使用时间线视图 (Timeline view)

What is it?

The timeline view displays information about Flutter frames. It consists of three parts, each increasing in granularity.

  • Frame rendering chart
  • Frame events chart
  • CPU profiler

The timeline view also supports importing and exporting of timeline data files. For more information, see the Import and export section.

Frame rendering chart

This chart is populated with individual frames as they are rendered in your application. Each bar in the chart represents a frame. The bars are color-coded to highlight the different portions of work that occur when rendering a Flutter frame: work from the UI thread and work from the GPU thread.

Screenshot from a timeline recording

Clicking a bar displays additional details about that frame.

UI

The UI thread executes Dart code in the Dart VM. This includes code from your application as well as the Flutter framework. When your app creates and displays a scene, the UI thread creates a layer tree, a lightweight object containing device-agnostic painting commands, and sends the layer tree to the GPU thread to be rendered on the device. Do not block this thread.

GPU

The GPU thread executes graphics code from the Flutter Engine. This thread takes the layer tree and displays it by talking to the GPU (graphic processing unit). You cannot directly access the GPU thread or its data, but if this thread is slow, it’s a result of something you’ve done in the Dart code. Skia, the graphics library, runs on this thread, which is sometimes called the rasterizer thread.

Sometimes a scene results in a layer tree that is easy to construct, but expensive to render on the GPU thread. In this case, you’ll need to figure out what your code is doing that is causing rendering code to be slow. Specific kinds of workloads are more difficult for the GPU. They might involve unnecessary calls to saveLayer(), intersecting opacities with multiple objects, and clips or shadows in specific situations.

For more information on profiling, see Identifying problems in the GPU graph.

Jank

The frame rendering chart shows jank with a red overlay. A frame is considered to be janky if it takes more than ~16 ms to complete. To achieve a frame rendering rate of 60 FPS (frames per second), each frame must render in ~16 ms or less. When this target is missed, you may experience UI jank or dropped frames.

For more information on how to analyze your app’s performance, see Flutter performance profiling.

Frame events chart

The frame events chart shows the event trace for a single frame. The top-most event spawns the event below it, and so on. The UI and GPU events are separate event flows, but they share a common timeline (displayed at the top of the frame chart). This timeline is strictly for the given frame. It does not reflect the clock shared by all frames.

Screenshot of timeline events for a frame

The flame chart supports zooming and panning. Scroll up and down to zoom in and out, respectively. To pan around, you can either click and drag the chart or scroll horizontally. You can click an event to view CPU profiling information in the CPU profiler, described in the next section.

CPU profiler

This section shows CPU profiling information for a specific event from the frame events chart (Build, Layout, Paint, etc).

CPU flame chart

This tab of the profiler shows CPU samples for the selected frame event (such as Layout in the following example). This chart should be viewed as a top-down stack trace, where the top-most stack frame calls the one below it. The width of each stack frame represents the amount of time it consumed the CPU. Stack frames that consume a lot of CPU time may be a good place to look for possible performance improvements.

Screenshot of a flame chart

Call tree

The call tree view shows the method trace for the CPU profile. This table is a top-down representation of the profile, meaning that a method can be expanded to show its callees.

Total time
Time the method spent executing its own code as well as the code for its callees.
Self time
Time the method spent executing only its own code.
Method
Name of the called method.
Source
File path for the method call site.

Screenshot of a call tree table

Bottom up

The bottom up view shows the method trace for the CPU profile but, as the name suggests, it’s a bottom-up representation of the profile. This means that each top-level method in the table is actually the last method in the call stack for a given CPU sample (in other words, it’s the leaf node for the sample).

In this table, a method can be expanded to show its callers.

Total time

Time the method spent executing its own code as well as the code for its callee.

Self time

For top-level methods in the bottom-up tree (leaf stack frames in the profile), this is the time the method spent executing only its own code. For sub nodes (the callers in the CPU profile), this is the self time of the callee when being called by the caller. Using the following example, the self time of the caller Element.updateSlotForChild.visit() is equal to the self time of the callee [Stub] OneArgCheckInLineCache when being called by the caller.

Method

Name of the called method.

Source

File path for the method call site.

Screenshot of a bottom up table

Import and export

DevTools supports importing and exporting timeline snapshots. Clicking the export button (upper-right corner above the frame rendering chart) downloads a snapshot of the current timeline state. To import a timeline snapshot, you can drag and drop the snapshot into DevTools from any page. Note the DevTools only supports importing files that were originally exported from DevTools.