と見るや【とみるや】

と見るや follows a plain-form clause and means "the instant someone saw that", "no sooner had they judged that": a judgment is made and a quick, decisive action follows at once. It is literary, at home in novels and news reports.

The clause before と見るや is what someone has concluded, which need not be something seen with the eyes: すき隙がある, かちめ勝ち目がない. The second half is a past action by the same person, told as narrative, so it is never a request or your own plan.

と見るや or や否や

やいなや否や follows a dictionary-form verb and is pure timing: one thing happens and the next follows at once. と見るや puts someone's judgment in between. The two combine as とみる見るやいなや否や, with the same meaning.

In conversation, とわかると or とおもったら思ったら followed by すぐ says the same thing.