【正文】 to be able go to a subsequent screen B and then use the BACK key to go back to screen A, then the screen A needs to be implemented as an activity. The one exception to this rule is if your application takes control of the BACK key and manages the navigation itself. 東北大學(xué) 東軟信息學(xué)院畢業(yè)設(shè)計（論文） 譯文 8 Tasks A task is the sequence of activities the user follows to acplish an objective, regardless of which applications the activities belong to. Until a new task is explicitly specified (see Interrupting the Task), all activities the user starts are considered to be part of the current task. It39。re doing (their task) to perform a different task, then are able to return to where they left off to plete the original task. The idea is that users can run multiple tasks simultaneously and switch between them. There are two primary ways to jump off to that other task — in both cases the user should be able to return to where they were before the interruption: User is interrupted by a notification – a notification appears and the user wants to act on it User deciding to perform another task – user just presses Home and starts an application Of course, there are exceptions to the rules. Beyond the two ways just mentioned, there is a third way to start a task, and that is by starting an activity that defines itself as a new task. Maps and Browser are two applications that do this. For example, choosing an address in an starts the Maps activity as a new task, and choosing a link in an starts the Browser activity as a new task. In these cases, the BACK key will return to the previous activity in a different task (Email), because it was not started from Home. 東北大學(xué) 東軟信息學(xué)院畢業(yè)設(shè)計（論文） 譯文 9 A Tour of Activities and Tasks The following examples illustrate basic principles for applications, activities, the activity stack, the BACK key, tasks and intents. It shows how the system responds to user actions such as starting activities and switching between tasks. With most of these examples you can follow along, launching activities on your device as indicated. Starting an Activity from Home Home is the starting place for most applications. (Some applications can be launched only from other applications.) When the user touches an icon in the application launcher (or a shortcut on the Home screen), the main activity for that application is launched into the foreground where it has user focus. Navigating Away from an Activity with BACK and HOME keys An activity can keep or lose its state depending on how the user leaves the activity — by the HOME or BACK key. By default, pressing the BACK key finishes (destroys) the current activity and displays the previous activity to the user. In addition, not all activities have the behavior that they are destroyed when BACK is pressed. When the user starts playing music in the Music application and then presses BACK, the application overrides the normal back behavior, preventing the player activity from being destroyed, and continues playing music, even though its activity is no longer visible — as a visual substitute, the Music application places a notification in the status bar so the user still has an easy way to get to the application to stop or control the music. Note that you can write an activity to stop when its screen is no longer visible, or to continue running in the background — the latter was chosen for the music player. Reusing an Activity When activity A starts activity B in a different application, activity B is said to be reused. This use case normally takes place because activity A is lacking a capability and can find it in activity B. Contacts ReUses Gallery to Get a Picture The Contacts activity has a field for a picture of a contact, but the Gallery is normally where pictures are kept. So Contacts can reuse the Gallery activity to get a picture. This is a good example of reuse of the Gallery activity. Replacing an Activity 東北大學(xué) 東軟信息學(xué)院畢業(yè)設(shè)計（論文） 譯文 10 This is the use case where activity A replaces activity B in a different application. This situation normally happens because activity A is better at doing the job than activity B. In other words, A and B are equivalent enough that A can replace B. This case stands in contrast with reusing an activity, where A and B are quite different activities and supplement each other. Multitasking As previously noted, when an activity has been launched, the user can go to Home and launch a second activity without destroying the first activity. This scenario demonstrates launching the Maps application. State 1 The user launches the View Map activity and searches for a map location. Let39。re waiting, so they press HOME, which does not interrupt the map39。s calendar (as indicated by the heavy outline). State 4 The user presses Home, then Maps to return to the map, which by now has fully loaded. The application launcher at Home has launched View Map and Day View activities into separate tasks, hence the system is multitasking — running multiple tasks. Switching Between Tasks This scenario shows how the user can switch between two tasks. In this example, the user writes a text message, attaches a picture, but before they are done they glance at their calendar. They then return to where they left off, attaching the picture and sending the message. 1. Start first task. You want to send a text message and attach a photo. You would choose: Home Messaging New message MENU Attach Pictures. This last step launches the picture gallery for picking a photo. Notice that picture gallery is