Transition X

Kotlin DSL for choreographing Android Transitions

TransitionManager makes it easy to animate simple changes to layout without needing to explicitly calculate and specify from and to like Animator or Animation expects. When you call TransitionManager.beginDelayedTransition(layout, transition) before updating a layout, the framework automatically does a diff on before and after states and animates the difference.

Transition X is intended to simplify construction of these Transition instances to take full advantage of the framework and provide a clear, concise, type safe and extensible DSL using Kotlin language features.

I highly recommend reading the introduction blog post on my blog.

Download

• Add repository to your project level build.gradle file.

allprojects {
    repositories {
        jcenter()
    }
}

• Add library dependency to module level build.gradle file.

dependencies{
    implementation 'in.arunkumarsampath:transition-x:1.1.0'
}

Introduction

As shown above, instead of creating XML files and later inflating them using TransitionInflator, it is possible to create Transition instances directly using tranistionSet{} block provided by the DSL.

With Transition X, the construction and usage can be greatly simplified with a prepareTransition extension added to ViewGroup.

For example:

constraintLayout.prepareTransition {
  fadeOut {
      startDelay = 100
  }
  moveResize {
    pathMotion = ArcMotion()
  }
  fadeIn()
  +textView // Add textView as target using '+' operator
  exclude<RecyclerView>() // Exclude all recyclerViews
  ease {
    standardEasing // Applies FastOutSlowInInterpolator
  }
}
// Performing layout changes here will be animated just like
// calling TransitionManager.beginDelayedTransition()

All blocks are type-safe and has IDE auto complete support thanks to Kotlin.

Getting Started

Writing your first transition

TransitionSet's can be built programmatically like shown below.

val transition = TransitionSet().apply {
  addTransition(ChangeBounds().apply {
    startDelay = 100  
    setPathMotion(ArcMotion())  
  })  
}

The Transition X equivalent would be:

val transition = transitionSet {   
  moveResize {   
    startDelay = 100  
    pathMotion = ArcMotion()  
  }  
}

Some of the transition names are opinionated to better express their intent and promote clear code. Here ChangeBounds transition usually animates a View's height, width, or location on screen hence the name moveResize to better convey what it does.

Working with custom transitions

In case you have a custom transition class and want to use with the DSL, it is easy to do so.

• If your transition has a public no arg constructor then the transition can be added using customTransition<Type: Transition>{} method, transition-x takes care of instantiating the transition. Below example shows usage of ChangeCardColor which animates a CardView's cardBackground property.

constraintLayout.prepareTransition {  
  customTransition<ChangeCardColor> {  
    +colorChangeCardView  
  }
}

• If your transition does not have public no arg constructor then, you can instantiate the transition yourself and then use customTransition(transition) {} instead to add the transition and configure it.

Accessing custom properties

In addition to the common properties like startDelay, interpolator, etc, if your transition has custom properties then customProperties {} block can be used.

constraintLayout.prepareTransition {
  customTransition<ChangeCardColor> {
    +colorChangeCardView  
    customProperties {   
      myProperty = "hi"
    }  
  }
}

Adding, removing and excluding targets

The DSL provides simplified syntax to deal with targets by talking to Transition's add/exclude/remove API.

• Use + operator or add() to add targets of type String (Transition Name) or View or Resource Id.

transitionSet {  
  +"TransitionName"  
  +userIconView
  add(userIconView)  
}

• Use - operator or remove() to remove targets of type String (Transition Name) or View or Resource Id.

transitionSet {  
  -"TransitionName"  
  -userIconView
  remove(userIconView)  
}

• exclude and excludeChildren methods are provided for excluding targets which can be useful in advanced transitions. It can be used on Views, Resource Ids or Type

transitionSet {  
  exclude<RecyclerView>()  
  exclude(R.id.accentBackground)
  excludeChildren(constraintLayout)  
}

Interpolators

• Interpolators can be directly added using interpolator property.

transitionSet {  
  moveResize()  
  slide()  
  interpolator = FastOutLinearInInterpolator()  
}

• Easing - DSL provides a dedicated ease block to add interpolators recommended by material design spec.
  • standardEasing - Recommended for views that move within visible area of the layout. Uses FastOutSlowInInterpolator
  • decelerateEasing - Recommended for views that appear/enter outside visible bounds of the layout. Uses LinearOutSlowInInterpolator
  • accelerateEasing - Recommended for Views that exit visible bounds of the layout. Uses FastOutLinearInInterpolator

transitionSet {  
  moveResize()  
  ease {  
    decelerateEasing  
  }  
}

Nesting transitions

Often, for fined grained transitions it it necessary to add different transition sets for different targets. It is simple to nest multiple transition sets just by using transitionSet {} recursively.

transitionSet {  
  auto {   
    +"View 1"  
  }  
  transitionSet {   
    moveResize()  
    slide()  
    +"View 2"  
  }  
  transitionSet {   
    sequentially()  
    fadeOut()  
    moveResize()  
    fadeIn()  
  }  
}

Adding listeners to transitions

Transition-X makes it easy to react to Transition lifecycle by providing lifecycle methods like onEnd, onStart which internally uses Transition.addListener.

Example:

rootCoordinatorLayout.prepareTransition {
    onStart { 
        // Transition Started!
    }
    moveResize {
        +image1
    }
    onEnd { 
        // Transition Ended!
    }
}

Additional transitions

The library packages additional transitions not present in the support library and the plan is to add more commonly used transitions to provide a full package. Currently the following transitions are packaged:

• ChangeText: Animates changes to a TextView.text property.
• ChangeColor: Animates changes to View.background if it is a ColorDrawable or changes to TextView.textColor if the target is a TextView.

Samples

Sample	DSL	Demo
Snackbar animation	Snackbar is anchored below FAB. moveResize is used on on FAB since its position changes. Slide is used on Snackbar since it's visibility changes. constraintLayout.prepareTransition { moveResize { +fab } slide { +snackbarMessage } ease { decelerateEasing } } snackbarMessage.toggleGone()
Cascade animation	It is possible to write normal logical code in the prepareTransition block. Here we add moveResize using loops and by adding a start delay based on position, we can emulate a cascade transition. constraintLayout.prepareTransition { texts.forEachIndexed { position, view -> moveResize { +view startDelay = ((position + 1) * 150).toLong() } } moveResize { +fab } ease { decelerateEasing } } // Layout changes (if (defaultState) constraint1 else constraint2) .applyTo(constraintLayout)
Custom Transition	In the following example, ChangeCardColor is a custom transition that animates cardBackgroundColor property of MaterialCardView . constraintLayout.prepareTransition { customTransition<ChangeCardColor> { +cardView } changeColor { +textView } duration = 1000 } // Layout changes cardView.setCardBackgroundColor(color) textView.setTextColor(calcForegroundWhiteOrBlack(color))
Arc motion	Here the imageView's gravity is changed from START | CENTER_VERTICAL to TOP | CENTER_HORIZONTAL. By using a pathMotion it is possible to control the motion of the animation to follow material guidelines' arc motion. frameLayout.prepareTransition { moveResize { pathMotion = ArcMotion() +userIconView } }
Advanced choreography	By using techniques above and coupling it with further customization via lifecycle listeners such as onEnd or onPause it is possible to have finer control over the entire transition process. In the example below, notice how different views are configured with different parameters for transition type, interpolation and ordering. constraintLayout.prepareTransition { auto { ease { standardEasing } exclude(metamorphosisDesc2) } transitionSet { fade() slide() ease { accelerateEasing } +metamorphosisDesc2 } changeImage { add(*imageViews) } onEnd { constraintLayout.prepareTransition { moveResize() changeText { +collapseButton changeTextBehavior = ChangeText.CHANGE_BEHAVIOR_OUT_IN } } collapseButton.setText(R.string.collapse) } duration = 300 } expandConstraint.applyTo(constraintLayout) metamorphosisDesc2.isGone = false metamorphosisDesc.isGone = true
Shared element transition	Transition instances created by the DSL can be directly used with activity.window.sharedElementEnterTransition or fragment.sharedElementEnterTransition. fragment.sharedElementEnterTransition = transitionSet { transitionSet { changeImage() moveResize() changeClipBounds() scaleRotate() ease { standardEasing } duration = 375 +cartItem.cartImageTransitionName() } transitionSet { ease { standardEasing } moveResize() scaleRotate() add(cartItem.name, cartItem.price) duration = 375 } }	Demo - WIP. Example
Animated Bottom Navigation	Bottom navigation animation implmentend using custom choreography instead of relying on AutoTransition. The implementation uses ConstraintLayout to define the layouts and then simply show/hides the labels and adds tint to the icons. TransitionManager does the rest. transitionSet { fadeOut() moveResize { startDelay = 50 ease { standardEasing } } fadeIn { startDelay = 50 } changeColor { navItems.map { it.text }.forEach { text -> add(text) } +constraintLayout } customTransition<ChangeImageTint> { navItems.map { it.icon }.forEach { icon -> add(icon) } } }

Tasks

• [x] Initial release of Kotlin DSL
• [x] Provide samples for Shared Element Transitions
• [x] Package common transition within the library module
• [ ] Add wiki with best practices and gotchas.

Contributions

Contributions are welcome! I would greatly appreciate creating an issue to discuss major changes before submitting a PR directly. How you can help:

• Improving test coverage.
• Finding the DSL not sufficient for your case? Create an issue so we can discuss.
• Adding more animation samples to the sample app.

License

Copyright 2019, Arunkumar.

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

   http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.