Observer Design Pattern Event-Driven Design
EECS3311: Software Design Fall 2017
CHEN-WEI WANG
Motivating Problem
● A weather station maintains weather data such as temperature, humidity, and pressure.
● Various kinds of applications on these weather data should regularly update their displays:
○ Condition: temperature in celsius and humidity in percentages.
○ Forecast: if expecting for rainy weather due to reduced pressure.
○ Statistics: minimum/maximum/average measures of temperature. 2 of 35
First Design: Weather Station
Whenever the display feature is called, retrieve the current
values of temperature, humidity, and/or pressure via the
weather data reference. 3 of 35
Implementing the First Design (1)
class WEATHER_DATA create make feature — Data
temperature: REAL humidity: REAL pressure: REAL
feature — Queries correct_limits(t,p,h: REAL): BOOLEAN
ensure
Result implies -36 <=t and t <= 60 Result implies 50 <= p and p <= 110 Result implies 0.8 <= h and h <= 100
feature -- Commands make (t, p, h: REAL)
require
correct limits(temperature, pressure, humidity)
ensure
temperature = t and pressure = p and humidity = h
invariant
correct limits(temperature, pressure, humidity)
end
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Implementing the First Design (2.1)
class CURRENT_CONDITIONS create make feature -- Attributes
temperature: REAL humidity: REAL weather_data: WEATHER_DATA
feature -- Commands make(wd: WEATHER_DATA)
ensure weather data = wd update
do temperature := weather_data.temperature humidity := weather_data.humidity
end
display
do update
io.put_string("Current Conditions: ")
io.put_real (temperature) ; io.put_string (" degrees C and ")
io.put_real (humidity) ; io.put_string (" percent humidity%N") end
end
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Implementing the First Design (2.2)
class FORECAST create make feature -- Attributes
current_pressure: REAL last_pressure: REAL weather_data: WEATHER_DATA
feature -- Commands
make(wd: WEATHER_DATA) ensure weather data = a weather data update
do last_pressure := current_pressure current_pressure := weather_data.pressure
end
display
do update
if current_pressure > last_pressure then
print(“Improving weather on the way!%N”) elseif current_pressure = last_pressure then
print(“More of the same%N”)
else print(“Watch out for cooler, rainy weather%N”) end
end end
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Implementing the First Design (2.3)
class STATISTICS create make feature — Attributes
weather_data: WEATHER_DATA current_temp: REAL
max, min, sum_so_far: REAL num_readings: INTEGER
feature — Commands make(wd: WEATHER_DATA)
ensure weather data = a weather data update
do current_temp := weather_data.temperature — Update min, max if necessary.
end
display
do update
print(“Avg/Max/Min temperature = “)
print(sum_so_far / num_readings + “/” + max + “/” min + “%N”)
end end
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Implementing the First Design (3)
1 class WEATHER_STATION create make 2 feature — Attributes
3
4
5 feature — Commands 6 make
cc: CURRENT_CONDITIONS ; fd: FORECAST ; sd: STATISTICS wd: WEATHER_DATA
7 8 9
do
create wd.make (9, 75, 25)
create cc.make (wd) ; create fd.make (wd) ; create sd.make(wd)
wd.set_measurements (15, 60, 30.4) cc.display ; fd.display ; sd.display
cc.display ; fd.display ; sd.display wd.set_measurements (11, 90, 20)
cc.display ; fd.display ; sd.display
10
11
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17 end 18 end
L14: Updates occur on cc, fd, sd even with the same data. 8 of 35
First Design: Good Design?
● Each application (CURRENT CONDITION, FORECAST, STATISTICS) cannot know when the weather data change.
⇒ All applications have to periodically initiate updates in order to keep the display results up to date.
∵ Each inquiry of current weather data values is a remote call.
∴ Waste of computing resources (e.g., network bandwidth)
when there are actually no changes on the weather data. ● To avoid such overhead, it is better to let:
○ Each application subscribe the weather data.
○ The weather station publish/notify new changes.
⇒ Updates on the application side occur only when necessary . 9 of 35
Observer Pattern: Architecture
● Observer (publish-subscribe) pattern: one-to-many relation. ○ Observers (subscribers) are attached to a subject (publisher).
○ The subject notify its attached observers about changes.
● Some interchangeable vocabulary: ○ subscribe ≈ attach ≈ register
○ unsubscribe ≈ detach ≈ unregister ○ publish ≈ notify
○ handle ≈ update 10 of 35
Observer Pattern: Weather Station
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Implementing the Observer Pattern (1.1)
deferred class
OBSERVER
feature — To be effected by a descendant up_to_date_with_subject: BOOLEAN
— Is this observer up to date with its subject?
deferred end
update
— Update the observer’s view of ‘s’
deferred
ensure
up_to_date_with_subject: up_to_date_with_subject end
end
Each effective descendant class of OBSERVDER should: ○ Define what weather data are required to be up-to-date.
○ Define how to update the required weather data.
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Implementing the Observer Pattern (1.2)
class CURRENT_CONDITIONS inherit OBSERVER
feature — Commands
make(a_weather_data: WEATHER_DATA) do weather_data := a_weather_data
weather data.attach (Current)
ensure weather_data = a_weather_data weather data.observers.has (Current)
end
feature — Queries
up_to_date_with_subject: BOOLEAN
ensure then Result = temperature = weather_data.temperature and
humidity = weather_data.humidity
update
do — Same as 1st design; Called only on demand
end
display
do — No need to update; Display contents same as in 1st design
end end
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Implementing the Observer Pattern (1.3)
class FORECAST inherit OBSERVER feature — Commands
make(a_weather_data: WEATHER_DATA) do weather_data := a_weather_data
weather data.attach (Current)
ensure weather_data = a_weather_data weather data.observers.has (Current)
end
feature — Queries
up_to_date_with_subject: BOOLEAN ensure then
Result = current_pressure = weather_data.pressure update
do — Same as 1st design; Called only on demand
end
display
do — No need to update; Display contents same as in 1st design
end end
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Implementing the Observer Pattern (1.4)
class STATISTICS inherit OBSERVER feature — Commands
make(a_weather_data: WEATHER_DATA) do weather_data := a_weather_data
weather data.attach (Current)
ensure weather_data = a_weather_data weather data.observers.has (Current)
end
feature — Queries
up_to_date_with_subject: BOOLEAN ensure then
Result = current_temperature = weather_data.temperature update
do — Same as 1st design; Called only on demand
end
display
do — No need to update; Display contents same as in 1st design
end end
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Implementing the Observer Pattern (2.1)
class SUBJECT create make feature — Attributes
observers : LIST[OBSERVER] feature — Commands
make
do create {LINKED_LIST[OBSERVER]} observers.make ensure no observers: observers.count = 0 end
feature — Invoked by an OBSERVER
attach (o: OBSERVER) — Add ‘o’ to the observers
require not yet attached: not observers.has (o)
ensure is attached: observers.has (o) end detach (o: OBSERVER) — Add ‘o’ to the observers
require currently attached: observers.has (o)
ensure is attached: not observers.has (o) end feature — invoked by a SUBJECT
notify — Notify each attached observer about the update. do across observers as cursor loop cursor.item.update end ensure all views updated:
across observers as o all o.item.up_to_date_with_subject end end
end
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Implementing the Observer Pattern (2.2)
class WEATHER_DATA
inherit SUBJECT rename make as make subject end create make
feature — data available to observers
temperature: REAL
humidity: REAL
pressure: REAL
correct_limits(t,p,h: REAL): BOOLEAN
feature — Initialization make (t, p, h: REAL)
do
make subject — initialize empty observers
set_measurements (t, p, h) end
feature — Called by weather station set_measurements(t, p, h: REAL)
require correct_limits(t,p,h) invariant
correct limits(temperature, pressure, humidity)
end
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Implementing the Observer Pattern (3)
1 class WEATHER_STATION create make 2 feature — Attributes
3
4
5 feature — Commands 6 make
cc: CURRENT_CONDITIONS ; fd: FORECAST ; sd: STATISTICS wd: WEATHER_DATA
7 8 9
10 11
do
create wd.make (9, 75, 25)
create cc.make (wd) ; create fd.make (wd) ; create sd.make(wd)
wd.set_measurements (15, 60, 30.4) wd.notify
cc.display ; fd.display ; sd.display wd.set_measurements (11, 90, 20)
wd.notify
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17 end 18 end
L13: cc, fd, sd make use of “cached” data values. 18 of 35
Observer Pattern: Limitation? (1)
● The observer design pattern is a reasonable solution to building a one-to-many relationship: one subject (publisher) and multiple observers (subscribers).
● But what if a many-to-many relationship is required for the application under development?
○ Multiple weather data are maintained by weather stations.
○ Each application observes all these weather data.
○ But, each application still stores the latest measure only. e.g., the statistics app stores one copy of temperature
○ Whenever some weather station updates the temperature of its associated weather data, all relevant subscribed applications (i.e., current conditions, statistics) should update their temperatures.
● How can the observer pattern solve this general problem? ○ Each weather data maintains a list of subscribed applications. ○ Each application is subscribed to multiple weather data.
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Observer Pattern: Limitation? (2)
What happens at runtime when building a many-to-many relationship using the observer pattern?
wd: WEATHER_DATA wd2: WEATHER_DATA
…
wdm : WEATHER_DATA wdm: WEATHER_DATA
application
Graph complexity, with m subjects and n observers? [ O( m ⋅ n ) ] 20 of 35
application2
…
applicationn
Event-Driven Design (1)
Here is what happens at runtime when building a many-to-many relationship using the event-driven design.
wd: WEATHER_DATA wd2: WEATHER_DATA
…
wdn : WEATHER_DATA wdn: WEATHER_DATA
publish subscribe change_on_temperature: EVENT
application
application2 …
applicationn applicationn
Graph complexity, with m subjects and n observers? Additional cost by adding a new subject?
Additional cost by adding a new observer?
Additional cost by adding a new event type?
)] [O(1)] [O(1)]
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[O(
[O(m + n)]
m+n
Event-Driven Design (2)
In an event-driven design :
● Each variable being observed (e.g., temperature,
humidity, pressure) is called a monitored variable.
e.g., A nuclear power plant (i.e., the subject) has its temperature and pressure being monitored by a shutdown system (i.e., an observer): as soon as values of these monitored variables exceed the normal threshold, the SDS will be notified and react by shutting down the plant.
● Each monitored variable is declared as an event :
○ An observer is attached/subscribed to the relevant events.
CURRENT CONDITION attached to events for temperature, humidity. FORECAST only subscribed to the event for pressure.
STATISTICS only subscribed to the event for temperature.
○ A subject notifies/publishes changes to the relevant events. 22 of 35
Event-Driven Design: Implementation
● Requirements for implementing an event-driven design are:
1. When an observer object is subscribed to an event, it attaches:
1.1 The reference/pointer to an update operation
Such reference/pointer is used for executions.
1.2 Itself (i.e., the context object for invoking the update operation)
2. For the subject object to publish an update to the event, it:
2.1 Iterates through all its observers (or listeners)
2.2 Uses the operation reference/pointer (attached earlier) to update the
corresponding observer.
● Both requirements can be satisfied by Eiffel and Java.
● We will compare how an event-driven design for the weather station problems is implemented in Eiffel and Java.
⇒ It’s much more convenient to do such design in Eiffel. 23 of 35
delayed
Event-Driven Design in Java (1)
1 2 3 4 5 6 7 8 9
10 11
12 13 14 15
public class Event {
Hashtable
void subscribe(Object listener, MethodHandle action) {
listenersActions.put( listener , action ); }
void publish(Object arg) {
for (Object listener : listenersActions.keySet()) {
MethodHandle action = listenersActions.get(listener); try {
action .invokeWithArguments( listener , arg); } catch (Throwable e) { }
} }
}
● L5: Both the delayed action reference and its context object (or call target) listener are stored into the table.
● L11: An invocation is made from retrieved listener and action. 24 of 35
Event-Driven Design in Java (2)
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public class WeatherData {
private double temperature;
private double pressure;
private double humidity;
public WeatherData(double t, double p, double h) {
setMeasurements(t, h, p); }
public static Event public static Event public static Event
= new Event(); = new Event();
= new Event();
public void setMeasurements(double t, double h, double p) { temperature = t;
humidity = h;
pressure = p;
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changeOnTemperature
changeOnTemperature
changeOnHumidity
changeOnPressure
.publish(temperature); .publish(humidity); .publish(pressure);
changeOnHumidity
changeOnPressure
} }
Event-Driven Design in Java (3)
1 2 3 4 5 6 7 8 9
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public class CurrentConditions {
private double temperature; private double humidity;
public void updateTemperature(double t) { temperature = t; } public void updateHumidity(double h) { humidity = h; } public CurrentConditions() {
MethodHandles.Lookup lookup = MethodHandles.lookup(); try {
MethodHandle ut = lookup.findVirtual( this.getClass(), “updateTemperature”, MethodType.methodType(void.class, double.class));
WeatherData.changeOnTemperature.subscribe(this, ut); MethodHandle uh = lookup.findVirtual(
this.getClass(), “updateHumidity”,
MethodType.methodType(void.class, double.class)); WeatherData.changeOnHumidity.subscribe(this, uh);
} catch (Exception e) { e.printStackTrace(); } }
public void display() { System.out.println(“Temperature: ” + temperature); System.out.println(“Humidity: ” + humidity); } }
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Event-Driven Design in Java (4)
1 2 3 4 5 6 7 8 9
10 11
public class WeatherStation {
public static void main(String[] args) {
WeatherData wd = new WeatherData(9, 75, 25); CurrentConditions cc = new CurrentConditions(); System.out.println(“=======”); wd.setMeasurements(15, 60, 30.4);
cc.display(); System.out.println(“=======”); wd.setMeasurements(11, 90, 20); cc.display();
}}
L4 invokes WeatherData.changeOnTemperature.subscribe(
cc, ‘‘updateTemperature handle’’) WeatherData.changeOnTemperature.publish(15)
L6 invokes
which in turn invokes
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‘‘updateTemperature handle’’.invokeWithArguments(cc, 15)
Event-Driven Design in Eiffel (1)
1 class EVENT [ARGUMENTS -> TUPLE ] 2 create make
3 feature — Initialization
4
5
6 feature
actions: LINKED_LIST[PROCEDURE[ARGUMENTS]] make do create actions.make end
7 8 9
subscribe (an_action: PROCEDURE[ARGUMENTS])
require action_not_already_subscribed: not actions.has(an_action) do actions.extend (an_action)
ensure action_subscribed: action.has(an_action) end
loop actions.item.call (args) ; actions.forth end
10
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12
13
14 end 15 end
publish (args: G)
do from actions.start until actions.after
● L1 constrains the generic parameter ARGUMENTS: any class that instantiates ARGUMENTS must be a descendant of TUPLE.
● L4: The type PROCEDURE encapsulates both the context object and the reference/pointer to some update operation.
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Event-Driven Design in Eiffel (2)
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class WEATHER_DATA create make
feature — Measurements
temperature: REAL ; humidity: REAL ; pressure: REAL correct_limits(t,p,h: REAL): BOOLEAN do … end
make (t, p, h: REAL) do … end
feature — Event for data changes
: EVENT[TUPLE[REAL]]once create Result end
: EVENT[TUPLE[REAL]]once create Result end : EVENT[TUPLE[REAL]]once create Result end
feature — Command set_measurements(t, p, h: REAL)
require correct_limits(t,p,h)
do temperature := t ; pressure := p ; humidity := h
.publish ([t]) .publish ([p]) .publish ([h])
end
invariant correct_limits(temperature, pressure, humidity) end
change on temperature
change on humidity
change on pressure
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change on temperature
change on humidity
change on pressure
Event-Driven Design in Eiffel (3)
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class CURRENT_CONDITIONS create make
feature — Initialization
make(wd: WEATHER_DATA) do
wd.change on temperature.subscribe (agent update_temperature)
wd.change on temperature.subscribe (agent update_humidity) end
feature
temperature: REAL
humidity: REAL
update_temperature (t: REAL) do temperature := t end update_humidity (h: REAL) do humidity := h end display do … end
end
● retrieves the pointer to cmd and its context object. ● L6 ≈
● Contrast L6 with L8–11 in Java class CurrentConditions. 30 of 35
agent cmd
. . . (agent Current .update temperature)
Event-Driven Design in Eiffel (4)
1 2 3 4 5 6 7 8 9
10 11 12
class WEATHER_STATION create make feature
cc: CURRENT_CONDITIONS make
do create wd.make (9, 75, 25)
create cc.make (wd)
wd.set measurements (15, 60, 30.4) cc.display
wd.set measurements (11, 90, 20) cc.display
end end
L6 invokes
wd.change on temperature.subscribe(
agent cc.update temperature) wd.change on temperature.publish([15])
which in turn invokes
L7 invokes
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cc.update temperature(15)
Event-Driven Design: Eiffel vs. Java
● Storing observers/listeners of an event ○ Java, in the Event class:
○ Eiffel, in the EVENT class:
● Creating and passing function pointers
○ Java, in the CurrentConditions class constructor:
Hashtable
actions: LINKED_LIST[PROCEDURE[ARGUMENTS]]
MethodHandle ut = lookup.findVirtual( this.getClass(), “updateTemperature”, MethodType.methodType(void.class, double.class));
WeatherData.changeOnTemperature.subscribe(this, ut);
○ Eiffel, in the CURRENT CONDITIONS class construction:
⇒ Eiffel’s type system has been better thought-out for . 32 of 35
wd.change on temperature.subscribe (agent update_temperature)
design
Index (1)
Motivating Problem
First Design: Weather Station Implementing the First Design (1) Implementing the First Design (2.1) Implementing the First Design (2.2) Implementing the First Design (2.3) Implementing the First Design (3)
First Design: Good Design?
Observer Pattern: Architecture Observer Pattern: Weather Station Implementing the Observer Pattern (1.1) Implementing the Observer Pattern (1.2) Implementing the Observer Pattern (1.3)
Implementing the Observer Pattern (1.4)
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Index (2)
Implementing the Observer Pattern (2.1) Implementing the Observer Pattern (2.2) Implementing the Observer Pattern (3) Observer Pattern: Limitation? (1) Observer Pattern: Limitation? (2) Event-Driven Design (1)
Event-Driven Design (2)
Event-Driven Design: Implementation Event-Driven Design in Java (1) Event-Driven Design in Java (2) Event-Driven Design in Java (3) Event-Driven Design in Java (4) Event-Driven Design in Eiffel (1)
Event-Driven Design in Eiffel (2)
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Index (3)
Event-Driven Design in Eiffel (3)
Event-Driven Design in Eiffel (4)
Event-Driven Design: Eiffel vs. Java
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