Constraint Layout – blog.fossasia.org

Creating Control Panel For Wave Generator using Constraint Layout

Post author:Avjeet
Post published:July 30, 2018
Post category:Android FOSSASIA GSoC Pocket Science Lab
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In the blog Creating the onScreen Monitor Using CardView I had created the monitors to view the wave properties in this blog we will create the UI of controlling panel that will be used for that monitors with multiple buttons for both analog and digital waveforms.

Which layout to choose?

In today’s world, there are millions of Android devices present with different screen sizes and densities and the major concern of an Android developer is to make the layout that fits all the devices and this task is really difficult to handle with a linear or relative layout with fixed dimensions.

To create a complex layout with lots of views inside the parent using linear layout we have to make use of the attribute layout_weight for their proper stretching and positioning, but such a complex layout require a lot of nesting of weights and android tries to avoid it by giving a warning :

Nested Weights are bad for performance

This is because layout_weight attribute requires a widget to be measured twice[1]. When a LinearLayout with non-zero weights is nested inside another LinearLayout with non-zero weights, then the number of measurements increases exponentially.

So, to overcome this issue we will make use of special type of layout “Constraint Layout” which was introduced in Google I/O 2016.

Features of Constraint Layout:-

It is similar to Relative Layout as all views are laid out according to their relationship with the sibling, but it is more flexible than Relative Layout.
It helps to flatten the view hierarchy for complex layouts.
This layout is created with the help of powerful tool provided by Android which has a palette on the left-hand side from where we can drag and drop the different widgets like TextView, ImageView, Buttons etc. and on the right-hand side it provides options for positioning, setting margins and other styling option like setting color, change text style etc.
It automatically adjusts the layout according to the screen size and hence doesn’t require the use of layout_weight attribute.

In following steps, I will create the controlling panel for Wave generator which is a complex layout with lots of buttons with the help of constraint layout.

Step 1: Add the dependency of the Constraint Layout in the Project

To use Constraint layout add the following to your build.gradle file and sync the project

dependencies {
    implementation "com.android.support.constraint:constraint-layout:1.1.0"
}

Step 2: Applying Guidelines to the layout

Guidelines[3] are anchors that won’t be displayed in your app, they are like one line of a grid above your layout and can be used to attach or constraint your widgets to it. They are only visible on your blueprint or preview editor. These will help to position and constraint the UI components on the screen easily.

For adding guidelines :

As shown in Figure 1 Right-click anywhere on the layout -> Select helpers -> Select horizontal or vertical guideline according to your need.

Figure 1 shows the horizontal guideline being added to the layout.

And for positioning the guideline we have to set the value of attribute layout_constraintGuide_percent

Let’s say we want the guideline to be at the middle of the screen so we’ll set :

app:layout_constraintGuide_percent=”0.50″

For my layout I have added three guideline :

One horizontal guideline at 50%
Two vertical guidelines at 30% and 65%

Doing this will bifurcate the screen into six square blocks as shown in below figure :

Figure 2 shows the blueprint of constraint layout containing two vertical and one horizontal guidelines with their percentage offset from respective bases

Step 3: Adding the buttons in the blocks

Until now we have created six squares blocks, now we have to put a button view in each of the boxes.

First drag and drop button view from the Palette (shown in Figure 3) on the left side inside the box.

Figure 3 shows the layout editor palette
Then we have to set constraints of this button by clicking on the small circle present on the middle of edges and dragging it onto the side of the block facing it.

Figure 4 shows the button widget getting constrained to sides
Set the layout_width and layout_height attribute of the button to be “0dp”, doing this the button will expand in all the direction occupying all the space with respect to the border it has been constrained with.

Figure 6 shows the button widget expanding to all the available space in the box

Similarly, adding buttons in all the square blocks and providing proper theme color we will have a blueprint and layout as shown in Figure 6.

Figure 6 shows the waveform panel blueprint and actual layout for analog waves with six buttons

Following the same steps until now, I have created the other controlling panel layout having buttons for digital waves as shown in Figure 7

Figure 7 shows other constraint layout for digital waves having seven buttons

Detailing and combining the panels to form Complete UI

After adding both the panels we have created in this layout inside the Wave Generator we have the layout as shown in Figure 8

Figure 8 shows the UI of Wave Generator as shown by a actual Android device in the PSLab app.

As we can see on adding the panels the button created inside the layout shrink so as to adapt to the screen and giving out a beautiful button-like appearance.

Resources

Few months ago, during the Google I/O conference, Google introduced a new set of tools for Android developers. Among them is a new Layout editor and a new layout called the ConstraintLayout.

I’ll be highlighting the key points in this brand new layout.

ConstraintLayout is available in a new Support Library that’s compatible with Android 2.3 (Gingerbread) and higher, but the new layout editor is available only in Android Studio 2.2 Preview.

Layout Editor & Constraints Overview.

The new layout editor in Android Studio 2.2 Preview is specially built for the ConstraintLayout. You can specify the constraints manually, or automatically reference within the layout editor.

Overview of Constraints?

A constraint is the description of how a view should be positioned relative to other items, in a layout. A constraint is typically defined for one or more sides by connecting the view to:

An anchor point, or another view,
An edge of the layout,
Or An invisible guide line.

Since each view within the layout is defined by associations to other views within the layout, it’s easier to achieve flat hierarchy for complex layouts.

In principle, the ConstraintLayout works very similar to the RelativeLayout, but uses various handles (or say anchors) for the constraints.

Figure B.

(Source)

Resize handle. The resize handle is the seen in the corners of the figure above, and it’s used to resize the view.
Side handle. The side handle is the in the figure above, and it’s used to specify the location of a widget. E.g using the left side handle to always be aligned to the right of another view, or the left of the ConstraintLayout itself.
Baseline handle. The baseline handle is the in the figure above. It is used to align the text of a view by the baseline of the text on another view.

Getting started with ConstraintLayout

Setup

Ensure that you’re running the AS 2.2 preview, and Android Support Repository version 32 or higher, it’s required before you can use the ConstraintLayout. Let’s get started.

First, you need to add the constraint layout library to your app’s dependencies within your build.gradle file:

dependencies {
    compile 'com.android.support.constraint:constraint-layout:1.0.0-alpha1'
}

Sync your project.

Add Constraints

There are typically two ways to create ConstraintLayout in AS. You can create a new XML layout and select the root element to be a ConstraintLayout or convert an existing layout into a ConstraintLayout as shown in the image below:

Once you have the ConstraintLayout setup, what is next is to add the constraints to the views within that layout.

As an example, drag an ImageView to the layout. The new layout builder will immediately ask to add a drawable or resource, select one from the options and press ok. Also drag a TextView unto the layout.

To create a constraint, drag the top side handle on the ImageView to the top of the ConstraintLayout. You can also drag from the top side handle of the TextView to the bottom handle of the ImageView

Using the Inspector Pane

Now that we’re able to add constraints, we will need to use the inspector. It’s on the right hand side of the layout builder and it lists various properties of the selected widget. Typically, it looks as shown below:

You can use the sliders to move the view by percentage along the x and y axes. You can also control the dimensions of the view from the inspector pane, by altering the values corresponding to the layout_width and layout_height fields.

Taking a closer look at the square widget on the inspector pane. It contains some more control over the dimensions of the views.

There are other modes of controlling the size of the view. Clicking on the inner lines in the image above help you cycle through the other modes.

Fixed mode: This allows you specify the width and height of the view.
Any size: This mode allows the image to fill up all the space required to fulfill that constraint. You can look at this like “match constraint”
Wrap content: This just expands to fill the content of the view. E.g text or image

Using Auto-connect to add constraints.

Autoconnect as the name suggests, automatically creates connections between views/widgets. It tries to create connections to neighboring views/widgets.
To enable autoconnect, look out for the alt text icon on the top bar of the layout editor.

Thats’s almost it for the constraint layout.
If you want, you can head over to the official documentation on it at http://tools.android.com/tech-docs/layout-editor

Cheers.