See Event (Ereignis).
One way for iframes to communicate with each other is by using the postMessage API stackoverflow 
. This allows you to send messages between windows or iframes even if they have different domains stackoverflow .
> To send a message from one iframe to another, you can dispatch an Event from the first iframe to its parent window that notifies it that the iframe is loaded. […]
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dispatch | BrE dɪˈspatʃ, AmE dəˈspætʃ | A transitive verb 1 (send off) schicken ▸ dispatch sb [to do sth] jmdn. entsenden [um etw. zu tun] (formal) 2 (get through) erledigen abschließen ‹Geschäft› erfüllen ‹Pflicht› 3 (kill) töten 4 (eat) verschlingen verputzen (informal) B noun 1 (official report) Bericht (Mask.) Depesche (Fem.) (archaic) ▸ they were mentioned in dispatches (Military) ≈ ihnen wurde förmliche Anerkennung ausgesprochen 2 (sending off) Absenden (Neutr.) (of troops, messenger, delegation) Entsendung (Fem.) (formal) 3 (killing) Tötung (Fem.) 4 (prompt execution) Erledigung (Fem.) 5 act with dispatch prompt handeln