https://source.android.com/ 
/*
 * Copyright (C) 2017 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package android.hardware;

import android.annotation.IntDef;
import android.os.MemoryFile;

import dalvik.system.CloseGuard;

import java.io.IOException;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.nio.channels.Channel;
import java.util.concurrent.atomic.AtomicBoolean;


Class representing a sensor direct channel. Use SensorManager.createDirectChannel(MemoryFile) or SensorManager.createDirectChannel(HardwareBuffer) to obtain an object. The channel object can be then configured (see configure(Sensor, int)) to start delivery of sensor events into shared memory buffer. 
/**
 * Class representing a sensor direct channel. Use
 * {@link SensorManager#createDirectChannel(android.os.MemoryFile)} or
 * {@link SensorManager#createDirectChannel(android.hardware.HardwareBuffer)}
 * to obtain an object. The channel object can be then configured
 * (see {@link #configure(Sensor, int)})
 * to start delivery of sensor events into shared memory buffer.
 */
public final class SensorDirectChannel implements Channel {

    // shared memory types

    
@hide
/** @hide */
    @Retention(RetentionPolicy.SOURCE)
    @IntDef(flag = true, prefix = { "TYPE_" }, value = {
            TYPE_MEMORY_FILE,
            TYPE_HARDWARE_BUFFER
    })
    public @interface MemoryType {}

    
Shared memory type ashmem, wrapped in MemoryFile object.

See Also:
  • createDirectChannel.createDirectChannel(MemoryFile)
/**
     * Shared memory type ashmem, wrapped in MemoryFile object.
     *
     * @see SensorManager#createDirectChannel(MemoryFile)
     */
    public static final int TYPE_MEMORY_FILE = 1;

    
Shared memory type wrapped by HardwareBuffer object.

See Also:
  • createDirectChannel.createDirectChannel(HardwareBuffer)
/**
     * Shared memory type wrapped by HardwareBuffer object.
     *
     * @see SensorManager#createDirectChannel(HardwareBuffer)
     */
    public static final int TYPE_HARDWARE_BUFFER = 2;

    // sensor rate levels

    
@hide
/** @hide */
    @Retention(RetentionPolicy.SOURCE)
    @IntDef(flag = true, prefix = { "RATE_" }, value = {
            RATE_STOP,
            RATE_NORMAL,
            RATE_FAST,
            RATE_VERY_FAST
    })
    public @interface RateLevel {}

    
Sensor stopped (no event output).

See Also:
  • configure(Sensor, int)
/**
     * Sensor stopped (no event output).
     *
     * @see #configure(Sensor, int)
     */
    public static final int RATE_STOP = 0;
    
Sensor operates at nominal rate of 50Hz.
The actual rate is expected to be between 55% to 220% of nominal rate, thus between 27.5Hz to
110Hz.

See Also:
  • configure(Sensor, int)
/**
     * Sensor operates at nominal rate of 50Hz.
     *
     * The actual rate is expected to be between 55% to 220% of nominal rate, thus between 27.5Hz to
     * 110Hz.
     *
     * @see #configure(Sensor, int)
     */
    public static final int RATE_NORMAL = 1; //50Hz
    
Sensor operates at nominal rate of 200Hz.
The actual rate is expected to be between 55% to 220% of nominal rate, thus between 110Hz to
440Hz.

See Also:
  • configure(Sensor, int)
/**
     * Sensor operates at nominal rate of 200Hz.
     *
     * The actual rate is expected to be between 55% to 220% of nominal rate, thus between 110Hz to
     * 440Hz.
     *
     * @see #configure(Sensor, int)
     */
    public static final int RATE_FAST = 2; // ~200Hz
    
Sensor operates at nominal rate of 800Hz.
The actual rate is expected to be between 55% to 220% of nominal rate, thus between 440Hz to
1760Hz.

See Also:
  • configure(Sensor, int)
/**
     * Sensor operates at nominal rate of 800Hz.
     *
     * The actual rate is expected to be between 55% to 220% of nominal rate, thus between 440Hz to
     * 1760Hz.
     *
     * @see #configure(Sensor, int)
     */
    public static final int RATE_VERY_FAST = 3; // ~800Hz

    
Determine if a channel is still valid. A channel is invalidated after close() is called. 
Returns:true if channel is valid.
/**
     * Determine if a channel is still valid. A channel is invalidated after {@link #close()} is
     * called.
     *
     * @return <code>true</code> if channel is valid.
     */
    @Override
    public boolean isOpen() {
        return !mClosed.get();
    }

    
@removed
/** @removed */
    @Deprecated
    public boolean isValid() {
        return isOpen();
    }

    
Close sensor direct channel.
Stop all active sensor in the channel and free sensor system resource related to channel.
Shared memory used for creating the direct channel need to be closed or freed separately.

See Also:
/**
     * Close sensor direct channel.
     *
     * Stop all active sensor in the channel and free sensor system resource related to channel.
     * Shared memory used for creating the direct channel need to be closed or freed separately.
     *
     * @see SensorManager#createDirectChannel(MemoryFile)
     * @see SensorManager#createDirectChannel(HardwareBuffer)
     */
    @Override
    public void close() {
        if (mClosed.compareAndSet(false, true)) {
            mCloseGuard.close();
            // actual close action
            mManager.destroyDirectChannel(this);
        }
    }

    
Configure sensor rate or stop sensor report. To start event report of a sensor, or change rate of existing report, call this function with rateLevel other than RATE_STOP. Sensor events will be added into a queue formed by the shared memory used in creation of direction channel. Each element of the queue has size of 104 bytes and represents a sensor event. Data structure of an element (all fields in little-endian): 
  offset   type                    name
------------------------------------------------------------------------
  0x0000   int32_t                 size (always 104)
  0x0004   int32_t                 sensor report token
  0x0008   int32_t                 type (see SensorType)
  0x000C   uint32_t                atomic counter
  0x0010   int64_t                 timestamp (see Event)
  0x0018   float[16]/int64_t[8]    data (data type depends on sensor type)
  0x0058   int32_t[4]              reserved (set to zero)

 There are no head or tail pointers. The sequence and frontier of new sensor events is determined by the atomic counter, which counts from 1 after creation of direct channel and increments 1 for each new event. Atomic counter will wrap back to 1 after it reaches UINT32_MAX, skipping value 0 to avoid confusion with uninitialized memory. The writer in sensor system will wrap around from the start of shared memory region when it reaches the end. If size of memory region is not a multiple of size of element (104 bytes), the residual is not used at the end. Function returns a positive sensor report token on success. This token can be used to differentiate sensor events from multiple sensor of the same type. For example, if there are two accelerometers in the system A and B, it is guaranteed different report tokens will be returned when starting sensor A and B. To stop a sensor, call this function with rateLevel equal RATE_STOP. If the sensor parameter is left to be null, this will stop all active sensor report associated with the direct channel specified. Function return 1 on success or 0 on failure. 
Params:
Throws:
Returns:* starting report or changing rate: positive sensor report token on success,
                                            0 on failure;
        * stopping report: 1 on success, 0 on failure.
/**
     * Configure sensor rate or stop sensor report.
     *
     * To start event report of a sensor, or change rate of existing report, call this function with
     * rateLevel other than {@link android.hardware.SensorDirectChannel#RATE_STOP}. Sensor events
     * will be added into a queue formed by the shared memory used in creation of direction channel.
     * Each element of the queue has size of 104 bytes and represents a sensor event. Data
     * structure of an element (all fields in little-endian):
     *
     * <pre>
     *   offset   type                    name
     * ------------------------------------------------------------------------
     *   0x0000   int32_t                 size (always 104)
     *   0x0004   int32_t                 sensor report token
     *   0x0008   int32_t                 type (see SensorType)
     *   0x000C   uint32_t                atomic counter
     *   0x0010   int64_t                 timestamp (see Event)
     *   0x0018   float[16]/int64_t[8]    data (data type depends on sensor type)
     *   0x0058   int32_t[4]              reserved (set to zero)
     * </pre>
     *
     * There are no head or tail pointers. The sequence and frontier of new sensor events is
     * determined by the atomic counter, which counts from 1 after creation of direct channel and
     * increments 1 for each new event. Atomic counter will wrap back to 1 after it reaches
     * UINT32_MAX, skipping value 0 to avoid confusion with uninitialized memory. The writer in
     * sensor system will wrap around from the start of shared memory region when it reaches the
     * end. If size of memory region is not a multiple of size of element (104 bytes), the residual
     * is not used at the end.  Function returns a positive sensor report token on success. This
     * token can be used to differentiate sensor events from multiple sensor of the same type. For
     * example, if there are two accelerometers in the system A and B, it is guaranteed different
     * report tokens will be returned when starting sensor A and B.
     *
     * To stop a sensor, call this function with rateLevel equal {@link
     * android.hardware.SensorDirectChannel#RATE_STOP}. If the sensor parameter is left to be null,
     * this will stop all active sensor report associated with the direct channel specified.
     * Function return 1 on success or 0 on failure.
     *
     * @param sensor A {@link android.hardware.Sensor} object to denote sensor to be operated.
     * @param rateLevel rate level defined in {@link android.hardware.SensorDirectChannel}.
     * @return * starting report or changing rate: positive sensor report token on success,
     *                                             0 on failure;
     *         * stopping report: 1 on success, 0 on failure.
     * @throws NullPointerException when channel is null.
     */
    public int configure(Sensor sensor, @RateLevel int rateLevel) {
        return mManager.configureDirectChannelImpl(this, sensor, rateLevel);
    }

    
@hide
/** @hide */
    SensorDirectChannel(SensorManager manager, int id, int type, long size) {
        mManager = manager;
        mNativeHandle = id;
        mType = type;
        mSize = size;
        mCloseGuard.open("SensorDirectChannel");
    }

    
@hide
/** @hide */
    int getNativeHandle() {
        return mNativeHandle;
    }

    
This function encode handle information in MemoryFile into a long array to be passed down to native methods. 
@hide
/**
     * This function encode handle information in {@link android.os.MemoryFile} into a long array to
     * be passed down to native methods.
     *
     * @hide */
    static long[] encodeData(MemoryFile ashmem) {
        int fd;
        try {
            fd = ashmem.getFileDescriptor().getInt$();
        } catch (IOException e) {
            fd = -1;
        }
        return new long[] { 1 /*numFds*/, 0 /*numInts*/, fd };
    }

    @Override
    protected void finalize() throws Throwable {
        try {
            if (mCloseGuard != null) {
                mCloseGuard.warnIfOpen();
            }

            close();
        } finally {
            super.finalize();
        }
    }

    private final AtomicBoolean mClosed = new AtomicBoolean();
    private final CloseGuard mCloseGuard = CloseGuard.get();
    private final SensorManager mManager;
    private final int mNativeHandle;
    private final long mSize;
    private final int mType;
}