Implementation Issues

• Implementation Variants
  • The Subject and Observer interfaces must be designed carefully so that the data can be passed/accessed efficiently to let observers know what changed in subject. There are two main variants:
• Variant1: Push Data
  • Subject passes the changed data to its observers:
    update(data1,data2,…)
  • The Observer interface may get complex because it must enable to pass in the changed data for all supported observers (whether the data is simple or complex).
• Variant2: Pull Data
  • Subject passes nothing but itself to its observers so that they can call back to get (pull) the required data from subject:
    update(this)
  • The Subject interface may get complex because it must enable all supported observers to access the needed data.
• "The pull model emphasizes the subject's ignorance of its observers, whereas the push model assumes subjects know something about their observers' needs." [GoF, p298]