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You're reading from Android Studio 4.1 Development Essentials – Java Edition

Product typeBook

Published inMay 2021

PublisherPackt

ISBN-139781801815161

Edition1st Edition

Tools

Android Studio

Concepts

Android Development

Author (1)

Neil Smyth

45. A MotionLayout KeyCycle Tutorial

The previous chapters introduced and demonstrated the concepts of integrating animation into Android app user interfaces using the MotionLayout container combined with the features of the Android Studio MotionLayout editor. The chapter entitled “An Introduction to MotionLayout” briefly mentioned the cycle (KeyCycle) and time cycle (KeyTimeCycle) key frames and explained how these can be used to implement animations involving large numbers of repetitive state changes.

This chapter will cover cycle key frames in more detail before demonstrating how to make use of them in an example project using Android Studio and the Cycle Editor.

45.1 An Overview of Cycle Keyframes

Clearly, position keyframes can be used to add intermediate state changes into the animation timeline. While this works well for small numbers of state changes, it would be cumbersome to implement in larger quantities. To make a button shake 50 times when tapped to indicate that an error occurred, for example, would involve the manual creation of 100 position keyframes to perform small clockwise and anti-clockwise rotations. Similarly, to implement a bouncing effect on a view as it moves across the screen would be an equally time consuming task.

For situations where state changes need to be performed repetitively, MotionLayout includes the Cycle and Time Cycle keyframes. Both perform the same tasks, with the exception that KeyCycle frames are based on frame positions within an animation path, while KeyTimeCycles are time-based in cycles per second (Hz).

Using these KeyCycle frames, the animation timeline is essentially divided up into subsections...

45.2 Using the Cycle Editor

Although not particularly complicated, it can take some time to get the exact cycle configuration you need by directly editing XML KeyCycle entries in the MotionScene file. In recognition of this fact, the Android engineers at Google have developed the Cycle Editor. This is a separate Java-based utility that is not yet part of Android Studio. The Cycle Editor allows you to visually design and test cycle key frame sets.

The Cycle Editor tool is provided in the form of a Java archive (jar) file which will require that the Java runtime be installed on your development system. Steps to install Java will vary depending on your operating system.

Once you have Java installed, the CycleEditor.jar file can be downloaded from the following URL:

https://github.com/googlesamples/android-ConstraintLayoutExamples/releases/download/1.0/CycleEditor.jar

Once downloaded, open a command-prompt or terminal window, change directory to the location of the jar file...

45.3 Creating the KeyCycleDemo Project

Select the Create New Project quick start option from the welcome screen and, within the resulting new project dialog, choose the Empty Activity template before clicking on the Next button.

Enter KeyCycleDemo into the Name field and specify com.ebookfrenzy.keycycledemo as the package name. Before clicking on the Finish button, change the Minimum API level setting to API 26: Android 8.0 (Oreo) and the Language menu to Java.

With the layout editor in Design mode and the activity_main.xml file open, right-click on the ConstraintLayout entry and select the Convert to MotionLayout menu option:

Figure 45-7

After making the selection, click on the Convert button in the confirmation dialog.

45.4 Configuring the Start and End Constraints

The objective of this tutorial is to animate the movement of a button from one side of the device screen to the other including KeyCycle effects that cause the view to also move up and down along the Y-axis. The first step is to configure the start and end constraints.

With the activity_main.xml file loaded into the MotionLayout editor, select and delete the default TextView widget. Make sure the Motion Layout box (marked E in Figure 45-9 below) is selected before dragging and dropping a Button view from the palette so that it is centered vertically and positioned along the left-hand edge of the layout canvas:

Figure 45-8

To configure the constraints for the start point, select the start constraint set entry in the editor window (marked A in Figure 45-9):

Figure 45-9

Next, select the button entry within the ConstraintSet list (B). With the button entry still selected, click on the edit button (C) and select Create...

45.5 Creating the Cycles

The next step is to use the Cycle Editor to generate the cycle key frames for the animation. With the Cycle Editor running, refer to the control panel shown in Figure 45-12 below:

Figure 45-12

Using the menu marked A, change the property to be modified from rotation to translationY.

Next, use the KeyCycle control (B) to select cycle 0 so that changes made elsewhere in the panel will be applied to the first cycle. Move the Period slider to 1 and the translationY slider to 60 as shown in Figure 45-13 (refer to the XML panel to see the precise setting for the translationY value as you move the slider):

Figure 45-13

To see the changes so far in the graph, select the File -> Parse XML menu option. Using the values listed in Table 45-2, configure the settings for KeyFrames 1 through 4 (keeping in mind that you have already configured the settings in the KeyCycle 0 column):

...

45.6 Previewing the Animation

The cycle-based animation may now be previewed from within the Cycle Editor tool. Start the animation running by clicking on the play button (marked A in Figure 45-15). To combine the cycles with horizontal movement, change the second menu (B) from Stationary to West to East. Also, take some time to experiment with the time and linearity settings (C and D).

Figure 45-15

45.7 Adding the KeyFrameSet to the MotionScene

Within the Cycle Editor, highlight and copy the KeyCycle elements from the XML panel and paste them into the Transition section of the res -> xml -> activity_main_scene.xml file within Android Studio so that they are placed between the existing KeyFrameSet markers. Note also the increased duration setting and the addition of an OnClick handler to initiate the animation:

<Transition

motion:constraintSetEnd="@+id/end"

motion:constraintSetStart="@id/start"

motion:duration="7000">

<KeyCycle

motion:framePosition="0"

motion:motionTarget="@+id/button"

...

45.8 Summary

Cycle key frames provide a useful way to build frame animations that involve potentially large numbers of state changes that match wave patterns. As outlined in this chapter, the process of generating these cycle key frames can be eased significantly by making use of the Cycle Editor application.

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Author (1)

Neil Smyth

Neil Smyth has over 25 years of experience in the IT industry, including roles in software development and enterprise-level UNIX and Linux system administration. In addition to a bachelor’s degree in information technology, he also holds A+, Security+, Network+, Project+, and Microsoft Certified Professional certifications and is a CIW Database Design Specialist. Neil is the co-founder and CEO of Payload Media, Inc. (a technical content publishing company), and the author of the Essentials range of programming and system administration books.
Read more about Neil Smyth

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