put target histori need updat histori pointer indic target param target target histori histori object target current target target didn allreadi count object model target target fig fig target owner target object old target weak refer histori current target old target old target fig fig old target owner old target old target model target target navig backward current target histori size uml listen add listen target histori add weak refer target current target resiz weak refer ref current target weak refer histori current target current target ref equal target size histori size current target size uml listen remov listen histori remov current target histori add weak refer target uml listen add listen target current target 