StateCallback

onActive

open fun onActive(session: CameraCaptureSession): Unit

This method is called when the session starts actively processing capture requests.

If capture requests are submitted prior to onConfigured being called, then the session will start processing those requests immediately after the callback, and this method will be immediately called after onConfigured.

If the session runs out of capture requests to process and calls onReady, then this callback will be invoked again once new requests are submitted for capture.

onCaptureQueueEmpty

open fun onCaptureQueueEmpty(session: CameraCaptureSession): Unit

This method is called when camera device's input capture queue becomes empty, and is ready to accept the next request.

Pending capture requests exist in one of two queues: the in-flight queue where requests are already in different stages of processing pipeline, and an input queue where requests wait to enter the in-flight queue. The input queue is needed because more requests may be submitted than the current camera device pipeline depth.

This callback is fired when the input queue becomes empty, and the camera device may have to fall back to the repeating request if set, or completely skip the next frame from the sensor. This can cause glitches to camera preview output, for example. This callback will only fire after requests queued by capture() or captureBurst(), not after a repeating request or burst enters the in-flight queue. For example, in the common case of a repeating request and a single-shot JPEG capture, this callback only fires when the JPEG request has entered the in-flight queue for capture.

By only sending a new capture or captureBurst when the input queue is empty, pipeline latency can be minimized.

This callback is not fired when the session is first created. It is different from onReady, which is fired when all requests in both queues have been processed.

onClosed

open fun onClosed(session: CameraCaptureSession): Unit

This method is called when the session is closed.

A session is closed when a new session is created by the parent camera device, or when the parent camera device is closed (either by the user closing the device, or due to a camera device disconnection or fatal error).

Once a session is closed, all methods on it will throw an IllegalStateException, and any repeating requests or bursts are stopped (as if stopRepeating() was called). However, any in-progress capture requests submitted to the session will be completed as normal.

onConfigureFailed

abstract fun onConfigureFailed(session: CameraCaptureSession): Unit

This method is called if the session cannot be configured as requested.

This can happen if the set of requested outputs contains unsupported sizes, or too many outputs are requested at once.

The session is considered to be closed, and all methods called on it after this callback is invoked will throw an IllegalStateException. Any capture requests submitted to the session prior to this callback will be discarded and will not produce any callbacks on their listeners.

onConfigured

abstract fun onConfigured(session: CameraCaptureSession): Unit

This method is called when the camera device has finished configuring itself, and the session can start processing capture requests.

If there are capture requests already queued with the session, they will start processing once this callback is invoked, and the session will call onActive right after this callback is invoked.

If no capture requests have been submitted, then the session will invoke onReady right after this callback.

If the camera device configuration fails, then onConfigureFailed will be invoked instead of this callback.

onReady

open fun onReady(session: CameraCaptureSession): Unit

This method is called every time the session has no more capture requests to process.

During the creation of a new session, this callback is invoked right after onConfigured if no capture requests were submitted to the session prior to it completing configuration.

Otherwise, this callback will be invoked any time the session finishes processing all of its active capture requests, and no repeating request or burst is set up.

onSurfacePrepared

open fun onSurfacePrepared(
    session: CameraCaptureSession, 
    surface: Surface
): Unit

This method is called when the buffer pre-allocation for an output Surface is complete.

Buffer pre-allocation for an output Surface is started by the prepare call. While allocation is underway, the Surface must not be used as a capture target. Once this callback fires, the output Surface provided can again be used as a target for a capture request.

In case of a error during pre-allocation (such as running out of suitable memory), this callback is still invoked after the error is encountered, though some buffers may not have been successfully pre-allocated.