[DAY 2] 2. Activating components

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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 app or another.

An intent is created with an Intent object, which defines a message to activate either a specific component (explicit intent) or a specific type of component (implicit intent).

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.

Unlike activities, services, and broadcast receivers, content providers are not activated by intents. Rather, they are 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).
• With Android 5.0 (API level 21) and later, you can use the JobScheduler class to schedule actions. For earlier Android versions, you can start a service (or give new instructions to an ongoing service) by passing an Intent to startService(). You can bind to the service by passing an Intent to bindService().
• You can initiate a broadcast by passing an Intent to methods such as sendBroadcast(), sendOrderedBroadcast(), or sendStickyBroadcast().
• You can perform a query to a content provider by calling query() on a ContentResolver.

The manifest file

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

The manifest does a number of things in addition to declaring the app's components, such as the following:

• Identifies any user permissions the app requires, such as Internet access or read-access to the user's contacts.
• Declares the minimum API Level required by the app, based on which APIs the app uses.
• Declares hardware and software features used or required by the app, such as a camera, bluetooth services, or a multitouch screen.
• Declares API libraries the app needs to be linked against (other than the Android framework APIs), such as the Google Maps library.

Declaring components

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>

In the <application> element, the android:icon attribute points to resources for an icon that identifies the app.

In the <activity> element, the android:name attribute specifies the fully qualified class name of the Activity subclass and the android:label attribute specifies a string to use as the user-visible label for the activity.

You must declare all app components using the following elements:

• <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().

 

Declaring component capabilities

As discussed above, in Activating components, you can use an Intent to start activities, services, and broadcast receivers. You can use an Intent by explicitly naming the target component (using the component class name) in the intent. You can also use an implicit intent, which describes the type of action to perform and, optionally, the data upon which you’d like to perform the action. The implicit intent 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, the user selects which one to use.

Caution: If you use an intent to start a Service, ensure that your app is secure by using an explicit intent. Using an implicit intent to start a service is a security hazard because you cannot be certain what service will respond to the intent, and the user cannot see which service starts. Beginning with Android 5.0 (API level 21), the system throws an exception if you call bindService() with an implicit intent. Do not declare intent filters for your services.

The system identifies the components that can respond to an intent by comparing the intent received to the intent filters provided in the manifest file of other apps on the device.

When you declare an activity in your app's manifest, you can optionally include intent filters that declare the capabilities of the activity so it can respond to intents from other apps. 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, if you build 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), as shown in the following example:

 

<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>

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.

 

Declaring app requirements

There are a variety of devices powered by Android and not all of them provide the same features and capabilities. To prevent your app from being installed on devices that lack features needed by your app, it's important that you clearly define a profile for the types of devices your app 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 apps from their device.

For example, if your app requires a camera and uses APIs introduced in Android 8.0 (API Level 26), you must declare these requirements.

The values for minSdkVersion and targetSdkVersion are set in your app module's build.gradle file:

 

android {
  ...
  defaultConfig {
    ...
    minSdkVersion 26
    targetSdkVersion 29
  }
}

Note: Don't set minSdkVersion and targetSdkVersion directly in the manifest file, since they will be overwritten by Gradle during the build process. 

Declare the camera feature directly in your app's manifest file:

 

<manifest ... >
    <uses-feature android:name="android.hardware.camera.any"
                  android:required="true" />
    ...
</manifest>

With the declarations shown in these examples, devices that do not have a camera or have an Android version lower than 8.0 cannot install your app from Google Play. However, you can declare that your app uses the camera, but does not require it. In that case, your app must set the required attribute to false and check at runtime whether the device has a camera and disable any camera features as appropriate.

 

App resources

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. For example, you can define animations, menus, styles, colors, and the layout of activity user interfaces with XML files. Using app resources makes it easy to update various characteristics of your app without modifying code. 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. For example, if your app contains an image file named logo.png (saved in the res/drawable/ directory), the SDK tools generate a resource ID named R.drawable.logo. This ID maps to an app-specific integer, 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 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 Android applies the appropriate language strings to your UI 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.

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. For example, you should create different layouts for your activities, depending on the device's screen orientation and size. 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 could 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.

 

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단어

1. asynchronous : 비동기

2. explicit : 명시적

3. implicit : 암시적

4. consequently : 그 결과, 따라서

5. capabilities : 기능, 능력, 역량

6. hazard : 위험

7. appropriate : 적절한

8. configurations : 환경 설정, 배열

9. qualifier : 한정자, 통과자 

10. portrait orientation : 세로 방향

11. landscape orientation : 가로 방향

12. aligned : 정렬된

 

 

 

 

 

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정리

1. An intent is created with an Intent object, which defines a message to activate either a specific component (explicit intent) or a specific type of component (implicit intent).

2.  The manifest is to inform the system about the app's components.

3. Using app resources is the ability to provide alternative resources for different device configurations without a source code.

 

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출처

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

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