When I take photos in the dark using my phone, I sometimes use the flash. However, the flash fires only at the instant of taking the photo, as with a traditional camera. This means that the preview is not accurate. For example, the flash often ends up overpowering the scene. If I'd known that before I take the photo, I'd have turned the flash off, and not end up with a wasted photo, irritated people around me with the flash for no good reason, etc.
To fix this, can I ask the phone to turn the LED on as soon as I've enabled flash, and leave it on until I exit the camera app or turn the flash off? That way, the preview will accurately reflect the photo I'll get. After all, LED flashes, unlike Xenon flashes, can be left on continuously.
I'm fine with a third party app if that's the solution.
I currently use an iPhone 6, but I'd like a solution that works for Android as well.