained
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"mini application".
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</p>
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<p>This approach allows an application to be broken into pieces, which
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can be reused and replaced in a variety of ways. Consider, for example,
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a "camera application." There are a number of things this application
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must do, each of which is provided by a separate activity: take a picture
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(creating a new image), browse through the existing images, display a
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specific image, etc. If the "contacts application" then wants to let the
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user associate an image with a person, it can simply launch the existing
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"take a picture" or "select an image" activity that is part of the camera
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application and attach the picture it gets back.
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</p>
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<p>Note that there is no hard relationship between tasks the user sees and
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applications the developer writes. A task can be composed of activities from
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multiple applications (such as the contact application using an activity in
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the camera application to get a picture for a person), and multiple active
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tasks may be running for the same application (such as editing e-mail messages
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to two different people). The way tasks are organized is purely a UI policy
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decided by the system; for example, typically a new task is started when the
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user goes to the application launcher and selects an application.
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</p>
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</body>
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</html>
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