經手東西的人
這一族最靠近字面：手上真的有東西。
noun · B2
Someone whose job is to touch, lift, carry, or move things — most often goods, food, or luggage.
搬運人員；經手人員
核心畫面
You can think of a handler as the one who takes hold of something and keeps it under control. Handle means to get your hands on a thing and deal with it; -er turns that into the one who does it. What sits in the hands changes from use to use. It can be a suitcase or a tray of food. It can be a working dog on a lead. It can be a famous person's day, or a spy's instructions. In computing it is not even a thing you can touch — just an event arriving, and a piece of code ready to catch it. Whatever it is, someone or something has it in hand.
handler 可以想成「把東西接過去、拿在手上控制」的那一個。handle 是動手拿起來處理，加上 -er 就變成…更多收起
handler 可以想成「把東西接過去、拿在手上控制」的那一個。handle 是動手拿起來處理，加上 -er 就變成做這件事的人。手上拿的東西每個用法都不一樣。可能是一個行李箱或一盤食物。可能是一條牽繩牽著的工作犬。可能是一個名人的行程，或是一個情報員要執行的指令。在程式裡甚至不是摸得到的東西，只是一個事件送進來，然後有一段程式等著接住。不管是什麼，都有人或有東西把它握在手上。From the word’s history
From Middle English handler, handiller: handle plus -er, the one who handles.
來自中古英語 handler、handiller，也就是 handle 加上 -er：動手處理的那個人。這個字面意思到今天還完全站得住。
Wiktionary entry for handler, packet source_ref kaikki:3f6f69a190036ae4:1434369744 (etymology_text: From Middle English handler, handiller, equivalent to handle + -er).
Three branches
手上牽著一個生命
手上的東西換成會動、會做決定的對象，所以「拿住」變成「帶著、指揮」。
noun · B2
Someone who trains a working animal and works beside it, especially a dog.
（工作動物的）訓練師、領犬員
程式裡接住事情的那段
同一個畫面搬進程式：手變成一段程式，東西變成一個事件。
noun · C1
In computing, a small piece of code that sits and waits for one kind of event or problem, and runs when that thing happens.
（程式的）處理常式、處理器