Anvil: move fast and break things

Dear users, those who were early followers of Anvil. I’m now heavily working on the next version of Anvil, but it’s likely to make you rewrite your code, and I’m deeply sorry about that.

Below I will describe why the changes are necessary and I hope you will find that your code would not be modified a lot and embrace the benefist it brings.

how anvil works now

Currently we have Renderables which have a view() method returning a virtual layout. Virtual layout is a tree of nodes, each node describing a view or a view group and all its attributes.

Then Anvil compares new virtual layout and the previously known one, and applies the changes between two virtual layouts to the actual view tree.

This approach is popular in web development where manipulating real DOM is slow, but comparing two javascript objects to minimize the number of DOM manipulations is known to be faster.

In Anvoid this approach brings some limitations, though:


I was thinking Anvil is “fast enough”. Rendering a simple layout of ~15 views took about 200us on a modern phone. Which means you can render this layout a few thousand times per second.

In practice, this allocates and releases lots of temporary objects, which results in GC pauses. Also, as your layouts grow - the rendering time seems to grow linearly so for complex layouts you may not even meet the 60 FPS deadline.

I thought the bottleneck would be countless lambdas that we create when building virtual node attributes (like setter for text, setter for OnClickListener etc). It turns out the slowest part is not the lambdas, but the collections. In each node we keep a list of child nodes and attributes. These lists are created on each rendering cycle and shortly after that they are collected by the GC.

To improve Anvil performance I started thinking about reusing previously known virtual layout as much as possible instead of allocating new objects, because the nature of Android layouts is that they are mostly constant (number and order of views/attributes remains mostly the same, only the values change).

functional vs imperative

Ok, when a new view is created we need to pick the next virtual view node from the previous tree. Same for the new attribute. We shall create a new node only if current attribute or view is different, otherwise we shall keep the previous one.

Which means now the order in which attribute and view builders are called begins to matter. It’s no longer a functional approach (when functions return nodes, top-level function collects the nodes and returns anoder node). It’s an imperative approach with a sequence of statements (as it was in Anvil/Kotlin).

Java guarantees that function agruments are evaluated left-to-right, but this is not enough to reconstruct the view tree. For example:


We have v() which indicates end of the view node, we have attributes, but we don’t have the indicator of the node beginning. This becomes cumbersome if we look at a group of views:


Here it’s impossible to say if text(“world”) belongs to SomeLayout or SomeView judging only from the order of method calls. So we need a special function that would be called first in every view function. In the new Anvil it will look like:

o (linearLayout(),

  o (textView(),
  o (button(),
     text("click me"),

Here “o” (or “x”) looks like a list bullet, but it acts as an end-of-view marker. View functions or a generic v(SomeView.class) act as a start-of-view marker.

If a user can use Java 8 lambdas - it becomes more readable:

linearLayout(() -> {

    textView(() -> {

    button(() -> {
        text("Click me");

Or in Kotlin (notice the lack of dashes - we are going to have first-class support of Kotlin, no extra Sugar.kt needed!)

linearLayout {
    textView {
    button {
        text("Click me")

Of course due to the imperative nature you would be able to use if/for loops, not only the ternary operations. In fact Kotlin users have been able to use them for a long time, now it comes to java as well.

So anvil is not about diffing two virtual layouts as React or Mithril do. It’s about manipulating real views in a lazy manner (still keeping the previous values set to the views). It’s still similar to a virtual layout, but it’s happening in a pipeline - thus the performance boost.


This changes a lot. The view() method now returns void, becuse the tree it contructs is already stored inside Anvil. Also, view() now manipulates the real views and layouts, which is more natural for Android development. However it does changes in a lazy way - only if the new value is different from the previous one.

This allows to simplify Anvil code a lot, and to increase performance 5-7 times. An average rendering cycle now takes 30us and that seems to be on the lower limit of my poor-man benchmarks, because simple string concatenation takes about the same time. And if your layout is huge, but constant - the rendering time will still be around 30-50 us!


Anvil is now being rewritten from scratch, you can see it on the forge branch at

Anvil core now only takes care about the virtual layout management. Using just the core forces you to write all attribute and view wrappers by hand.

Of course generated attributes and views will still be there, but moved to a separate module. So are the tests. We can now run tests in Java 8 and Kotlin, keeping core code to be Java6-compliant.

It’s still arguable how to distribute various versions of attributes and views for each of the API levels - any suggestions are welcome.

So the plan is as follows:

If I get enough free time it should take a week or two, so be in touch!

P.S. Anyone willing to help - let me know!

I hope you’ve enjoyed this article. You can follow – and contribute to – on Github, Twitter or subscribe via rss.

Sep 13, 2015