建一个简单的用户界面

Create a Linear Layout（创建一个线性布局）

<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>

注释：android:orientation="vertical"水平线性布局,"horizontal"垂直水平线性

match_parent：This value declares that the view should expand its width or height to match the width or height of the parent view.

Add a Text Field（添加一个text）

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

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

Add String Resources

String resources allow you to manage all UI text in a single location, which makes it easier to find and update text.
默认情况下string resource 文件位于t res/values/strings.xml.

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

Add a Button

 <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

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

weight表示横向占比：如果有两个view（分别为view1，view2），view1的weight值设为2，view2的weight值设为1，那么view1占2/3,view2占1/3;
weight的默认值为0，所以如果在多个views中只设置一个weight值为1，其他均为0，那么weight值设为1的那个view会占全其他空间。

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>