Input源码解读——从"Show tabs"开始

本文基于Android T版本源码，梳理当用户在开发者选项中开启Show tabs功能后显示第点按操作的视觉反馈的原理，来进一步了解Android Input系统

首先是设置应用(Settings)提供的开发者选项画面响应点击，将Show taps选项对应的设置Key SHOW_TOUCHES的 ON 值通过android.provder.Settings接口写入到保存系统设置数据的SettingsProvier中。

// packages/apps/Settings/src/com/android/settings/development/ShowTapsPreferenceController.java
public class ShowTapsPreferenceController extends DeveloperOptionsPreferenceController ... {
    ...
    @Override
    public boolean onPreferenceChange(Preference preference, Object newValue) {
        final boolean isEnabled = (Boolean) newValue;
        Settings.System.putInt(mContext.getContentResolver(),
                Settings.System.SHOW_TOUCHES, isEnabled ? SETTING_VALUE_ON : SETTING_VALUE_OFF);
        return true;
    }
    ...
}

负责管理输入的系统服务InputManagerService在启动之际，会监听设置中的 SHOW_TOUCHES字段的变化，在设置产生变化的时候调用native侧的代码进行处理。

// frameworks/base/services/core/java/com/android/server/input/InputManagerService.java
public class InputManagerService extends IInputManager.Stub... {
    ...
    public void start() {
        ...
        registerShowTouchesSettingObserver();
        ...
    }

    private void registerShowTouchesSettingObserver() {
        mContext.getContentResolver().registerContentObserver(
                Settings.System.getUriFor(Settings.System.SHOW_TOUCHES), true,
                new ContentObserver(mHandler) {
                    @Override
                    public void onChange(boolean selfChange) {
                        updateShowTouchesFromSettings();
                    }
                }, UserHandle.USER_ALL);
    }

    private void updateShowTouchesFromSettings() {
        int setting = getShowTouchesSetting(0);
        mNative.setShowTouches(setting != 0);
    }
    ...

// frameworks/base/services/core/java/com/android/server/input/NativeInputManagerService.java
public interface NativeInputManagerService {
    ...
    void setShowTouches(boolean enabled);
    ...
}


// frameworks/base/services/core/jni/com_android_server_input_InputManagerService.cpp
class NativeInputManager : public virtual RefBase, ...{
    ...
    void setShowTouches(bool enabled);
    ...
}

void NativeInputManager::setShowTouches(bool enabled) {
    { // acquire lock
        AutoMutex _l(mLock);

        if (mLocked.showTouches == enabled) {
            return;
        }

        ALOGI("Setting show touches feature to %s.", enabled ? "enabled" : "disabled");
        mLocked.showTouches = enabled;
    } // release lock

    mInputManager->getReader().requestRefreshConfiguration(
            InputReaderConfiguration::CHANGE_SHOW_TOUCHES);
}

这里的mInputManager是InputManagerInterface对象实例，InputManager是InputManagerInterface和子类，所以通过mInputManager可以连接NativeInputManager和InputReader。

这里向负责读取事件的InputReader发出更新配置的请求，配置变更的Type为 CHANGE_SHOW_TOUCHES。

InputReader接收到配置变化的Type之后，会根据记录待刷新配置的变量 mConfigurationChangesToRefresh判断当前是否已经在刷新过程中。

如果尚未处于刷新中，则更新mConfigurationChangesToRefresh的值，并唤醒EventHub进行配置刷新。

// frameworks/native/services/inputflinger/reader/InputReader.cpp
void InputReader::requestRefreshConfiguration(uint32_t changes) {
    std::scoped_lock _l(mLock);

    if (changes) {
        bool needWake = !mConfigurationChangesToRefresh;
        mConfigurationChangesToRefresh |= changes;

        if (needWake) {
            mEventHub->wake();
        }
    }
}

InputManagerService过来的刷新请求最终需要InputReader线程来处理。

可是 InputReader 线程处在从EventHub中读取事件和没有事件时便调用epoll_wait进入等待状态的循环当中。

所以为了让其立即处理配置变化，需要EventHub的手动唤醒。

// frameworks/native/services/inputflinger/reader/EventHub.cpp
void EventHub::wake() {
    ALOGV("wake() called");

    ssize_t nWrite;
    do {
        nWrite = write(mWakeWritePipeFd, "W", 1);
    } while (nWrite == -1 && errno == EINTR);

    if (nWrite != 1 && errno != EAGAIN) {
        ALOGW("Could not write wake signal: %s", strerror(errno));
    }
}

size_t EventHub::getEvents(int timeoutMillis, RawEvent* buffer, size_t bufferSize) {
    ...
    for (;;) {
        ...
        int pollResult = epoll_wait(mEpollFd, mPendingEventItems, EPOLL_MAX_EVENTS, timeoutMillis);
        ...
    }
    ...
}

EventHub唤醒后处于等待状态的getEvents会结束，之后InputReader线程会进入下次循环即loopOnce。

其首先将检查是否存在待刷新的配置变化changes，存在的话调用refreshConfigurationLocked让InputDevice去重新适配变化。

// frameworks/native/services/inputflinger/reader/InputReader.cpp
void InputReader::loopOnce() {
    ...
    std::vector<InputDeviceInfo> inputDevices;
    { // acquire lock
        ...
        uint32_t changes = mConfigurationChangesToRefresh;
        if (changes) {
            mConfigurationChangesToRefresh = 0;
            timeoutMillis = 0;
            refreshConfigurationLocked(changes);
        } else if (mNextTimeout != LLONG_MAX) {
            nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
            timeoutMillis = toMillisecondTimeoutDelay(now, mNextTimeout);
        }
    } // release lock

    size_t count = mEventHub->getEvents(timeoutMillis, mEventBuffer, EVENT_BUFFER_SIZE);
    ...
}

需要留意，refreshConfigurationLocked在调用InputDevice进一步处理之前需要先获取配置的变化放入mConfig中。

// frameworks/native/services/inputflinger/reader/InputReader.cpp
void InputReader::refreshConfigurationLocked(uint32_t changes) {
    mPolicy->getReaderConfiguration(&mConfig);
    ...

    if (changes & InputReaderConfiguration::CHANGE_MUST_REOPEN) {
        mEventHub->requestReopenDevices();
    } else {
        for (auto& devicePair : mDevices) {
            std::shared_ptr<InputDevice>& device = devicePair.second;
            device->configure(now, &mConfig, changes);
        }
    }
    ...
}

InputDevice配置变化

InputDevice的configure需要处理很多配置变化，比如键盘布局、麦克风等。对于Show taps的变化关注调用 InputMapper的congfigure即可。

// frameworks/native/services/inputflinger/reader/InputDevice.cpp
void InputDevice::configure(nsecs_t when, const InputReaderConfiguration* config,
                            uint32_t changes) {
    ...
    if (!isIgnored()) {
        ...
        for_each_mapper([this, when, config, changes](InputMapper& mapper) {
            mapper.configure(when, config, changes);
            mSources |= mapper.getSources();
        });
        ...
    }
}

TouchInputMapper 进一步处理

众多输入事件的物理数据需要对应的InputMapper来转化为上层能识别的事件类型。比如识别键盘输入的 KeyboardInputMapper、识别震动的VibratorInputMapper等等。

现在的触摸屏都支持多点触控，所以是MultiTouchInputMapper来处理的。可MultiTouchInputMapper没有复写 configure()，而是沿用由父类TouchInputMapper的共通处理。

// frameworks/native/services/inputflinger/reader/mapper/TouchInputMapper.cpp
void TouchInputMapper::configure(nsecs_t when, const InputReaderConfiguration* config,
                                 uint32_t changes) {
    ...
    bool resetNeeded = false;
    if (!changes ||
        (changes &
         (InputReaderConfiguration::CHANGE_DISPLAY_INFO |
          InputReaderConfiguration::CHANGE_POINTER_CAPTURE |
          InputReaderConfiguration::CHANGE_POINTER_GESTURE_ENABLEMENT |
          InputReaderConfiguration::CHANGE_SHOW_TOUCHES |
          InputReaderConfiguration::CHANGE_EXTERNAL_STYLUS_PRESENCE))) {
        // Configure device sources, display dimensions, orientation and
        // scaling factors.
        configureInputDevice(when, &resetNeeded);
    }
    ...
}

TouchInputMapper会依据changes的类型进行对应处理，对于SHOW_TOUCHES的变化需要调用configureInputDevice进一步处理。

configureInputDevice进行多个参数的测量和配置，其中和Show taps相关的是PointerController的创建，该类是 Mouse、Taps、Pointer location 等系统 Touch 显示的专用类。

// frameworks/native/services/inputflinger/reader/mapper/TouchInputMapper.cpp
void TouchInputMapper::configureInputDevice(nsecs_t when, bool* outResetNeeded) {
    ...
    // Create pointer controller if needed, and keep it around if Pointer Capture is enabled to
    // preserve the cursor position.
    if (mDeviceMode == DeviceMode::POINTER ||
        (mDeviceMode == DeviceMode::DIRECT && mConfig.showTouches) ||
        (mParameters.deviceType == Parameters::DeviceType::POINTER &&
         mConfig.pointerCaptureRequest.enable)) {
        if (mPointerController == nullptr) {
            mPointerController = getContext()->getPointerController(getDeviceId());
        }
        if (mConfig.pointerCaptureRequest.enable) {
            mPointerController->fade(PointerControllerInterface::Transition::IMMEDIATE);
        }
    } else {
        mPointerController.reset();
    }
    ...
}

这里调用InputReaderContext#getPointerController，InputReader::ContextImpl是InputReaderContext的子类，所以会回调到InputReader开启PointerController的创建和初始化。

// frameworks/native/services/inputflinger/reader/InputReader.cpp
std::shared_ptr<PointerControllerInterface> InputReader::ContextImpl::getPointerController(
        int32_t deviceId) {
    // lock is already held by the input loop
    return mReader->getPointerControllerLocked(deviceId);
}

std::shared_ptr<PointerControllerInterface> InputReader::getPointerControllerLocked(
        int32_t deviceId) {
    std::shared_ptr<PointerControllerInterface> controller = mPointerController.lock();
    if (controller == nullptr) {
        controller = mPolicy->obtainPointerController(deviceId);
        mPointerController = controller;
        updatePointerDisplayLocked();
    }
    return controller;
}

这里调用InputReaderPolicyInterface#obtainPointerController，而NativeInputManager是InputReaderPolicyInterface的子类。

// frameworks/base/services/core/jni/com_android_server_input_InputManagerService.cpp
std::shared_ptr<PointerControllerInterface> NativeInputManager::obtainPointerController(
        int32_t /* deviceId */) {
    ...
    std::shared_ptr<PointerController> controller = mLocked.pointerController.lock();
    if (controller == nullptr) {
        ensureSpriteControllerLocked();

        controller = PointerController::create(this, mLooper, mLocked.spriteController);
        mLocked.pointerController = controller;
        updateInactivityTimeoutLocked();
    }

    return controller;
}

PointerController 构建的同时需要构建持有的 MouseCursorController。

// frameworks/base/libs/input/PointerController.cpp
std::shared_ptr<PointerController> PointerController::create( ... ) {
    std::shared_ptr<PointerController> controller = std::shared_ptr<PointerController>(
            new PointerController(policy, looper, spriteController));
    ...
    return controller;
}

PointerController::PointerController( ... )
      : mContext(policy, looper, spriteController, *this), mCursorController(mContext) {
    std::scoped_lock lock(mLock);
    mLocked.presentation = Presentation::SPOT;
    ...
}

obtainPointerController执行完之后调用updatePointerDisplayLocked执行PointerController的初始化。

初始化 PointerController

调用PointerController的setDisplayViewport传入显示用的DisplayViewPort。

// frameworks/native/services/inputflinger/reader/InputReader.cpp
void InputReader::updatePointerDisplayLocked() {
    ...
    std::optional<DisplayViewport> viewport =
            mConfig.getDisplayViewportById(mConfig.defaultPointerDisplayId);
    if (!viewport) {
        ...
        viewport = mConfig.getDisplayViewportById(ADISPLAY_ID_DEFAULT);
    }
    ...
    controller->setDisplayViewport(*viewport);
}

setDisplayViewport需要持有的MouseCursorController进一步初始化。

// frameworks/base/libs/input/PointerController.cpp
void PointerController::setDisplayViewport(const DisplayViewport& viewport) {
    ...
    mCursorController.setDisplayViewport(viewport, getAdditionalMouseResources);
}

MouseCursorController需要获取Display相关的参数，并执行两个重要步骤：loadResourcesLocked/updatePointerLocked

// frameworks/base/libs/input/MouseCursorController.cpp
void MouseCursorController::setDisplayViewport(const DisplayViewport& viewport,
                                               bool getAdditionalMouseResources) {
    ...
    // Reset cursor position to center if size or display changed.
    if (oldViewport.displayId != viewport.displayId || oldDisplayWidth != newDisplayWidth ||
        oldDisplayHeight != newDisplayHeight) {
        float minX, minY, maxX, maxY;
        if (getBoundsLocked(&minX, &minY, &maxX, &maxY)) {
            mLocked.pointerX = (minX + maxX) * 0.5f;
            mLocked.pointerY = (minY + maxY) * 0.5f;
            // Reload icon resources for density may be changed.
            loadResourcesLocked(getAdditionalMouseResources);
        ...
        }
    } else if (oldViewport.orientation != viewport.orientation) {
        ...
    }

    updatePointerLocked();
}

加载 Pointer 相关资源

// frameworks/base/libs/input/MouseCursorController.cpp
void MouseCursorController::loadResourcesLocked(bool getAdditionalMouseResources) REQUIRES(mLock) {
    ...
    policy->loadPointerResources(&mResources, mLocked.viewport.displayId);
    policy->loadPointerIcon(&mLocked.pointerIcon, mLocked.viewport.displayId);
    ...
}

省略诸多细节，loadPointerResources将通过InputManagerService的JNI端以及PointerIcon的JNI端创建PointerIcon实例，并读取显示的资源。

getSystemIcon则是负责的函数，其将读取系统资源里名为Pointer的Style，并读取指针对应的资源 ID。

// frameworks/base/core/java/android/view/PointerIcon.java
    public static PointerIcon getSystemIcon(@NonNull Context context, int type) {
        ...
        int typeIndex = getSystemIconTypeIndex(type);
        if (typeIndex == 0) {
            typeIndex = getSystemIconTypeIndex(TYPE_DEFAULT);
        }

        int defStyle = sUseLargeIcons ?
                com.android.internal.R.style.LargePointer : com.android.internal.R.style.Pointer;
        TypedArray a = context.obtainStyledAttributes(null,
                com.android.internal.R.styleable.Pointer,
                0, defStyle);
        int resourceId = a.getResourceId(typeIndex, -1);
        ...
        icon = new PointerIcon(type);
        if ((resourceId & 0xff000000) == 0x01000000) {
            icon.mSystemIconResourceId = resourceId;
        } else {
            icon.loadResource(context, context.getResources(), resourceId);
        }
        systemIcons.append(type, icon);
        return icon;
    }

    private static int getSystemIconTypeIndex(int type) {
        switch (type) {
            ...
            case TYPE_SPOT_TOUCH:
                return com.android.internal.R.styleable.Pointer_pointerIconSpotTouch;
            ...
            default:
                return 0;
        }
    }

资源 ID 为 pointer_spot_touch_icon。

<!-- frameworks/base/core/res/res/drawable/pointer_spot_touch_icon.xml -->
<?xml version="1.0" encoding="utf-8"?>
<pointer-icon xmlns:android="http://schemas.android.com/apk/res/android"
    android:bitmap="@drawable/pointer_spot_touch"
    android:hotSpotX="16dp"
    android:hotSpotY="16dp" />

其指向的图片就是如下熟悉的 Spot png：pointer_spot_touch.png。之后的loadPointerIcon阶段会将该图片解析成 Bitmap 并被管理在SpriteIcon中。

而SpriteIcon在updatePointerLocked阶段会被存放到SpriteController中，等待显示的调度。

// frameworks/base/libs/input/MouseCursorController.cpp
void MouseCursorController::updatePointerLocked() REQUIRES(mLock) {
    if (!mLocked.viewport.isValid()) {
        return;
    }
    sp<SpriteController> spriteController = mContext.getSpriteController();
    spriteController->openTransaction();

    ...
    if (mLocked.updatePointerIcon) {
        if (mLocked.requestedPointerType == mContext.getPolicy()->getDefaultPointerIconId()) {
            mLocked.pointerSprite->setIcon(mLocked.pointerIcon);
        ...
        }
        mLocked.updatePointerIcon = false;
    }

    spriteController->closeTransaction();
}

点击的时候EventHub#getEvents会产生事件，InputReader#loopOnce会调用processEventsLocked处理事件。

// frameworks/native/services/inputflinger/reader/InputReader.cpp
void InputReader::loopOnce() {
    ...
    size_t count = mEventHub->getEvents(timeoutMillis, mEventBuffer, EVENT_BUFFER_SIZE);

    { // acquire lock
        ...
        if (count) {
            processEventsLocked(mEventBuffer, count);
        }
        ....
    } // release lock
    ...
}

之后调用InputMapper开始加工事件，并在TouchInputMapper#cookAndDispatch的时候调用updateTouchSpots更新 PointerController的一些参数。

// frameworks/native/services/inputflinger/reader/mapper/TouchInputMapper.cpp
void TouchInputMapper::updateTouchSpots() {
    ...
    mPointerController->setPresentation(PointerControllerInterface::Presentation::SPOT);
    mPointerController->fade(PointerControllerInterface::Transition::GRADUAL);

    mPointerController->setButtonState(mCurrentRawState.buttonState);
    setTouchSpots(mCurrentCookedState.cookedPointerData.pointerCoords,
                  mCurrentCookedState.cookedPointerData.idToIndex,
                  mCurrentCookedState.cookedPointerData.touchingIdBits, mViewport.displayId);
}

其中比较关键的setTouchSpots是显示Taps的关键步骤，准备 x、y 坐标和压力值。

在 Reader 而不是 Dispatch、更不是 ViewRootImpl 的时候处理的原因在于：Read 到事件即显示可以更早地响，同时不用占用 App 进程。

// frameworks/native/services/inputflinger/reader/mapper/TouchInputMapper.cpp
void TouchInputMapper::setTouchSpots(const PointerCoords* spotCoords, const uint32_t* spotIdToIndex,
                                     BitSet32 spotIdBits, int32_t displayId) {
    std::array<PointerCoords, MAX_POINTERS> outSpotCoords{};

    for (BitSet32 idBits(spotIdBits); !idBits.isEmpty();) {
        const uint32_t index = spotIdToIndex[idBits.clearFirstMarkedBit()];
        float x = spotCoords[index].getX();
        float y = spotCoords[index].getY();
        float pressure = spotCoords[index].getAxisValue(AMOTION_EVENT_AXIS_PRESSURE);
        ...
    }

    mPointerController->setSpots(outSpotCoords.data(), spotIdToIndex, spotIdBits, displayId);
}

其后PointerController会通过TouchSpotController创建Spot实例向其发送updateSprite请求。最后回调 SpriteController调用setIcon处理。

// frameworks/base/libs/input/TouchSpotController.cpp
void TouchSpotController::Spot::updateSprite(const SpriteIcon* icon, float x, float y,
                                             int32_t displayId) {
    sprite->setLayer(Sprite::BASE_LAYER_SPOT + id);
    ...
    if (icon != mLastIcon) {
        mLastIcon = icon;
        if (icon) {
            sprite->setIcon(*icon);
            sprite->setVisible(true);
        } else {
            sprite->setVisible(false);
        }
    }
}


// frameworks/base/libs/input/SpriteController.cpp
void SpriteController::SpriteImpl::setIcon(const SpriteIcon& icon) {
    AutoMutex _l(mController->mLock);
    ...
    invalidateLocked(dirty);
}

void SpriteController::SpriteImpl::invalidateLocked(uint32_t dirty) {
    ...
    if (!wasDirty) {
        mController->invalidateSpriteLocked(this);
    }
}

void SpriteController::invalidateSpriteLocked(const sp<SpriteImpl>& sprite) {
    bool wasEmpty = mLocked.invalidatedSprites.isEmpty();
    mLocked.invalidatedSprites.push(sprite);
    if (wasEmpty) {
        if (mLocked.transactionNestingCount != 0) {
            mLocked.deferredSpriteUpdate = true;
        } else {
            mLooper->sendMessage(mHandler, Message(MSG_UPDATE_SPRITES));
        }
    }
}

MSG_UPDATE_SPRITES经过 Handler 回调doUpdateSprites，将取出封装在SpriteUpdate中的SpriteIcon并执行 draw。

// frameworks/base/libs/input/SpriteController.cpp
void SpriteController::doUpdateSprites() {
    ...
    for (size_t i = 0; i < numSprites; i++) {
        SpriteUpdate& update = updates.editItemAt(i);

        if ((update.state.dirty & DIRTY_BITMAP) && update.state.surfaceDrawn) {
            update.state.surfaceDrawn = false;
            update.surfaceChanged = surfaceChanged = true;
        }

        if (update.state.surfaceControl != NULL && !update.state.surfaceDrawn
                && update.state.wantSurfaceVisible()) {
            sp<Surface> surface = update.state.surfaceControl->getSurface();
            if (update.state.icon.draw(surface)) {
                update.state.surfaceDrawn = true;
                update.surfaceChanged = surfaceChanged = true;
            }
        }
    }
    ...
    updates.clear();
}

最后，SpriteIcon将取出Bitmap描画到Surface的Canvas上去。

// frameworks/base/libs/input/SpriteIcon.cpp
bool SpriteIcon::draw(sp<Surface> surface) const {
    ...
    graphics::Paint paint;
    paint.setBlendMode(ABLEND_MODE_SRC);

    graphics::Canvas canvas(outBuffer, (int32_t)surface->getBuffersDataSpace());
    canvas.drawBitmap(bitmap, 0, 0, &paint);
    ...
    status = surface->unlockAndPost();
    if (status) {
        ALOGE("Error %d unlocking and posting sprite surface after drawing.", status);
    }
    return !status;
}

通过一个框图简单回顾一下整个流程。

可以看到，简简单单的 Show taps 功能，从设置、配置、刷新再到显示，经历了多个进程、多个模块的协力。