Kotlin - A Guide to using ConstraintLayout in Android Studio

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A Guide to the Android ConstraintLayoutWorking with ConstraintLayout Chains and Ratios in Android Studio

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As mentioned more than once in previous chapters, Google has made significant changes to the Android Studio Layout Editor tool, many of which were made solely to support user interface layout design using ConstraintLayout. Now that the basic concepts of ConstraintLayout have been outlined in the previous chapter, this chapter will explore these concepts in more detail while also outlining the ways in which the Layout Editor tool allows ConstraintLayout-based user interfaces to be designed and implemented.

Design and Layout Views

The chapter entitled “A Guide to the Android Studio Layout Editor Tool” explained that the Android Studio Layout Editor tool provides two ways to view the user interface layout of an activity in the form of Design and Layout (also known as blueprint) views. These views of the layout may be displayed individually or, as in Figure 25-1, side by side:

As3.0 layout design and blueprint constraints.png

The Design view (positioned on the left in the above figure) presents a “what you see is what you get” representation of the layout, wherein the layout appears as it will within the running app. The Layout view, on the other hand, displays a blueprint style of view where the widgets are represented by shaded outlines. As can be seen in Figure 25-1 above, Layout view also displays the constraint connections (in this case opposing constraints used to center a button within the layout). These constraints are also overlaid onto the Design view when a specific widget in the layout is selected or when the mouse pointer hovers over the design area as illustrated in Figure 25-2:

As3.0 layout design constraints.png

The appearance of constraint connections in both views can be change using the toolbar menu shown in Figure 25-3:

As3.0 layout show constraints.png

In addition to the two modes of displaying the user interface layout, the Layout Editor tool also provides three different ways of establishing the constraints required for a specific layout design.

Autoconnect Mode

Autoconnect, as the name suggests, automatically establishes constraint connections as items are added to the layout. Autoconnect mode may be enabled and disabled using the toolbar button indicated in Figure 25-4:

As3.0 layout autoconnect button.png

Autoconnect mode uses algorithms to decide the best constraints to establish based on the position of the widget and the widget’s proximity to both the sides of the parent layout and other elements in the layout. In the event that any of the automatic constraint connections fail to provide the desired behavior, these may be changed manually as outlined later in this chapter.

Inference Mode

Inference mode uses a heuristic approach involving algorithms and probabilities to automatically implement constraint connections after widgets have already been added to the layout. This mode is usually used when the Autoconnect feature has been turned off and objects have been added to the layout without any constraint connections. This allows the layout to be designed simply by dragging and dropping objects from the palette onto the layout canvas and making size and positioning changes until the layout appears as required. In essence this involves “painting” the layout without worrying about constraints. Inference mode may also be used at any time during the design process to fill in missing constraints within a layout.

Constraints are automatically added to a layout when the Infer constraints button (Figure 25-5) is clicked:

As3.0 layout inference button.png

As with Autoconnect mode, there is always the possibility that the Layout Editor tool will infer incorrect constraints, though these may be modified and corrected manually.

Manipulating Constraints Manually

The third option for implementing constraint connections is to do so manually. When doing so, it will be helpful to understand the various handles that appear around a widget within the Layout Editor tool. Consider, for example, the widget shown in Figure 25-6:

As3.0 layout constraint elements.png

Clearly the spring-like lines (A) represent established constraint connections leading from the sides of the widget to the targets. The small square markers (B) in each corner of the object are resize handles which, when clicked and dragged, serve to resize the widget. The small circle handles (C) located on each side of the widget are the side constraint anchors. To create a constraint connection, click on the handle and drag the resulting line to the element to which the constraint is to be connected (such as a guideline or the side of either the parent layout or another widget) as outlined in Figure 25-7. When connecting to the side of another widget, simply drag the line to the side constraint handle of that widget and, when it turns green, release the line.

As3.0 layout drag constraint.png

An additional marker indicates the anchor point for baseline constraints whereby the content within the widget (as opposed to outside edges) is used as the alignment point. To display this marker, simply click on the button displaying the letters ‘ab’ (referenced by marker D in Figure 25-6). To establish a constraint connection from a baseline constraint handle, simply hover the mouse pointer over the handle until it begins to flash before clicking and dragging to the target (such as the baseline anchor of another widget as shown in Figure 25-8). When the destination anchor begins to flash green, release the mouse button to make the constraint connection:

As3.0 layout baseline constraint.png

To hide the baseline anchors, simply click on the baseline button a second time.

Adding Constraints in the Inspector

Constraints may also be added to a view within the Android Studio Layout Editor tool using the Inspector panel located in the Attributes tool window as shown in Figure 25-9. The square in the center represents the currently selected view and the areas around the square the constraints, if any, applied to the corresponding sides of the view:

As3.0 constraint inspector.png

The absence of a constraint on a side of the view is represented by a dotted line leading to a blue circle containing a plus sign (as is the case with the bottom edge of the view in the above figure). To add a constraint, simply click on this blue circle and the layout editor will add a constraint connected to what it considers to be the most appropriate target within the layout.

Deleting Constraints

To delete an individual constraint, simply click within the anchor to which it is connected. The constraint will then be deleted from the layout (when hovering over the anchor it will glow red to indicate that clicking will perform a deletion):

As3.0 layout delete connection.png

Alternatively, remove all of the constraints on a widget by selecting it and clicking on the Delete All Constraints button which appears next to the widget when it is selected in the layout as highlighted in Figure 25-11:

As3.0 layout delete all connections.png

To remove all of the constraints from every widget in a layout, use the toolbar button highlighted in Figure 25-12:

As3.0 layout delete all connections in layout.png

Adjusting Constraint Bias

In the previous chapter, the concept of using bias settings to favor one opposing constraint over another was outlined. Bias within the Android Studio Layout Editor tool is adjusted using the Inspector located in the Attributes tool window and shown in Figure 25-13. The two sliders indicated by the arrows in the figure are used to control the bias of the vertical and horizontal opposing constraints of the currently selected widget.

As3.0 layout bias sliders.png

Understanding ConstraintLayout Margins Constraints can be used in conjunction with margins to implement fixed gaps between a widget and another element (such as another widget, a guideline or the side of the parent layout). Consider, for example, the horizontal constraints applied to the Button object in Figure 25-14:

As3.0 layout centered button.png

As currently configured, horizontal constraints run to the left and right edges of the parent ConstraintLayout. As such, the widget has opposing horizontal constraints indicating that the ConstraintLayout layout engine has some discretion in terms of the actual positioning of the widget at runtime. This allows the layout some flexibility to accommodate different screen sizes and device orientation. The horizontal bias setting is also able to control the position of the widget right up to the right-hand side of the layout. Figure 25-15, for example, shows the same button with 100% horizontal bias applied:

As3.0 layout 100 percent bias.png

ConstraintLayout margins can appear at the end of constraint connections and represent a fixed gap into which the widget cannot be moved even when adjusting bias or in response to layout changes elsewhere in the activity. In Figure 25-16, the right-hand constraint now includes a 50dp margin into which the widget cannot be moved even though the bias is still set at 100%.

As3.0 layout margin constraint.png

Existing margin values on a widget can be modified from within the Inspector. As can be seen in Figure 25-17, a dropdown menu is being used to change the right-hand margin on the currently selected widget to 16dp. Alternatively, clicking on the current value also allows a number to be typed into the field.

As3.0 layout inspector set margin.png

The default margin for new constraints can be changed at any time using the option in the toolbar highlighted in Figure 25-18:

As3.0 layout set all margins.png

The Importance of Opposing Constraints and Bias As discussed in the previous chapter, opposing constraints, margins and bias form the cornerstone of responsive layout design in Android when using the ConstraintLayout. When a widget is constrained without opposing constraint connections, those constraints are essentially margin constraints. This is indicated visually within the Layout Editor tool by solid straight lines accompanied by margin measurements as shown in Figure 25-19.

As3.0 layout opposing constraint.png

The above constraints essentially fix the widget at that position. The result of this is that if the device is rotated to landscape orientation, the widget will no longer be visible since the vertical constraint pushes it beyond the top edge of the device screen (as is the case in Figure 25-20). A similar problem will arise if the app is run on a device with a smaller screen than that used during the design process.

As3.0 layout opposing constraint landscape.png

When opposing constraints are implemented, the constraint connection is represented by the spring-like jagged line (the spring metaphor is intended to indicate that the position of the widget is not fixed to absolute X and Y coordinates):

As3.0 layout opposing constraint bias.png

In the above layout, vertical and horizontal bias settings have been configured such that the widget will always be positioned 90% of the distance from the bottom and 35% from the left-hand edge of the parent layout. When rotated, therefore, the widget is still visible and positioned in the same location relative to the dimensions of the screen:

As3.0 layout opposing constraint landscape bias.png

When designing a responsive and adaptable user interface layout, it is important to take into consideration both bias and opposing constraints when manually designing a user interface layout and making corrections to automatically created constraints.

Configuring Widget Dimensions

The inner dimensions of a widget within a ConstraintLayout can also be configured using the Inspector. As outlined in the previous chapter, widget dimensions can be set to wrap content, fixed or match constraint modes. The prevailing settings for each dimension on the currently selected widget are shown within the square representing the widget in the Inspector as illustrated in Figure 25-23:

As3.0 layout widget dims.png

In the above figure, both the horizontal and vertical dimensions are set to wrap content mode (indicated by the inward pointing chevrons). The inspector uses the following visual indicators to represent the three dimension modes:

As3.0 layout widget dims fixed.png - Fixed Size

As3.0 layout widget dims match.png - Match Constraint

As3.0 layout widget dims wrap.png - Wrap Content

To change the current setting, simply click on the indicator to cycle through the three different settings. When the dimension of a view within the layout editor is set to match constraint mode, the corresponding sides of the view are drawn with the spring-like line instead of the usual straight lines. In Figure 25-24, for example, only the width of the view has been set to match constraint:

As3.0 constraint view with match constraints.png

In addition, the size of a widget can be expanded either horizontally or vertically to the maximum amount allowed by the constraints and other widgets in the layout using the Expand horizontally and Expand vertically options. These are accessible by right clicking on a widget within the layout and selecting the Organize option from the resulting menu (Figure 25-25). When used, the currently selected widget will increase in size horizontally or vertically to fill the available space around it.

As3.0 layout expand.png

Adding Guidelines

Guidelines provide additional elements to which constraints may be anchored. Guidelines are added by right-clicking on the layout and selecting either the Add Vertical Guideline or Add Horizontal Guideline menu option or using the toolbar menu options as shown in Figure 25-26:

As3.0 layout add guidelines.png

Once added, a guideline will appear as a dashed line in the layout and may be moved simply by clicking and dragging the line. To establish a constraint connection to a guideline, click in the constraint handler of a widget and drag to the guideline before releasing. In Figure 25-27, the left sides of two Buttons are connected by constraints to a vertical guideline.

The position of a vertical guideline can be specified as an absolute distance from either the left or the right of the parent layout (or the top or bottom for a horizontal guideline). The vertical guideline in the above figure, for example, is positioned 96dp from the left-hand edge of the parent.

As3.0 layout guideline.png

Alternatively, the guideline may be positioned as a percentage of overall width or height of the parent layout. To switch between these three modes, select the guideline and click on the circle at the top or start of the guideline (depending on whether the guideline is vertical or horizontal). Figure 25-28, for example, shows a guideline positioned based on percentage:

As3.0 layout guideline percent.png

Adding Barriers

Barriers are added by right-clicking on the layout and selecting either the Add Vertical Barrier or Add Horizontal Barrier menu option, or using the toolbar menu options as shown in Figure 25-29:

As3.0 adding barrier.png

Once a barrier has been added to the layout, it will appear as an entry in the Component Tree panel:

As3.0 barrier in component tree.png

To add views as reference views (in other words, the views that control the position of the barrier), simply drag the widgets from within the Component Tree onto the barrier entry. In Figure 25-31, for example, widgets named textView1 and textView2 have been assigned as the reference widgets for barrier1:

As3.0 barrier in component tree with widgets.png

After the reference views have been added, the barrier needs to be configured to specify the direction of the barrier in relation those views. This is the barrier direction setting and is defined within the Attributes tool window when the barrier is selected in the Component Tree panel:

As3.0 barrier direction property.png

The following figure shows a layout containing a barrier declared with textView1 and textView2 acting as the reference views and textview3 as the constrained view. Since the barrier is pushing from the end of the reference views towards the constrained view, the barrier direction has been set to end:

As3.0 barrier example.png

Widget Group Alignment

The Android Studio Layout Editor tool provides a range of alignment actions that can be performed when two or more widgets are selected in the layout. Simply shift-click on each of the widgets to be included in the action, right-click on the layout and make a selection from the many options displayed in the Align menu:

As3.0 layout align menu.png

As shown in Figure 25-35 below, these options are also available as buttons in the Layout Editor toolbar:

As3.0 layout toolbar align menu.png

Converting other Layouts to ConstraintLayout

For existing user interface layouts that make use of one or more of the other Android layout classes (such as RelativeLayout or LinearLayout), the Layout Editor tool provides an option to convert the user interface to use the ConstraintLayout.

When the Layout Editor tool is open and in Design mode, the Component Tree panel is displayed beneath the Palette. To convert a layout to ConstraintLayout, locate it within the Component Tree, right-click on it and select the Convert <current layout> to Constraint Layout menu option:

As3.0 layout convert to constraint layout.png

When this menu option is selected, Android Studio will convert the selected layout to a ConstraintLayout and use inference to establish constraints designed to match the layout behavior of the original layout type.


A redesigned Layout Editor tool combined with ConstraintLayout makes designing complex user interface layouts with Android Studio a relatively fast and intuitive process. This chapter has covered the concepts of constraints, margins and bias in more detail while also exploring the ways in which ConstraintLayout-based design has been integrated into the Layout Editor tool.

You are reading a sample chapter from the Android Studio 3.0 / Android 8 Edition book.

Purchase the fully updated Android Studio 3.2 / Android 9 / Jetpack Edition of this publication in eBook ($29.99) or Print ($45.99) format

Android Studio 3.2 Development Essentials - Kotlin Edition Print and eBook (ePub/PDF/Kindle) editions contain 96 chapters and over 800 pages

Buy Print Preview Book

PreviousTable of ContentsNext
A Guide to the Android ConstraintLayoutWorking with ConstraintLayout Chains and Ratios in Android Studio