Android官方文档系列

APP基础

2016-07-22  本文已影响41人  Sinchunk

https://developer.android.com/guide/components/fundamentals.html

Android SDK tools 把你的代码、数据和资源文件编译成一个APK文件。这个APK文件包含了APP需要的所有文件，而且Android设备使用APK文件来安装APP。

一旦安装完成，每个Android APP都生存于自己的沙盒中：

虽然每个app都有自己生存的沙盒，但是他们之间也可以交流或者获取系统服务：

APP组件

Android有4种组件，分别是Activities、Services、Content providers、Broadcast receivers。下面简要介绍一下这四种组件。

Activities
Services
Content providers
Broadcast receivers

A unique aspect of the Android system design is that any app can start another app’s component.

When the system starts a component, it starts the process for that app (if it's not already running) and instantiates the classes needed for the component.

Because the system runs each app in a separate process with file permissions that restrict access to other apps, your app cannot directly activate a component from another app. The Android system, however, can. So, to activate a component in another app, 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.

激活组件

Three of the four component types—activities, services, and broadcast receivers—are activated by an asynchronous message called anintent. 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 app 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). 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 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).

他们各自有几种方法来激活组件：

AndroidManifest.xml文件

Before the Android system can start an app component, the system must know that the component exists by reading the app's AndroidManifest.xml file (the "manifest" file). Your app must declare all its components in this file, which must be at the root of the app project directory.

声明组件

The primary task of the manifest is to inform the system about the app'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>

You must declare all app components this way:

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()](https://developer.android.com/reference/android/content/Context.html#registerReceiver(android.content.BroadcastReceiver, android.content.IntentFilter)).

声明组件功能

An implicit intent simply describes the type of action to perform (and, optionally, the data upon which you’d like to perform the action) and allows 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 apps on the device.

For example, if you've built an email app with an activity for composing a new email, you can declare an intent filter to respond to "send" intents (in order to send a new email) like this:

<manifest ... >
    ...
    <application ... >
        <activity android:name="com.example.project.ComposeEmailActivity">
            <intent-filter>
                <action android:name="android.intent.action.SEND" />
                <data android:type="*/*" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>
    </application>
</manifest>

Then, if another app creates an intent with the ACTION_SEND action and passes it to startActivity(), the system may start your activity so the user can draft and send an email.

APP资源文件

An Android app 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 app. Using app resources makes it easy to update various characteristics of your app without modifying code and—by providing sets of alternative resources—enables you to optimize your app 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 app code or from other resources defined in XML.

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.

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. 例如：
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.

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