由浅入深学习android input系统 - input事件采集（InputReader）

Posted 2021-06-29 许佳佳233

概述

前面讲到，系统进程是通过InputDispatcher将事件分发给app进程的。

前文地址：由浅入深学习android input系统（三） - InputChannel解析

那么InputDispatcher的事件又是从哪里来的呢？
android是怎么收到硬件层的事件的呢？
本文将对此进行探索。

InputReader

本身的答案就是在InputReader这个类中，InputReader会启动一个线程一直循环处理逻辑，主要做两件事：

1. 获取硬件设备的事件
2. 将获取到的原始数据进行处理后传递给InputDispatcher

void InputReader::loopOnce() {
————————————————省略
    size_t count = mEventHub->getEvents(timeoutMillis, mEventBuffer, EVENT_BUFFER_SIZE);
————————————————省略
        if (count) {
            processEventsLocked(mEventBuffer, count);
        }
————————————————省略
}

InputReader获取硬件事件

InputReader获取硬件事件是通过mEventHub的getEvents()方法。

mEventHub监听的是/dev/input目录下的设备节点，对这块内容由兴趣的读者可以自行看下mEventHub源码。

代码逻辑如下：

1. 启动一个for循环去获取事件，如果没有事件就会上锁，一直到有事件进来才会继续执行逻辑。
2. 如果有设备已经关闭，需要从监听的设备中删除，并且将”设备关闭“作为一个事件记录到event中。
3. 如果有新的设备添加进来，需要加入监听设备的链表，并且将”设备添加“作为事件记录到event中。
4. 遍历发生的硬件事件，首先找到事件对应的硬件，然后从该硬件读取事件相关的数据存储到event中。
5. 最终返回发生事件的个数。（方法外本身就有对event对象的引用，因此不需要将event传出去。）

size_t EventHub::getEvents(int timeoutMillis, RawEvent* buffer, size_t bufferSize) {
    ALOG_ASSERT(bufferSize >= 1);

    AutoMutex _l(mLock);

    struct input_event readBuffer[bufferSize];

    RawEvent* event = buffer;
    size_t capacity = bufferSize;
    bool awoken = false;
    for (;;) {
        nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);

        // Reopen input devices if needed.
        if (mNeedToReopenDevices) {
            mNeedToReopenDevices = false;

            ALOGI("Reopening all input devices due to a configuration change.");

            closeAllDevicesLocked();
            mNeedToScanDevices = true;
            break; // return to the caller before we actually rescan
        }

        // Report any devices that had last been added/removed.
        while (mClosingDevices) {
            Device* device = mClosingDevices;
            ALOGV("Reporting device closed: id=%d, name=%s\\n",
                 device->id, device->path.string());
            mClosingDevices = device->next;
            event->when = now;
            event->deviceId = device->id == mBuiltInKeyboardId ? BUILT_IN_KEYBOARD_ID : device->id;
            event->type = DEVICE_REMOVED;
            event += 1;
            delete device;
            mNeedToSendFinishedDeviceScan = true;
            if (--capacity == 0) {
                break;
            }
        }

        if (mNeedToScanDevices) {
            mNeedToScanDevices = false;
            scanDevicesLocked();
            mNeedToSendFinishedDeviceScan = true;
        }

        while (mOpeningDevices != NULL) {
            Device* device = mOpeningDevices;
            ALOGV("Reporting device opened: id=%d, name=%s\\n",
                 device->id, device->path.string());
            mOpeningDevices = device->next;
            event->when = now;
            event->deviceId = device->id == mBuiltInKeyboardId ? 0 : device->id;
            event->type = DEVICE_ADDED;
            event += 1;
            mNeedToSendFinishedDeviceScan = true;
            if (--capacity == 0) {
                break;
            }
        }

        if (mNeedToSendFinishedDeviceScan) {
            mNeedToSendFinishedDeviceScan = false;
            event->when = now;
            event->type = FINISHED_DEVICE_SCAN;
            event += 1;
            if (--capacity == 0) {
                break;
            }
        }

        // Grab the next input event.
        bool deviceChanged = false;
        while (mPendingEventIndex < mPendingEventCount) {
            const struct epoll_event& eventItem = mPendingEventItems[mPendingEventIndex++];
            if (eventItem.data.u32 == EPOLL_ID_INOTIFY) {
                if (eventItem.events & EPOLLIN) {
                    mPendingINotify = true;
                } else {
                    ALOGW("Received unexpected epoll event 0x%08x for INotify.", eventItem.events);
                }
                continue;
            }

            if (eventItem.data.u32 == EPOLL_ID_WAKE) {
                if (eventItem.events & EPOLLIN) {
                    ALOGV("awoken after wake()");
                    awoken = true;
                    char buffer[16];
                    ssize_t nRead;
                    do {
                        nRead = read(mWakeReadPipeFd, buffer, sizeof(buffer));
                    } while ((nRead == -1 && errno == EINTR) || nRead == sizeof(buffer));
                } else {
                    ALOGW("Received unexpected epoll event 0x%08x for wake read pipe.",
                            eventItem.events);
                }
                continue;
            }

            ssize_t deviceIndex = mDevices.indexOfKey(eventItem.data.u32);
            if (deviceIndex < 0) {
                ALOGW("Received unexpected epoll event 0x%08x for unknown device id %d.",
                        eventItem.events, eventItem.data.u32);
                continue;
            }

            Device* device = mDevices.valueAt(deviceIndex);
            if (eventItem.events & EPOLLIN) {
                int32_t readSize = read(device->fd, readBuffer,
                        sizeof(struct input_event) * capacity);
                if (readSize == 0 || (readSize < 0 && errno == ENODEV)) {
                    // Device was removed before INotify noticed.
                    ALOGW("could not get event, removed? (fd: %d size: %" PRId32
                            " bufferSize: %zu capacity: %zu errno: %d)\\n",
                            device->fd, readSize, bufferSize, capacity, errno);
                    deviceChanged = true;
                    closeDeviceLocked(device);
                } else if (readSize < 0) {
                    if (errno != EAGAIN && errno != EINTR) {
                        ALOGW("could not get event (errno=%d)", errno);
                    }
                } else if ((readSize % sizeof(struct input_event)) != 0) {
                    ALOGE("could not get event (wrong size: %d)", readSize);
                } else {
                    int32_t deviceId = device->id == mBuiltInKeyboardId ? 0 : device->id;

                    size_t count = size_t(readSize) / sizeof(struct input_event);
                    for (size_t i = 0; i < count; i++) {
                        struct input_event& iev = readBuffer[i];
                        ALOGV("%s got: time=%d.%06d, type=%d, code=%d, value=%d",
                                device->path.string(),
                                (int) iev.time.tv_sec, (int) iev.time.tv_usec,
                                iev.type, iev.code, iev.value);


                        if (iev.type == EV_MSC) {
                            if (iev.code == MSC_ANDROID_TIME_SEC) {
                                device->timestampOverrideSec = iev.value;
                                continue;
                            } else if (iev.code == MSC_ANDROID_TIME_USEC) {
                                device->timestampOverrideUsec = iev.value;
                                continue;
                            }
                        }
                        if (device->timestampOverrideSec || device->timestampOverrideUsec) {
                            iev.time.tv_sec = device->timestampOverrideSec;
                            iev.time.tv_usec = device->timestampOverrideUsec;
                            if (iev.type == EV_SYN && iev.code == SYN_REPORT) {
                                device->timestampOverrideSec = 0;
                                device->timestampOverrideUsec = 0;
                            }
                            ALOGV("applied override time %d.%06d",
                                    int(iev.time.tv_sec), int(iev.time.tv_usec));
                        }


                        event->when = nsecs_t(iev.time.tv_sec) * 1000000000LL
                                + nsecs_t(iev.time.tv_usec) * 1000LL;
                        ALOGV("event time %" PRId64 ", now %" PRId64, event->when, now);

                        if (event->when >= now + 10 * 1000000000LL) {
                            // Double-check.  Time may have moved on.
                            nsecs_t time = systemTime(SYSTEM_TIME_MONOTONIC);
                            if (event->when > time) {
                                ALOGW("An input event from %s has a timestamp that appears to "
                                        "have been generated using the wrong clock source "
                                        "(expected CLOCK_MONOTONIC): "
                                        "event time %" PRId64 ", current time %" PRId64
                                        ", call time %" PRId64 ".  "
                                        "Using current time instead.",
                                        device->path.string(), event->when, time, now);
                                event->when = time;
                            } else {
                                ALOGV("Event time is ok but failed the fast path and required "
                                        "an extra call to systemTime: "
                                        "event time %" PRId64 ", current time %" PRId64
                                        ", call time %" PRId64 ".",
                                        event->when, time, now);
                            }
                        }
                        event->deviceId = deviceId;
                        event->type = iev.type;
                        event->code = iev.code;
                        event->value = iev.value;
                        event += 1;
                        capacity -= 1;
                    }
                    if (capacity == 0) {
                        // The result buffer is full.  Reset the pending event index
                        // so we will try to read the device again on the next iteration.
                        mPendingEventIndex -= 1;
                        break;
                    }
                }
            } else if (eventItem.events & EPOLLHUP) {
                ALOGI("Removing device %s due to epoll hang-up event.",
                        device->identifier.name.string());
                deviceChanged = true;
                closeDeviceLocked(device);
            } else {
                ALOGW("Received unexpected epoll event 0x%08x for device %s.",
                        eventItem.events, device->identifier.name.string());
            }
        }
        if (mPendingINotify && mPendingEventIndex >= mPendingEventCount) {
            mPendingINotify = false;
            readNotifyLocked();
            deviceChanged = true;
        }

        // Report added or removed devices immediately.
        if (deviceChanged) {
            continue;
        }

        // Return now if we have collected any events or if we were explicitly awoken.
        if (event != buffer || awoken) {
            break;
        }
        mPendingEventIndex = 0;

        mLock.unlock(); // release lock before poll, must be before release_wake_lock
        release_wake_lock(WAKE_LOCK_ID);

        int pollResult = epoll_wait(mEpollFd, mPendingEventItems, EPOLL_MAX_EVENTS, timeoutMillis);

        acquire_wake_lock(PARTIAL_WAKE_LOCK, WAKE_LOCK_ID);
        mLock.lock(); // reacquire lock after poll, must be after acquire_wake_lock

        if (pollResult == 0) {
            // Timed out.
            mPendingEventCount = 0;
            break;
        }

        if (pollResult < 0) {
            mPendingEventCount = 0;
            if (errno != EINTR) {
                ALOGW("poll failed (errno=%d)\\n", errno);
                usleep(100000);
            }
        } else {
            // Some events occurred.
            mPendingEventCount = size_t(pollResult);
        }
    }

    return event - buffer;
}

InputReader处理硬件事件

硬件的事件有很多种，包括”设备输入“、”设备添加“、”设备删除“等。
”设备输入“中也包括”键盘事件“和”触摸事件“。
我们本文主要探索”设备输入“中的”触摸事件“。

调用的任务栈如下：

• InputReader.loopOnce()
• InputReader.processEventsLocked()
• InputReader.processEventsForDeviceLocked()
• InputDevice.process()
• InputMapper.process()
  等同于TouchInputMapper.process()
• TouchInputMapper.sync()
• TouchInputMapper.processRawTouches()
• TouchInputMapper.cookAndDispatch()
• TouchInputMapper.dispatchTouches()
• TouchInputMapper.dispatchMotion()
• getListener().notifyMotion()
  等同于InputDispatcher.notifyMotion()
• InputDispatcher.dispatchOnce()

逻辑大致如下：

1. InputReader在接收到事件之后将原始数据交给InputDevice处理。
2. 每个InputDevice会有自己对应的处理事件的Mapper，InputDevice会把事件交给InputDevice继续处理，此处探索的是触摸事件，因此是TouchInputMapper。
   （InputDevice还有SwitchInputMapper,KeyBoardInputMapper等，分别对应不同的输入方式）
3. TouchInputMapper中会将原始数据转化成InputDispatcher可以接收的数据后，将数据传给InputDispatcher，然后唤醒InputDispatcher线程。最终会在InputDispatcher的dispatchOnce()中处理。

对InputDispatcher还不了解的读者可以看下笔者前几篇文章：

继续探索

问题：

1. InputReader什么时候初始化？
2. InputDispatcher什么时候初始化？

有兴趣的读者可以关注下笔者后续的文章，或者自行阅读下相关源码。