Question

Ways to avoid calls to saveLayer():

• To implement fading in an image, consider using the FadeInImage widget, which applies a gradual opacity using the GPU’s fragment shader. For more information, see Opacity.
• To create a rectangle with rounded corners, instead of applying a clipping rectangle, consider using the borderRadius property offered by many of the widget classes.

Render grids and lists lazily

Use the lazy methods, with callbacks, when building large grids or lists. That way only the visible portion of the screen is built at startup time.

Also see:

• Working with long lists in the Cookbook
• Creating a ListView that loads one page at a time by AbdulRahman AlHamali
• Listview.builder API

Build and display frames in 16ms

Since there are two separate threads for building and rendering, you have 16ms for building, and 16ms for rendering on a 60Hz display. If latency is a concern, build and display a frame in 16ms or less. Note that means built in 8ms or less, and rendered in 8ms or less, for a total of 16ms or less. If missing frames (jankyness) is a concern, then 16ms for each of the build and render stages is OK.

If your frames are rendering in well under 16ms total in a profile build, you likely don’t have to worry about performance even if some performance pitfalls apply, but you should still aim to build and render a frame as fast as possible. Why?

• Lowering the frame render time below 16ms may not make a visual difference, but it will improve battery life and thermal issues.
• It may run fine on your device, but consider performance for the lowest device you are targeting.
• When 120fps devices become widely available, you’ll want to render frames in under 8ms (total) in order to provide the smoothest experience.

If you are wondering why 60fps leads to a smooth visual experience, see the video Why 60fps?

Pitfalls

If you need to tune your app’s performance, or perhaps the UI isn’t as smooth as you expect, the Flutter plugin for your IDE can help. In the Flutter Performance window, enable the Show widget rebuild information check box. This feature helps you detect when frames are being rendered and displayed in more than 16ms. Where possible, the plugin provides a link to a relevant tip.

The following behaviors might negatively impact your app’s performance.

• Avoid using the Opacity widget, and particularly avoid it in an animation. Use AnimatedOpacity or FadeInImage instead. For more information, see Performance considerations for opacity animation.

• When using an AnimatedBuilder, avoid putting a subtree in the builder function that builds widgets that don’t depend on the animation. This subtree is rebuilt for every tick of the animation. Instead, build that part of the subtree once and pass it as a child to the AnimatedBuilder. For more information, see Performance optimizations.

• Avoid clipping in an animation. If possible, pre-clip the image before animating it.

• Avoid using constructors with a concrete List of children (such as Column() or ListView()) if most of the children are not visible on screen to avoid the build cost.

Resources

For more performance info, see the following resources:

• Performance optimizations in the AnimatedBuilder API page
• Performance considerations for opacity animation in the Opacity API page
• Child elements’ lifecycle and how to load them efficiently, in the ListView API page
• Performance considerations of a StatefulWidget