构建一个简单的用户界面

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This lesson teaches you to

1. Create a Linear Layout
2. Add a Text Field
3. Add String Resources
4. Add a Button
5. Make the Input Box Fill in the Screen Width

You should also read

• Layouts

在这一课，你会创建一个布局，包括一个文本框和一个按钮 . 在下一课，当点击按钮，文本框的文字信息会发送到另一个activity，这是你App的响应 .

安卓应用的图形用户界面的创建是使用了 View and ViewGroup 对象. View 对象通常是UI部件，比如 buttons 或者 text fields. ViewGroup 对象是不可见的view容器，它定义了子view如何布局，比如在grid里面或者垂直列表。

安卓提供了XML去对应 View 和 ViewGroup 的子类，这样你可以在XML里面使用UI元素定义UI.

Layouts are subclasses of the ViewGroup. In this exercise, you'll work with a LinearLayout.

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.

Figure 1. Illustration of how ViewGroup objects form branches in the layout and contain other View objects.

Create a Linear Layout

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1. 在res/layout 下面，打开 content_my.xml 文件.

   当你创建项目的时候，选择的 BlankActivity 模板包含了 content_my.xml 。这个XML文件定义了一个 RelativeLayout root view and a TextView child view.

2. In the Preview pane, click the Hide icon  to close the Preview pane.

   In Android Studio, when you open a layout file, you’re first shown the Preview pane. Clicking elements in this pane opens the WYSIWYG tools in the Design pane. For this lesson, you’re going to work directly with the XML.

3. Delete the <TextView> element.
4. Change the <RelativeLayout> element to <LinearLayout>.
5. Add the android:orientation attribute and set it to "horizontal"（水平）.
6. Remove the android:padding attributes and the tools:context attribute.

The result looks like this:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:app="http://schemas.android.com/apk/res-auto"    xmlns:tools="http://schemas.android.com/tools"    android:orientation="horizontal"    android:layout_width="match_parent"    android:layout_height="match_parent"    app:layout_behavior="@string/appbar_scrolling_view_behavior"    tools:showIn="@layout/activity_my">

LinearLayout（线性布局） is a view group (a subclass（子类） of ViewGroup) that lays out child views in either a vertical or horizontal orientation（方向）, as specified by the android:orientation attribute.  LinearLayout 每一个子元素在屏幕上在按顺序显示.另外两个属性, android:layout_width and android:layout_height, 为了明确大小，这两个属性都是都是必须的.

LinearLayout 在布局中是根视图,它必须通过设置高度和宽度为"match_parent" 去填充整个屏幕。这个值（match_parent）声明了视图应该扩大宽度或者高度去匹配它的父视图。

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

添加一个文本框Text Field

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正如每一个视图对象（View object）, 你必须定义一些XML属性来明确 EditText 对象的属性.

1. 在 content_my.xml 文件, within the <LinearLayout> element, define an <EditText> element with the id attribute set to @+id/edit_message.
2. Define the layout_width and layout_height attributes as wrap_content.
3. Define a hint attribute as a string object named edit_message.

The <EditText> element should read as follows:

<EditText android:id="@+id/edit_message"    android:layout_width="wrap_content"    android:layout_height="wrap_content"    android:hint="@string/edit_message" />

Here are the <EditText> attributes you added:

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).

Resource Objects

A resource object is 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.javafile each time you compile your app. You should never modify this file by hand.

For more information, read the guide toProviding Resources.

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. With the resource ID 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 EditTextelement 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

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By default, your Android project includes a string resource file at res/values/strings.xml. Here, you'll add a new string named "edit_message" and set the value to "Enter a message."

1. In Android Studio, from the res/values directory, open strings.xml.
2. Add a line for a string named "edit_message" with the value, "Enter a message".
3. Add a line for a string named "button_send" with the value, "Send".

   You'll create the button that uses this string in the next section.

The result for strings.xml looks like this:

<?xml version="1.0" encoding="utf-8"?><resources>    <string name="app_name">My First App</string>    <string name="edit_message">Enter a message</string>    <string name="button_send">Send</string>    <string name="action_settings">Settings</string></resources>

For text in the user interface, always specify each string as a resource. String resources allow you to manage all UI text in a single location, which makes the text easier to find and update. Externalizing the strings also allows you to localize your app to different languages by providing alternative definitions for each string resource.

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

Add a Button

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1. In Android Studio, from the res/layout directory, edit the content_my.xml file.
2. Within the <LinearLayout> element, define a <Button> element immediately following the <EditText>element.
3. Set the button's width and height attributes to "wrap_content" so the button is only as big as necessary to fit the button's text label.
4. Define the button's text label with the android:text attribute; set its value to the button_send string resource you defined in the previous section.

Your <LinearLayout> should look like this:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"    xmlns:app="http://schemas.android.com/apk/res-auto"    xmlns:tools="http://schemas.android.com/tools"    android:orientation="horizontal"    android:layout_width="match_parent"    android:layout_height="match_parent"    app:layout_behavior="@string/appbar_scrolling_view_behavior"    tools:showIn="@layout/activity_my">        <EditText android:id="@+id/edit_message"          android:layout_width="wrap_content"          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>

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

The layout is currently designed so that both the EditText and Button widgets are only as big as necessary to fit their content, as Figure 2 shows.

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. It would be nice to fill the unused screen width with the text field. You can do this inside a LinearLayout with theweight 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 soda, 1 part syrup" means two-thirds of the drink is soda. 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.

Make the Input Box Fill in the Screen Width

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To fill the remaining space in your layout with the EditText element, do the following:

1. In the content_my.xml file, assign the <EditText> element's layout_weight attribute a value of 1.
2. Also, assign <EditText> element's layout_width attribute a value of 0dp.

   <EditText    android:layout_weight="1"    android:layout_width="0dp"    ... />

   To improve the layout efficiency when you specify the weight, you should change the width of the EditTextto 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.

   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 it fills the remaining space in the LinearLayout.

Here’s how your complete content_my.xmllayout file should now look:

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"   xmlns:app="http://schemas.android.com/apk/res-auto"   xmlns:tools="http://schemas.android.com/tools"   android:orientation="horizontal"   android:layout_width="match_parent"   android:layout_height="match_parent"   app:layout_behavior="@string/appbar_scrolling_view_behavior"   tools:showIn="@layout/activity_my">    <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>

Run Your App

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This layout is applied by the default Activity class that the SDK tools generated when you created the project. Run the app to see the results:

• In Android Studio, from the toolbar, click Run .
• Or from a command line, change directories to the root of your Android project and execute:

  $ ant debugadb install -r app/build/outputs/apk/app-debug.apk

Continue to the next lesson to learn how to respond to button presses, read content from the text field, start another activity, and more.