【正文】 長(zhǎng)春理工大學(xué)光電信息學(xué)院畢業(yè)設(shè)計(jì) 1 Application Fundamentals Android applications are written in the Java programming language. The piled Java code — along with any data and resource files required by the application — is bundled by the aapt tool into an Android package, an archive file marked by an .apk suffix. This file is the vehicle for distributing the application and installing it on mobile devices。 it39。s the file users download to their devices. All the code in a single .apk file is considered to be one application. In many ways, each Android application lives in its own world: 1. By default, every application runs in its own Linux process. Android starts the process when any of the application39。s code needs to be executed, and shuts down the process when it39。s no longer needed and system resources are required by other applications. 2. Each process has its own virtual machine (VM), so application code runs in isolation from the code of all other applications. 3. By default, each application is assigned a unique Linux user ID. Permissions are set so that the application39。s files are visible only to that user and only to the application itself — although there are ways to export them to other applications as well. It39。s possible to arrange for two applications to share the same user ID, in which case they will be able to see each other39。s files. To conserve system resources, applications with the same ID can also arrange to run in the same Linux process, sharing the same VM. Application Components A central feature of Android is that one application can make use of elements of other applications (provided those applications permit it). For example, if your application needs to display a scrolling list of images and another application has developed a suitable scroller and made it available to others, you can call upon that scroller to do the work, rather than develop your own. Your application doesn39。t incorporate the code of the other application or link to it. Rather, it simply starts up that piece of the other application when the need arises. For this to work, the system must be able to start an application process when any part of it is needed, and instantiate the Java objects for that part. Therefore, unlike applications on most other systems, Android applications don39。t have a single entry point for everything in the application (no main() function, for example). Rather, they have essential ponents that the system can instantiate and run as needed. There are four 長(zhǎng)春理工大學(xué)光電信息學(xué)院畢業(yè)設(shè)計(jì) 2 types of ponents: Activities An activity presents a visual user interface for one focused endeavor the user can undertake. For example, an activity might present a list of menu items users can choose from or it might display photographs along with their captions. A text messaging application might have one activity that shows a list of contacts to send messages to, a second activity to write the message to the chosen contact, and other activities to review old messages or change settings. Though they work together to form a cohesive user interface, each activity is independent of the others. Each one is implemented as a subclass of the Activity base class. An application might consist of just one activity or, like the text messaging application just mentioned, it may contain several. What the activities are, and how many there are depends, of course, on the application and its design. Typically, one of the activities is marked as the first one that should be presented to the user when the application is launched. Moving from one activity to another is acplished by having the current activity start the next one. Each activity is given a default window to draw in. Typically, the window fills the screen, but it might be smaller than the screen and float on top of other windows. An activity can also make use of additional windows — for example, a popup dialog that calls for a user response in the midst of the activity, or a window that presents users with vital information when they select a particular item onscreen. The visual content of the window is provided by a hierarchy of views — objects derived from the base View class. Each view controls a particular rectangular space within the window. Parent views contain and organize the layout of their children. Leaf views (those at the bottom of the hierarchy) draw in the rectangles they control and respond to user actions directed at that space. Thus, views are where the activity39。s interaction with the user takes place. For example, a view might display a small i