Android Camera2 API中createCaptureSession的Handler参数含义及疑问

Great question—this is a super common point of confusion when diving into Camera2, so let’s break down what’s going on here.

First, what does the Handler parameter in createCaptureSession do? #

The Handler argument tells the system which thread should execute the callbacks in your CameraCaptureSession.StateCallback. If you pass null, the system defaults to using the Looper of the thread where you called createCaptureSession itself.

Why does the Camera2Basic example pass null here? #

Look at where createCaptureSession is called in the sample: it’s inside the onOpened callback of CameraDevice.StateCallback. And how was that CameraDevice.StateCallback set up? When opening the camera with cameraManager.openCamera(), the sample passes mBackgroundHandler as the third parameter. That means the onOpened callback runs on the background thread tied to mBackgroundHandler.

So when createCaptureSession is called inside onOpened, the current thread is already the background thread. Passing null here just tells the system to keep using that same thread’s Looper for the CameraCaptureSession.StateCallback callbacks. It’s functionally identical to passing mBackgroundHandler—the sample just uses null as a shorthand since it’s redundant to specify the same thread we’re already on.

Why do other methods like setRepeatingRequest or imageReader.setOnImageAvailableListener use mBackgroundHandler? #

These calls often happen from the UI thread (or a different thread than the background camera thread). For example, you might trigger setRepeatingRequest in response to a user button tap on the UI. If you didn’t pass mBackgroundHandler here, the callbacks (like capture results) would run on the UI thread, which can cause jank or block UI updates.

By explicitly passing mBackgroundHandler, you ensure all heavy camera-related work (processing capture results, handling image data from ImageReader) stays on the dedicated background thread, keeping the UI smooth.

Quick recap #

• For createCaptureSession: Passing null works because the method is already called on the background thread, so callbacks stay there automatically.
• For other camera methods: Explicitly pass mBackgroundHandler to force callbacks onto the background thread, even if the method is called from the UI thread.

内容的提问来源于stack exchange，提问作者varunkr