安卓应用基础中英文对照外文翻译文献

When the system starts a component, it starts the process for that application (if it's not

already running) and instantiates the classes needed for the component. For example, if your application starts the activity in the camera application that captures a photo, that activity runs in the process that belongs to the camera application, not in your application's process. Therefore, unlike applications on most other systems, Android applications don't have a single entry point (there's no main() function, for example).

Because the system runs each application in a separate process with file permissions that

restrict access to other applications, your application cannot directly activate a component from another application. The Android system, however, can. So, to activate a component in another application, you must deliver a message to the system that specifies your intent to start a particular component. The system then activates the component for you.

Activating Components

Three of the four component types—activities, services, and broadcast receivers—are

activated by an asynchronous message called an intent. Intents bind individual components to each other at runtime (you can think of them as the messengers that request an action from other components), whether the component belongs to your application or another.

An intent is created with an Intent object, which defines a message to activate either a

specific component or a specific type of component—an intent can be either explicit or implicit, respectively.

For activities and services, an intent defines the action to perform (for example, to \

or \the component being started might need to know). For example, an intent might convey a request for an activity to show an image or to open a web page. In some cases, you can start an activity to receive a result, in which case, the activity also returns the result in an Intent (for example, you can issue an intent to let the user pick a personal contact and have it returned to you—the return intent includes a URI pointing to the chosen contact).

For broadcast receivers, the intent simply defines the announcement being broadcast (for

example, a broadcast to indicate the device battery is low includes only a known action string that indicates \

The other component type, content provider, is not activated by intents. Rather, it is

activated when targeted by a request from a ContentResolver. The content resolver handles all direct transactions with the content provider so that the component that's performing transactions with the provider doesn't need to and instead calls methods on

the ContentResolver object. This leaves a layer of abstraction between the content provider and the component requesting information (for security).

There are separate methods for activating each type of component: You can start an activity (or give it something new to do) by passing

an Intent to startActivity() or startActivityForResult() (when you want the activity to return a result).

You can start a service (or give new instructions to an ongoing service) by passing an Intent to startService(). Or you can bind to the service by passing an Intent tobindService().

You can initiate a broadcast by passing an Intent to methods like sendBroadcast(), sendOrderedBroadcast(), or sendStickyBroadcast().

You can perform a query to a content provider by calling query() on a ContentResolver.

For more information about using intents, see the Intents and Intent Filters document. More information about activating specific components is also provided in the following documents: Activities, Services, BroadcastReceiver and Content Providers.

Declaring components

The primary task of the manifest is to inform the system about the application's

components. For example, a manifest file can declare an activity as follows: ...

In the element, the android:icon attribute points to resources for an icon

that identifies the application.

In the element, the android:name attribute specifies the fully qualified class

name of the Activity subclass and the android:label attributes specifies a string to use as the user-visible label for the activity.

You must declare all application components this way: 1、 elements for activities 2、 elements for services

3、 elements for broadcast receivers 4、 elements for content providers

Activities, services, and content providers that you include in your source but do not

declare in the manifest are not visible to the system and, consequently, can never run. However, broadcast receivers can be either declared in the manifest or created dynamically in code (as BroadcastReceiver objects) and registered with the system by calling registerReceiver().

Declaring component capabilities

As discussed above, in Activating Components, you can use an Intent to start activities,

services, and broadcast receivers. You can do so by explicitly naming the target component (using the component class name) in the intent. However, the real power of intents lies in the concept of intent actions. With intent actions, you simply describe the type of action you want to perform (and optionally, the data upon which you’d like to perform the action) and allow the system to find a component on the device that can perform the action and start it. If there are multiple components that can perform the action described by the intent, then the user selects which one to use.

The way the system identifies the components that can respond to an intent is by

comparing the intent received to the intent filters provided in the manifest file of other applications on the device.

When you declare a component in your application's manifest, you can optionally include

intent filters that declare the capabilities of the component so it can respond to intents from other applications. You can declare an intent filter for your component by adding an element as a child of the component's declaration element.

For example, an email application with an activity for composing a new email might

declare an intent filter in its manifest entry to respond to \email). An activity in your application can then create an intent with the “send” action (ACTION_SEND), which the system matches to the email application’s “send” activity and launches it when you invoke the intent with startActivity().

For more about creating intent filters, see the Intents and Intent Filters document.

Declaring application requirements

There are a variety of devices powered by Android and not all of them provide the same

features and capabilities. In order to prevent your application from being installed on devices that lack features needed by your application, it's important that you clearly define a profile for the types of devices your application supports by declaring device and software requirements in your manifest file. Most of these declarations are informational only and the system does not read them, but external services such as Google Play do read them in order to provide filtering for users when they search for applications from their device.

For example, if your application requires a camera and uses APIs introduced in Android

2.1 (API Level 7), you should declare these as requirements in your manifest file. That way, devices that do not have a camera and have an Android version lower than 2.1 cannot install your application from Google Play.

However, you can also declare that your application uses the camera, but does

not require it. In that case, your application must perform a check at runtime to determine if the device has a camera and disable any features that use the camera if one is not available.