Reduce spinner flickering

lib/dry/cli/ansi.rb

@@ -31,6 +31,12 @@ def self.carriage_return = CR
       EL2 = "\e[2K"
       def self.erase_line = EL2
 
+      # Erase from the cursor to the end of the line
+      # @api public
+      # @since x.y.z
+      EL0 = "\e[K"
+      def self.erase_to_end_of_line = EL0
+
       # Reset all styles
       # @api public
       # @since x.y.z

lib/dry/cli/spinner.rb

@@ -68,14 +68,18 @@ def run(format = "%{spinner}", fps: self.fps, stream: $stderr, &)
           frames = self.frames.cycle
 
           ticker = Ticker.start(interval: 1.0 / fps) do
-            stream.raw.print ANSI.clear_line
+            # Draw over the previous frame, then erase what is left of it. Erasing the whole line
+            # first would show a blank line in terminals that redraw between writes.
+            stream.raw.print ANSI.carriage_return
 
             data = {spinner: frames.next}
             before_tick.call(stream, data)
 
             stream.print format % data
 
             after_tick.call(stream)
+
+            stream.raw.print ANSI.erase_to_end_of_line
           end
         end
 

lib/dry/cli/spinner/dsl.rb

@@ -50,8 +50,9 @@ def handlers
       # @since x.y.z
       def run(&block) = @events[:run] = block
 
-      # Executed before each tick of the spinner. The line has been erased, so the current state
-      # of the stream will be a blank slate.
+      # Executed before each tick of the spinner. The cursor is at the start of the line. The line
+      # still shows the previous frame, but anything you write draws over it, and any old text left
+      # to the right is erased after {#after_tick} runs.
       #
       # This runs in a separate thread, so do not `throw :exit` or raise {Dry::CLI::ExitError} from
       # it. Do that from the {#run} block instead.

spec/unit/dry/cli/ansi_spec.rb

@@ -37,6 +37,12 @@
     end
   end
 
+  describe ".erase_to_end_of_line" do
+    it "returns the ANSI escape sequence to erase from the cursor to the end of the line" do
+      expect(described_class.erase_to_end_of_line).to eq("\e[K")
+    end
+  end
+
   describe ".enter_alt_screen" do
     it "returns the ANSI escape sequence to switch to the alternate screen buffer" do
       expect(described_class.enter_alt_screen).to eq("\e[?1049h")
@@ -143,6 +149,7 @@
       expect(described_class::CNORM).to eq("\e[?25h")
       expect(described_class::CR).to eq("\r")
       expect(described_class::EL2).to eq("\e[2K")
+      expect(described_class::EL0).to eq("\e[K")
       expect(described_class::SGR0).to eq("\e[0m")
       expect(described_class::SMCUP).to eq("\e[?1049h")
       expect(described_class::RMCUP).to eq("\e[?1049l")

spec/unit/dry/cli/spinner_spec.rb

@@ -151,7 +151,7 @@ def render_ticks(count = 1)
       end
       render_ticks
 
-      expect(terminal.string).to include "#{Dry::CLI::ANSI.clear_line}<frame>"
+      expect(terminal.string).to include "\r<frame>\e[K"
     end
 
     it "does not need the DSL to register a worker" do
@@ -175,8 +175,29 @@ def render_ticks(count = 1)
     it "cycles through the frames on each tick" do
       spinner.run(stream: terminal) { render_ticks(2) }
 
-      expect(terminal.string).to include "#{Dry::CLI::ANSI.clear_line}|"
-      expect(terminal.string).to include "#{Dry::CLI::ANSI.clear_line}/"
+      expect(terminal.string).to include "\r|\e[K"
+      expect(terminal.string).to include "\r/\e[K"
+    end
+
+    it "draws over the previous frame and then erases to the end of the line" do
+      spinner.run("%{spinner} Loading", stream: terminal) { render_ticks(2) }
+
+      expect(terminal.string).to eq(
+        Dry::CLI::ANSI.hide_cursor +
+          "\r| Loading\e[K" \
+          "\r/ Loading\e[K" +
+          Dry::CLI::ANSI.clear_line +
+          Dry::CLI::ANSI.show_cursor
+      )
+    end
+
+    it "does not erase the whole line before drawing a frame" do
+      spinner.run(stream: terminal) { render_ticks(2) }
+
+      ticks = terminal.string.delete_prefix(Dry::CLI::ANSI.hide_cursor)
+        .delete_suffix(Dry::CLI::ANSI.clear_line + Dry::CLI::ANSI.show_cursor)
+
+      expect(ticks).to_not include Dry::CLI::ANSI.erase_line
     end
   end