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Build a Simple User Interface

Juni 21, 2017 0

In this lesson, you'll use the Android Studio Layout Editor to create a layout that includes a text box and a button. In the next lesson, you'll make the app respond to the button tap by sending the content of the text box to another activity.
Figure 1. Screenshot of the final layout
The user interface for an Android app is built using a hierarchy of layouts (ViewGroup objects) and widgets (View objects). Layouts are invisible containers that control how its child views are positioned on the screen. Widgets are UI components such as buttons and text boxes.
Figure 2. Illustration of how ViewGroup objects form branches in the layout and contain View objects
Android provides an XML vocabulary for ViewGroup and View classes, so most of your UI is defined in XML files. However, instead of teaching you to write some XML, this lesson shows you how to create a layout using Android Studio's Layout Editor, which makes it easy to build a layout by drag-and-dropping views.

Open the Layout Editor

Note: This lesson expects you are using Android Studio 2.3 or higher and you've followed the previous lesson to create your Android project.
To get started, set up your workspace as follows:
  1. In Android Studio's Project window, open app > res > layout > activity_main.xml.
  2. To make more room for the Layout Editor, hide the Project window by selecting View > Tool Windows > Project (or click Project on the left side of Android Studio).
  3. If your editor shows the XML source, click the Design tab at the bottom of the window.
  4. Click Show Blueprint so only the blueprint layout is visible.
  5. Make sure Show Constraints is on. The tooltip in the toolbar should read Hide Constraints (because they're now showing).
  6. Make sure Autoconnect is off. The tooltip in the toolbar should read Turn On Autoconnect (because it's now off).
  7. Click Default Margins in the toolbar and select 16 (you can still adjust the margin for each view later).
  8. Click Device in Editor in the toolbar and select Pixel XL.
Your editor should now look as shown in figure 3.
Figure 3. The Layout Editor showing activity_main.xml
The Component Tree window on the bottom-left side shows the layout's hierarchy of views. In this case, the root view is a ConstraintLayout, containing just one TextView object.
ConstraintLayout is a layout that defines the position for each view based on constraints to sibling views and the parent layout. In this way, you can create both simple and complex layouts with a flat view hieararchy. That is, it avoids the need for nested layouts (a layout inside a layout, as shown in figure 2), which can increase the time required to draw the UI.
Figure 4. Illustration of two views positioned inside ConstraintLayout
For example, you can declare the following layout (in figure 4):
  • View A appears 16dp from the top of the parent layout.
  • View A appears 16dp from the left of the parent layout.
  • View B appears 16dp to the right of view A.
  • View B is aligned to the top of view A.
In the following sections, you'll build a layout similar to this.

Add a text box

Figure 5. The text box is constrained to the top and left of the parent layout
  1. First, you need to remove what's already in the layout. So click TextView in the Component Tree window, and then press Delete.
  2. From the Palette window on the left, click Text in the left pane, and then drag Plain Text into the design editor and drop it near the top of the layout. This is an EditText widget that accepts plain text input.
  3. Click the view in the design editor. You can now see the resizing handles on each corner (squares), and the constraint anchors on each side (circles). For better control, you might want to zoom in on the editor to 75% or higher using the buttons in the toolbar.
  4. Click-and-hold the anchor on the top side, and then drag it up until it snaps to the top of the layout and release. That's a constraint—it specifies the view should be 16dp from the top of the layout (because you set the default margins to 16dp).
  5. Similarly, create a constraint from the left side of the view to the left side of the layout.
The result should look like the screenshot in figure 5.

Add a button

Figure 6. The button is constrained to the right side of the text box and its baseline
  1. From the Palette window, click Widgets in the left pane, and then drag Button into the design editor and drop it near the right side.
  2. Create a constraint from the left side of the button to the right side of the text box.
  3. To constrain the views in a horizontal alignment, you need to create a constraint between the text baselines. So click the button, and then click Baseline Constraint , which appears in the design editor directly below the selected view. The baseline anchor appears inside the button. Click-and-hold on this anchor and then drag it to the baseline anchor that appears in the text box.
The result should look like the screenshot in figure 6.
Note: You can also create a horizontal alignment using the top or bottom edges, but the button includes padding around its image, so the visual alignment is wrong if you align these views that way.

Change the UI strings

Click Show Design in the toolbar to preview the UI. Notice that the text input is pre-filled with "Name" and the button is labeled "Button." So now you'll change these strings.
  1. Open the Project window and then select res > values > strings.xml. This is a string resources file where you should specify all your UI strings. Doing so allows you to manage all UI strings in a single location, which makes it easier to find, update, and localize (compared to hard-coding strings in your layout or app code).
  2. Click Open editor at the top of the editor window. This opens the Translations Editor, which provides a simple interface for adding and editing your default strings, and helps keep all your translated strings organized.
  3. Figure 7. The dialog to add a new string
    Click Add Key to create a new string as the "hint text" for the text box.
    1. Enter "edit_message" for the key name.
    2. Enter "Enter a message" for the value.
    3. Click OK.
  4. Add another key named "button_send" with a value of "Send."
Now you can set these strings for each view. So return to the layout file by clicking activity_main.xml in the tab bar, and add the strings as follows:
  1. Click the text box in the layout and, if the Properties window isn't already visible on the right, click Properties on the right sidebar.
  2. Locate the hint property and then click Pick a Resource to the right of the text box. In the dialog that appears, double-click on edit_message from the list.
  3. Still viewing the text box properties, also delete the value for the text property (currently set to "Name").
  4. Now click the button in the layout, locate the text property, click Pick a Resource, and then select button_send.

Make the text box size flexible

To create a layout that's responsive to different screen sizes, you'll now make the text box stretch to fill all remaining horizontal space (after accounting for the button and margins).
Figure 8. Click to change the width and remove the right margin
Figure 9. The text box stretches to fill the remaining space
  1. Create a constraint from the right side of the button to the right side of the parent layout. This now defines the total width that the two views have available (which you can now fill with the text box).
  2. Add a constraint from the right side of the text box to the left side of the button. It might look like that's already there, but you're actually adding a bidirectional constraint between the two views. So both views are constrained to each other. This is called a chain (as indicated by the chain betwen the views), and it enables some extra layout options.
  3. Open the Properties window for the text box and then click the width indicator until set to Match Constraints, as indicated by callout 1 in figure 8. "Match constraints" means that the width is now determined by the horizontal constraints and margins. Therefore, the text box stretches to fill the horizontal space.
  4. However, the two views are separated by 32dp instead of 16dp, because both views have margins. So while still viewing the text box properties, change the right margin to 0, as indicated by callout 2 in figure 8.
Now the layout is done and should appear as shown in figure 9.
If it seems your layout did not turn out as expected, click below to see what your the XML should look like and compare it to what you see in the Text tab.
For more information about chains and all the other things you can do with ConstraintLayout, read Build a Responsive UI with ConstraintLayout.

Run the app

If your app is already installed on the device from the previous lesson, simply click Apply Changes in the toolbar to update the app with the new layout. Or click Run to install and run the app.



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