【正文】 an activity in a separate application. At this point, before you have picked a picture, you decide to stop and glance at your calendar, which is a separate task. Because the current activity has no button to go directly to the Calendar, you need to start from Home. 東北大學(xué) 東軟信息學(xué)院畢業(yè)設(shè)計(jì)（論文） 譯文 11 2. Start second task. You choose Home Calendar to look at a calendar event. Calendar launches from Home as a new task because the application launcher creates a new task for each application it launches. 3. Switch to first task and plete it. When done looking at the Calendar, you can return to attaching the picture by starting the root activity again for that task: choose Home Messaging, which takes you not to Messaging, but directly to the Picture gallery, where you left off. You can then pick a photo, which is added to the message, you send the message and you39。t be reused, don39。re writing an activity that you don39。s just no need for intent filters. Intent filters are published to all other applications, so if you make an intent filter, what you39。s up to you to determine how users start your application and the activities in it. As an application is a set of activities, the user can start these activities from Home or from another application. Launch your main activity from an icon at Home If your application can run standalone, it should probably be started by the user touching an icon in application launcher (typically implemented as a sliding drawer on the Home screen), or from a shortcut icon on the Home screen, or from the task 東北大學(xué) 東軟信息學(xué)院畢業(yè)設(shè)計(jì)（論文） 譯文 12 switcher. (The mechanism for this is for the activity to have an intent filter with action MAIN and category LAUNCHER.) Launch your activity from within another application Perhaps your activities are meant for reuse. For example, many applications have data they want to share with other users. Activities that can share data with other users include , text messaging and uploading to a public website. If one or more of your activities can be an alternative to an existing activity in another application, you can make it available to users at the point they request that activity. For example, if your activity can send data to others (such as by , text messaging, or uploading), consider setting up that activity to appear as a choice to the user. To give a specific example, Gallery enables a user to view and share pictures. When the user chooses Share from the menus, the system pares the Share request (an Intent object) to available activities (by looking at their intent filters) and displays choices to share. In this case, it matches Email, Gmail, Messaging and Picasa. If your activity can send a picture or upload it to a website, all it needs to do is make itself available for sharing (by setting its intent filter). Another activity can start your activity either with or without expecting a result back. o Start an activity expecting a result This approach is closed loop, where the activity being started must either return a valid result or be canceled. In the previous examples of sharing a photo from a Gallery, the user ends up back in the Gallery after pleting the send or upload procedure. These are examples of starting an activity external to the Gallery. (Such an activity is started with startActivityForResult().) o Start an activity not expecting a result This approach is openended. An example is choosing an house address in an message (or web page), where the Maps activity is started to map the location. No result from maps is expected to be returned to the message。t use dialog boxes in place of notifications If your background service needs to notify a user, use the standard notification system — don39。t take over the BACK key unless you absolutely need to As a user navigates from one activity to the next, the system adds them to the activity stack. This forms a navigation history that is accessible with the BACK key. Most activities are relatively limited in scope, with just one set of data, such as viewing a list of contacts, posing an , or taking a photo. But what if your application is one big activity with several pages of content and needs finergrained control of the BACK key? Examples of such Google applications are the Browser, which can have several web pages open at once, and Maps, which can have several layers of geographic data to switch between. Both of these applications take control of the BACK key and maintain their own internal back stacks that operate only when these applications have focus. For example, Maps uses layers to present different information on a map to the user: displaying the location of a search result, displaying locations of friends, and 東北大學(xué) 東軟信息學(xué)院畢業(yè)設(shè)計(jì)（論文） 譯文 14 displaying a line for a street path providing direction between points. Maps stores these layers in its own history so the BACK key can return to a previous layer. Similarly, Browser uses browser windows to present different web pages to the user. Each window has its own navigation history, equivalent to tabs in a browser in a desktop operating system (such as Windows, Macintosh or Linux). For example, if you did a Google web search in one window of the Android Browser, clicking on a link in the search results displays a web page in that same window, and then pressing BACK would to the search results page. Pressing BACK goes to a previous window only if the current window was launched from that previous window. If the user keeps pressing back, they will eventually leave the browser activity and return Home.