Android 应用开发培训 PDF 指南

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Android Developer Training PDF 格式文档概述 Android Developer Training PDF 格式文档是 Google 官方提供的 Android 开发者培训文档的 PDF 格式版本，该文档将官方网站上的内容编辑成了一个便于查看的 PDF 文档。 Android 开发者培训文档的主要内容包括 Android 应用程序开发的基础知识、Android 应用程序开发的进阶知识、Android 应用程序开发的高级知识等。该文档涵盖了 Android 应用程序开发的各个方面，包括 Android 基础知识、用户界面设计、数据存储、网络编程、多媒体处理、安全性等。 Android 开发者培训文档的优点包括： * 官方认证：该文档是 Google 官方提供的，确保了文档的权威性和准确性。 * 全面性：该文档涵盖了 Android 应用程序开发的各个方面，无论是基础知识还是高级知识。 * 易读性：该文档的内容清晰易懂，适合不同水平的开发者。 * 实践性：该文档提供了许多实践项目和代码示例，帮助开发者快速掌握 Android 应用程序开发的技能。 Android 开发者培训文档的结构包括： * 基础知识：介绍 Android 应用程序开发的基础知识，包括 Android 基础知识、用户界面设计、数据存储等。 * 进阶知识：介绍 Android 应用程序开发的进阶知识，包括网络编程、多媒体处理、安全性等。 * 高级知识：介绍 Android 应用程序开发的高级知识，包括 Android sistema 架构、Android NDK 等。 Android 开发者培训文档的license 信息包括： * Creative Commons 2.5 Attribution License：该License 适用于 Android Open Source Project 的一些部分。 * Apache 2.0 License：该License 适用于代码示例。 * Creative Commons Attribution-NonCommercial 3.0 Unported License：该License 适用于该文档的其他部分。 Android Developer Training PDF 格式文档是 Android 应用程序开发者的必备资源，该文档提供了全面、实践的 Android 应用程序开发知识，帮助开发者快速掌握 Android 应用程序开发的技能。

Running Your App

Content from developer.android.com/training/basics/firstapp/running-app.html through their Creative Commons Attribution 2.5 license

• Make sure the Android SDK platform-tools/ directory is included in your PATH environment variable, then

execute:

adb install bin/MyFirstApp-debug.apk

•

• On the emulator, locate MyFirstActivity and open it.

That's how you build and run your Android app on the emulator! To start developing, continue to the next lesson.

Building a Simple User Interface

Content from developer.android.com/training/basics/firstapp/building-ui.html through their Creative Commons Attribution 2.5 license

5. Building a Simple User Interface

Content from developer.android.com/training/basics/firstapp/building-ui.html through their Creative Commons Attribution 2.5 license

The graphical user interface for an Android app is built using a

hierarchy of View and ViewGroup objects. View objects are

usually UI widgets such as buttons or text fields and ViewGroup

objects are invisible view containers that define how the child

views are laid out, such as in a grid or a vertical list.

Android provides an XML vocabulary that corresponds to the

subclasses of View and ViewGroup so you can define your UI

in XML using a hierarchy of UI elements.

Figure 1. Illustration of how ViewGroup objects form branches

in the layout and contain other View objects.

In this lesson, you'll create a layout in XML that includes a text field and a button. In the following lesson, you'll respond

when the button is pressed by sending the content of the text field to another activity.

Create a Linear Layout

Open the activity_main.xml file from the res/layout/ directory.

Note: In Eclipse, when you open a layout file, you’re first shown the Graphical Layout editor. This is an editor that helps

you build layouts using WYSIWYG tools. For this lesson, you’re going to work directly with the XML, so click the

activity_main.xml tab at the bottom of the screen to open the XML editor.

The BlankActivity template you chose when you created this project includes the activity_main.xml file with a

RelativeLayout root view and a TextView child view.

First, delete the <TextView> element and change the <RelativeLayout> element to <LinearLayout>. Then add

the android:orientation attribute and set it to "horizontal". The result looks like this:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

xmlns:tools="http://schemas.android.com/tools"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:orientation="horizontal" >

</LinearLayout>

LinearLayout is a view group (a subclass of ViewGroup) that lays out child views in either a vertical or horizontal

orientation, as specified by the android:orientation attribute. Each child of a LinearLayout appears on the

screen in the order in which it appears in the XML.

The other two attributes, android:layout_width and android:layout_height, are required for all views in order

to specify their size.

Because the LinearLayout is the root view in the layout, it should fill the entire screen area that's available to the app

by setting the width and height to "match_parent". This value declares that the view should expand its width or height

to match the width or height of the parent view.

For more information about layout properties, see the Layout guide.

Add a Text Field

To create a user-editable text field, add an <EditText> element inside the <LinearLayout>.

Like every View object, you must define certain XML attributes to specify the EditText object's properties. Here’s how

you should declare it inside the <LinearLayout> element:

<EditText android:id="@+id/edit_message"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:hint="@string/edit_message" />

This lesson teaches you to

• Create a Linear Layout

• Add a Text Field

• Add String Resources

• Add a Button

•

Make the Input Box Fill in the Screen Width

You should also read

• Layouts

Alternative Layouts

Declaring your UI layout in XML rather than

runtime code is useful for several reasons, but

it's especially important so you can create

different layouts for different screen sizes. For

example, you can create two versions of a

layout and tell the system to use one on

"small" screens and the other on "large"

screens. For more information, see the class

about Supporting Different Devices.

Building a Simple User Interface

Content from developer.android.com/training/basics/firstapp/building-ui.html through their Creative Commons Attribution 2.5 license

About these attributes:

android:id

This provides a unique identifier for the view, which you

can use to reference the object from your app code,

such as to read and manipulate the object (you'll see

this in the next lesson).

The at sign (@) is required when you're referring to any

resource object from XML. It is followed by the resource

type (id in this case), a slash, then the resource name

(edit_message).

The plus sign (+) before the resource type is needed

only when you're defining a resource ID for the first time.

When you compile the app, the SDK tools use the ID

name to create a new resource ID in your project's

gen/R.java file that refers to the EditText element.

Once the resource ID is declared once this way, other

references to the ID do not need the plus sign. Using the

plus sign is necessary only when specifying a new

resource ID and not needed for concrete resources such

as strings or layouts. See the sidebox for more

information about resource objects.

android:layout_width and android:layout_height

Instead of using specific sizes for the width and height, the "wrap_content" value specifies that the view

should be only as big as needed to fit the contents of the view. If you were to instead use "match_parent",

then the EditText element would fill the screen, because it would match the size of the parent

LinearLayout. For more information, see the Layouts guide.

android:hint

This is a default string to display when the text field is empty. Instead of using a hard-coded string as the value,

the "@string/edit_message" value refers to a string resource defined in a separate file. Because this refers

to a concrete resource (not just an identifier), it does not need the plus sign. However, because you haven't

defined the string resource yet, you’ll see a compiler error at first. You'll fix this in the next section by defining

the string.

Note: This string resource has the same name as the element ID: edit_message. However, references to

resources are always scoped by the resource type (such as id or string), so using the same name does not

cause collisions.

Add String Resources

When you need to add text in the user interface, you should always specify each string as a resource. String resources

allow you to manage all UI text in a single location, which makes it easier to find and update text. Externalizing the

strings also allows you to localize your app to different languages by providing alternative definitions for each string

resource.

By default, your Android project includes a string resource file at res/values/strings.xml. Add a new string named

"edit_message" and set the value to "Enter a message." (You can delete the "hello_world" string.)

While you’re in this file, also add a "Send" string for the button you’ll soon add, called "button_send".

The result for strings.xml looks like this:

<?xml version="1.0" encoding="utf-8"?>

<string name="app_name">My First App</string>

<string name="edit_message">Enter a message</string>

<string name="action_settings">Settings</string>

<string name="title_activity_main">MainActivity</string>

</resources>

For more information about using string resources to localize your app for other languages, see the Supporting Different

Devices class.

Add a Button

Now add a <Button> to the layout, immediately following the <EditText> element:

About resource objects

A resource object is simply a unique integer

name that's associated with an app resource,

such as a bitmap, layout file, or string.

Every resource has a corresponding resource

object defined in your project's gen/R.java

file. You can use the object names in the R

class to refer to your resources, such as when

you need to specify a string value for the

android:hint attribute. You can also create

arbitrary resource IDs that you associate with

a view using the android:id attribute, which

allows you to reference that view from other

code.

The SDK tools generate the R.java each

time you compile your app. You should never

modify this file by hand.

For more information, read the guide to

Providing Resources.

Building a Simple User Interface

Content from developer.android.com/training/basics/firstapp/building-ui.html through their Creative Commons Attribution 2.5 license

<Button

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="@string/button_send" />

The height and width are set to "wrap_content" so the button is only as big as necessary to fit the button's text. This

button doesn't need the android:id attribute, because it won't be referenced from the activity code.

Make the Input Box Fill in the Screen Width

The layout is currently designed so that both the EditText and Button widgets are only as big as necessary to fit their

content, as shown in figure 2.

Figure 2. The EditText and Button widgets have their widths set to "wrap_content".

This works fine for the button, but not as well for the text field, because the user might type something longer. So, it

would be nice to fill the unused screen width with the text field. You can do this inside a LinearLayout with the weight

property, which you can specify using the android:layout_weight attribute.

The weight value is a number that specifies the amount of remaining space each view should consume, relative to the

amount consumed by sibling views. This works kind of like the amount of ingredients in a drink recipe: "2 parts vodka, 1

part coffee liqueur" means two-thirds of the drink is vodka. For example, if you give one view a weight of 2 and another

one a weight of 1, the sum is 3, so the first view fills 2/3 of the remaining space and the second view fills the rest. If you

add a third view and give it a weight of 1, then the first view (with weight of 2) now gets 1/2 the remaining space, while

the remaining two each get 1/4.

The default weight for all views is 0, so if you specify any weight value greater than 0 to only one view, then that view fills

whatever space remains after all views are given the space they require. So, to fill the remaining space in your layout

with the EditText element, give it a weight of 1 and leave the button with no weight.

<EditText

android:layout_weight="1"

... />

In order to improve the layout efficiency when you specify the weight, you should change the width of the EditText to

be zero (0dp). Setting the width to zero improves layout performance because using "wrap_content" as the width

requires the system to calculate a width that is ultimately irrelevant because the weight value requires another width

calculation to fill the remaining space.

<EditText

android:layout_weight="1"

android:layout_width="0dp"

... />

Figure 3 shows the result when you assign all weight to the EditText element.

Figure 3. The EditText widget is given all the layout weight, so fills the remaining space in the LinearLayout.

Here’s how your complete layout file should now look:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

xmlns:tools="http://schemas.android.com/tools"

android:layout_width="match_parent"

android:layout_height="match_parent"

android:orientation="horizontal">

<EditText android:id="@+id/edit_message"

android:layout_weight="1"

android:layout_width="0dp"

android:layout_height="wrap_content"

android:hint="@string/edit_message" />

<Button

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="@string/button_send" />

</LinearLayout>

This layout is applied by the default Activity class that the SDK tools generated when you created the project, so you

can now run the app to see the results:

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