Observer Pattern Implementor

Example 1: Spreadsheet Model with Multiple Chart Views

Scenario: A spreadsheet has a data model that three chart widgets (bar chart, pie chart, line chart) read directly. Currently the model calls barChart.refresh(), pieChart.refresh(), and lineChart.refresh() in its SetCell method. Adding a fourth chart requires modifying the model.

Trigger: "Every new chart type requires touching the spreadsheet model."

Process:

Output: SpreadsheetModel has no reference to any chart class. New charts attach themselves; model never changes.

Example 2: Clock with Multiple Display Observers

Scenario: A ClockTimer (concrete subject) notifies time displays. A DigitalClock and AnalogClock both observe the same timer. This is the canonical sample from the GoF text.

Trigger: "Both clock displays must always show the same time, but neither should know about the other."

Key implementation details:

void DigitalClock::Update(Subject* theChangedSubject) {
    if (theChangedSubject == _subject) {
        Draw();
    }
}
void DigitalClock::Draw() {
    int hour   = _subject->GetHour();
    int minute = _subject->GetMinute();
    // render digital display
}

Output: ClockTimer::Tick() calls Notify(); both displays update independently without knowing each other exist.

Example 3: Multi-Subject Dashboard with ChangeManager

Scenario: A financial dashboard has three data subjects: PriceSubject, VolumeSubject, and PositionSubject. A RiskSummaryObserver observes all three. When a trade executes, all three subjects change in one operation. Without ChangeManager, RiskSummaryObserver.Update fires three times for one logical event.

Trigger: "Our risk view recalculates three times per trade because it observes three subjects."

Process:

Output: One trade → three subject changes → RiskSummaryObserver.Update called exactly once.