Add condition and command support to watch command

README.md

@@ -565,9 +565,21 @@ The `<...>` notation means the argument.
   * Note that this feature is super slow.
 * `catch <Error>`
   * Set breakpoint on raising `<Error>`.
+* `catch ... if: <expr>`
+  * stops only if `<expr>` is true as well.
+* `catch ... pre: <command>`
+  * runs `<command>` before stopping.
+* `catch ... do: <command>`
+  * stops and run `<command>`, and continue.
 * `watch @ivar`
   * Stop the execution when the result of current scope's `@ivar` is changed.
   * Note that this feature is super slow.
+* `watch ... if: <expr>`
+  * stops only if `<expr>` is true as well.
+* `watch ... pre: <command>`
+  * runs `<command>` before stopping.
+* `watch ... do: <command>`
+  * stops and run `<command>`, and continue.
 * `del[ete]`
   * delete all breakpoints.
 * `del[ete] <bpnum>`

lib/debug/breakpoint.rb

@@ -328,12 +328,15 @@ def to_s
   end
 
   class WatchIVarBreakpoint < Breakpoint
-    def initialize ivar, object, current
+    def initialize ivar, object, current, cond: nil, command: nil
       @ivar = ivar.to_sym
       @object = object
-      @key = [:watch, @ivar].freeze
+      @key = [:watch, object.object_id, @ivar].freeze
 
       @current = current
+
+      @cond = cond
+      @command = command
       super()
     end
 
@@ -343,7 +346,10 @@ def watch_eval
         begin
           @prev = @current
           @current = result
-          suspend
+
+          if @cond.nil? || @object.instance_eval(@cond)
+            suspend
+          end
         ensure
           remove_instance_variable(:@prev)
         end

lib/debug/session.rb

@@ -556,6 +556,12 @@ def process_command line
 
       # * `catch <Error>`
       #   * Set breakpoint on raising `<Error>`.
+      # * `catch ... if: <expr>`
+      #   * stops only if `<expr>` is true as well.
+      # * `catch ... pre: <command>`
+      #   * runs `<command>` before stopping.
+      # * `catch ... do: <command>`
+      #   * stops and run `<command>`, and continue.
       when 'catch'
         check_postmortem
 
@@ -570,11 +576,17 @@ def process_command line
       # * `watch @ivar`
       #   * Stop the execution when the result of current scope's `@ivar` is changed.
       #   * Note that this feature is super slow.
+      # * `watch ... if: <expr>`
+      #   * stops only if `<expr>` is true as well.
+      # * `watch ... pre: <command>`
+      #   * runs `<command>` before stopping.
+      # * `watch ... do: <command>`
+      #   * stops and run `<command>`, and continue.
       when 'wat', 'watch'
         check_postmortem
 
         if arg && arg.match?(/\A@\w+/)
-          @tc << [:breakpoint, :watch, arg]
+          repl_add_watch_breakpoint(arg)
         else
           show_bps
           return :retry
@@ -1286,6 +1298,14 @@ def repl_add_catch_breakpoint arg
       add_bp bp
     end
 
+    def repl_add_watch_breakpoint arg
+      expr = parse_break arg.strip
+      cond = expr[:if]
+      cmd = ['watch', expr[:pre], expr[:do]] if expr[:pre] || expr[:do]
+
+      @tc << [:breakpoint, :watch, expr[:sig], cond, cmd]
+    end
+
     def add_catch_breakpoint pat
       bp = CatchBreakpoint.new(pat)
       add_bp bp

lib/debug/thread_client.rb

@@ -646,8 +646,8 @@ def make_breakpoint args
 
         bp
       when :watch
-        ivar, object, result = args[1..]
-        WatchIVarBreakpoint.new(ivar, object, result)
+        ivar, object, result, cond, command = args[1..]
+        WatchIVarBreakpoint.new(ivar, object, result, cond: cond, command: command)
       else
         raise "unknown breakpoint: #{args}"
       end
@@ -890,7 +890,7 @@ def wait_next_action_
             bp = make_breakpoint args
             event! :result, :method_breakpoint, bp
           when :watch
-            ivar = args[1]
+            ivar, cond, command = args[1..]
             result = frame_eval(ivar)
 
             if @success_last_eval
@@ -900,7 +900,7 @@ def wait_next_action_
                 else
                   current_frame.self
                 end
-              bp = make_breakpoint [:watch, ivar, object, result]
+              bp = make_breakpoint [:watch, ivar, object, result, cond, command]
               event! :result, :watch_breakpoint, bp
             else
               event! :result, nil

test/debug/watch_test.rb

@@ -11,7 +11,7 @@ def program
          3|
          4|   def initialize(name)
          5|     @name = name
-         6|     binding.break(do: "watch @name")
+         6|     binding.b
          7|   end
          8| end
          9|
@@ -33,21 +33,76 @@ def program
     def test_debugger_only_stops_when_the_ivar_of_instance_changes
       debug_code(program) do
         type 'continue'
-        # stops at binding.break
-        assert_line_text('Student#initialize(name="John")')
-        # stops when @name changes
+        type 'watch @name'
         assert_line_text(/#0  BP - Watch  #<Student:.*> @name = John/)
         type 'continue'
+        assert_line_text(/Stop by #0  BP - Watch  #<Student:.*> @name = John -> Josh/)
+        type 'continue'
       end
     end
 
     def test_watch_command_isnt_repeatable
       debug_code(program) do
         type 'continue'
+        type 'watch @name'
         type ''
         assert_no_line_text(/duplicated breakpoint/)
         type 'quit!'
       end
     end
+
+    def test_watch_works_with_command
+      debug_code(program) do
+        type 'continue'
+        type 'watch @name pre: p "1234"'
+        assert_line_text(/#0  BP - Watch  #<Student:.*> @name = John/)
+        type 'continue'
+        assert_line_text(/1234/)
+        type 'continue'
+      end
+
+      debug_code(program) do
+        type 'continue'
+        type 'watch @name do: p "1234"'
+        assert_line_text(/#0  BP - Watch  #<Student:.*> @name = John/)
+        type 'b 21'
+        type 'continue'
+        assert_line_text(/1234/)
+        type 'continue'
+      end
+    end
+
+    class ConditionTest < TestCase
+      def program
+        <<~RUBY
+         1| class Student
+         2|   attr_accessor :name, :age
+         3|
+         4|   def initialize(name, age)
+         5|     @name = name
+         6|     @age = age
+         7|     binding.b(do: "watch @age if: name == 'Sean'")
+         8|   end
+         9| end
+        10|
+        11| stan = Student.new("Stan", 30)
+        12| stan.age += 1
+        13| # only stops for Sean's age change
+        14| sean = Student.new("Sean", 25)
+        15| sean.age += 1
+        16|
+        17| a = 1 # additional line for line tp
+        RUBY
+      end
+
+      def test_condition_is_evaluated_in_the_watched_object
+        debug_code(program) do
+          type 'continue'
+          assert_line_text(/Stop by #\d  BP - Watch  #<Student:.*> @age = 25 -> 26/)
+          type 'continue'
+          assert_finish
+        end
+      end
+    end
   end
 end