android学习_文档记录

文档中记录了很长的一些除了系统主要的四个组件外的一些其他的相关信息，。我大概看了看，没怎么看懂，所以，先留下来，以后用到了再来查吧。 哈。、

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 "view" or "send" something) and may specify the URI of the data to act on (among other things that 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 "battery is low").

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

 to 

bindService()

.
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.

The Manifest File

Before the Android system can start an application component, the system must know that the component exists by reading the application's 

AndroidManifest.xml

 file (the "manifest" file).
Your application must declare all its components in this file, which must be at the root of the application project directory.

The manifest does a number of things in addition to declaring the application's components, such as:
Identify any user permissions the application requires, such as Internet access or read-access to the user's contacts.
Declare the minimum API Level required by the application, based
on which APIs the application uses.
Declare hardware and software features used or required by the application, such as a camera, bluetooth services, or a multitouch screen.
API libraries the application needs to be linked against (other than the Android framework APIs), such as theGoogle
Maps library.
And more

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:

<?xml version="1.0" encoding="utf-8"?>
<manifest ... >
    <application android:icon="@drawable/app_icon.png" ... >
        <activity android:name="com.example.project.ExampleActivity"
                  android:label="@string/example_label" ... >
        </activity>
        ...
    </application>
</manifest>

In the 

<application>

 element,
the 

android:icon

 attribute points to resources for an icon that identifies the application.

In the 

<activity>

 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:

<activity>

 elements
for activities

<service>

 elements
for services

<receiver>

 elements
for broadcast receivers

<provider>

 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()

.

For more about how to structure the manifest file for your application, see The AndroidManifest.xml Filedocumentation.

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 

<intent-filter>

 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 "send" intents (in order to send 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.

Here are some of the important device characteristics that you should consider as you design and develop your application:
Screen size and densityIn order to categorize devices by their screen type, Android defines two characteristics for each device: screen size (the physical dimensions of the screen) and screen density (the physical density of the pixels on the
screen, or dpi—dots per inch). To simplify all the different types of screen configurations, the Android system generalizes them into select groups that make them easier to target.
The screen sizes are: small, normal, large, and extra large.

The screen densities are: low density, medium density, high density, and extra high density.
By default, your application is compatible with all screen sizes and densities, because the Android system makes the appropriate adjustments to your UI layout and image resources. However, you should create specialized
layouts for certain screen sizes and provide specialized images for certain densities, using alternative layout resources, and by declaring in your manifest exactly which screen sizes your application supports with the 

<supports-screens>

 element.
For more information, see the Supporting Multiple Screens document.
Input configurationsMany devices provide a different type of user input mechanism, such as a hardware keyboard, a trackball, or a five-way navigation pad. If your application requires a particular kind of input hardware, then you should declare
it in your manifest with the 

<uses-configuration>

 element.
However, it is rare that an application should require a certain input configuration.Device featuresThere are many hardware and software features that may or may not exist on a given Android-powered device, such as a camera, a light sensor, bluetooth, a certain version of OpenGL, or the fidelity of the touchscreen. You
should never assume that a certain feature is available on all Android-powered devices (other than the availability of the standard Android library), so you should declare any features used by your application with the 

<uses-feature>

 element.Platform VersionDifferent Android-powered devices often run different versions of the Android platform, such as Android 1.6 or Android 2.3. Each successive version often includes additional APIs not available in the previous version. In
order to indicate which set of APIs are available, each platform version specifies an API Level(for
example, Android 1.0 is API Level 1 and Android 2.3 is API Level 9). If you use any APIs that were added to the platform after version 1.0, you should declare the minimum API Level in which those APIs were introduced using the 

<uses-sdk>

 element.
It's important that you declare all such requirements for your application, because, when you distribute your application on Google Play, the store uses these declarations to filter which applications are available on each device. As such, your application
should be available only to devices that meet all your application requirements.

For more information about how Google Play filters applications based on these (and other) requirements, see the Filters on Google
Play document.

Application Resources

An Android application is composed of more than just code—it requires resources that are separate from the source code, such as images, audio files, and anything relating to the visual presentation of the application. For example, you should define animations,
menus, styles, colors, and the layout of activity user interfaces with XML files. Using application resources makes it easy to update various characteristics of your application without modifying code and—by providing sets of alternative resources—enables
you to optimize your application for a variety of device configurations (such as different languages and screen sizes).

For every resource that you include in your Android project, the SDK build tools define a unique integer ID, which you can use to reference the resource from your application code or from other resources defined in XML. For example, if your application contains
an image file named 

logo.png

 (saved in the 

res/drawable/

 directory), the SDK tools generate a resource ID named 

R.drawable.logo

,
which you can use to reference the image and insert it in your user interface.

One of the most important aspects of providing resources separate from your source code is the ability for you to provide alternative resources for different device configurations. For example, by defining UI strings in XML, you can translate the strings into
other languages and save those strings in separate files. Then, based on a language qualifier that you append to the resource directory's name (such as 

res/values-fr/

 for French
string values) and the user's language setting, the Android system applies the appropriate language strings to your UI.

Android supports many different qualifiers for your alternative resources. The qualifier is a short string that you include in the name of your resource directories in order to define the device configuration for which those resources should be used.
As another example, you should often create different layouts for your activities, depending on the device's screen orientation and size. For example, when the device screen is in portrait orientation (tall), you might want a layout with buttons to be vertical,
but when the screen is in landscape orientation (wide), the buttons should be aligned horizontally. To change the layout depending on the orientation, you can define two different layouts and apply the appropriate qualifier to each layout's directory name.
Then, the system automatically applies the appropriate layout depending on the current device orientation.

For more about the different kinds of resources you can include in your application and how to create alternative resources for various device configurations, see the Application
Resources developer guide.