[Android] 输入系统（三）：加载按键映射映射表基本概念映射表加载过程

96 阅读 0 评论 64 点赞

我是靠谱客的博主诚心台灯，最近开发中收集的这篇文章主要介绍[Android] 输入系统（三）：加载按键映射映射表基本概念映射表加载过程，觉得挺不错的，现在分享给大家，希望可以做个参考。

概述

映射表基本概念

由于Android调用getEvents得到的key是linux发送过来的scan code，而Android处理的是类似于KEY_UP这种统一类型的key code，因此需要有映射表把scan code转换成key code。映射表在板子上的位置是/system/usr/keylayout/xxx.kl，先看一下映射表是什么样子的，下面截选了一段。

key 2
1
key 3
2
key 4
3
key 5
4
key 6
5
key 7
6
key 8
7
key 9
8
key 10
9
key 11
0
key 28
DPAD_CENTER
key 102
HOME
key 103
DPAD_UP
WAKE_DROPPED
key 105
DPAD_LEFT
WAKE_DROPPED
key 106
DPAD_RIGHT
WAKE_DROPPED
key 108
DPAD_DOWN
WAKE_DROPPED
key 111
DEL
key 113
VOLUME_MUTE
key 114
VOLUME_DOWN
key 115
VOLUME_UP
key 116
POWER

可以看到每行都是一个映射项，映射项格式如下：

key [scan code] [key label] [flag label] [flag label] ...

key是关键字，表明这个映射项是作为键值映射
scan code是从linux device取得的键值
key label是把scan code映射到key code中间的关键字，通过该关键字可以得到key code。
flag label即按键的标记的关键字，通过flag label可以得到flag，一行映射项后面可以有多个flag label

从3和4可以知道，还有一个key label到key code的过程，以及flag label到flag的过程

另外，映射表是设备相关的。由于不同设备发送到Android的scan code可能会不同，因此每个设备需要用自身对应的映射表才能正确解析出key code。

映射表加载过程

1. 获取设备相关信息

在构造EventHub的时候，就决定了需要扫描输入设备。然后会在第一次getEvents进行一次扫描。

扫描输入设备主要有两个目的：

得到该设备的各种信息，如：设备名称，设备版本，设备产品码等，这些信息都可以作为该设备的标识。
知道该设备所发送事件的类型，如：按键事件，触控事件，滑动事件，开关事件，xy坐标等；通过所发送事件的类型，就能定位出设备的类型。

EventHub::EventHub(void) :
mNeedToScanDevices(true),
{...}
size_t EventHub::getEvents(int timeoutMillis, RawEvent* buffer, size_t bufferSize) {
if (mNeedToScanDevices) {
mNeedToScanDevices = false;
scanDevicesLocked();
mNeedToSendFinishedDeviceScan = true;
}
}
void EventHub::scanDevicesLocked() {
status_t res = scanDirLocked(DEVICE_PATH);
if(res < 0) {
ALOGE("scan dir failed for %sn", DEVICE_PATH);
}
if (mDevices.indexOfKey(VIRTUAL_KEYBOARD_ID) < 0) {
createVirtualKeyboardLocked();
}
}

扫描的目录是/dev/input，linux中每加入一个输入设备，都会在该目录下创建设备文件。

status_t EventHub::scanDirLocked(const char *dirname)
{
char devname[PATH_MAX];
char *filename;
DIR *dir;
struct dirent *de;
dir = opendir(dirname);
if(dir == NULL)
return -1;
strcpy(devname, dirname);
filename = devname + strlen(devname);
*filename++ = '/';
while((de = readdir(dir))) {
if(de->d_name[0] == '.' &&
(de->d_name[1] == '' ||
(de->d_name[1] == '.' && de->d_name[2] == '')))
continue;
strcpy(filename, de->d_name);
openDeviceLocked(devname);
}
closedir(dir);
return 0;
}

在openDeviceLocked中就能清晰分析出扫描设备的两个目的

status_t EventHub::openDeviceLocked(const char *devicePath) {
int fd = open(devicePath, O_RDWR | O_CLOEXEC);
// Get device name.
if(ioctl(fd, EVIOCGNAME(sizeof(buffer) - 1), &buffer) < 1) {
//fprintf(stderr, "could not get device name for %s, %sn", devicePath, strerror(errno));
} else {
buffer[sizeof(buffer) - 1] = '';
identifier.name.setTo(buffer);
}
// Get device driver version.
int driverVersion;
if(ioctl(fd, EVIOCGVERSION, &driverVersion)) {
ALOGE("could not get driver version for %s, %sn", devicePath, strerror(errno));
close(fd);
return -1;
}
struct input_id inputId;
if(ioctl(fd, EVIOCGID, &inputId)) {
ALOGE("could not get device input id for %s, %sn", devicePath, strerror(errno));
close(fd);
return -1;
}
identifier.bus = inputId.bustype;
identifier.product = inputId.product;
identifier.vendor = inputId.vendor;
identifier.version = inputId.version;
...
Device* device = new Device(fd, deviceId, String8(devicePath), identifier);
// Figure out the kinds of events the device reports.
ioctl(fd, EVIOCGBIT(EV_KEY, sizeof(device->keyBitmask)), device->keyBitmask);
ioctl(fd, EVIOCGBIT(EV_ABS, sizeof(device->absBitmask)), device->absBitmask);
ioctl(fd, EVIOCGBIT(EV_REL, sizeof(device->relBitmask)), device->relBitmask);
ioctl(fd, EVIOCGBIT(EV_SW, sizeof(device->swBitmask)), device->swBitmask);
ioctl(fd, EVIOCGBIT(EV_LED, sizeof(device->ledBitmask)), device->ledBitmask);
ioctl(fd, EVIOCGBIT(EV_FF, sizeof(device->ffBitmask)), device->ffBitmask);
ioctl(fd, EVIOCGPROP(sizeof(device->propBitmask)), device->propBitmask);
//mouse device?
if (test_bit(BTN_MOUSE, device->keyBitmask)
&& test_bit(REL_X, device->relBitmask)
&& test_bit(REL_Y, device->relBitmask)) {
device->classes |= INPUT_DEVICE_CLASS_CURSOR;
}
// See if this is a touch pad.
// Is this a new modern multi-touch driver?
if (test_bit(ABS_MT_POSITION_X, device->absBitmask)
&& test_bit(ABS_MT_POSITION_Y, device->absBitmask)) {
// Some joysticks such as the PS3 controller report axes that conflict
// with the ABS_MT range.
Try to confirm that the device really is
// a touch screen.
if (test_bit(BTN_TOUCH, device->keyBitmask) || !haveGamepadButtons) {
device->classes |= INPUT_DEVICE_CLASS_TOUCH | INPUT_DEVICE_CLASS_TOUCH_MT;
}
// Is this an old style single-touch driver?
} else if (test_bit(BTN_TOUCH, device->keyBitmask)
&& test_bit(ABS_X, device->absBitmask)
&& test_bit(ABS_Y, device->absBitmask)) {
device->classes |= INPUT_DEVICE_CLASS_TOUCH;
}
// See if this device is a joystick.
// Assumes that joysticks always have gamepad buttons in order to distinguish them
// from other devices such as accelerometers that also have absolute axes.
if (haveGamepadButtons) {
uint32_t assumedClasses = device->classes | INPUT_DEVICE_CLASS_JOYSTICK;
for (int i = 0; i <= ABS_MAX; i++) {
if (test_bit(i, device->absBitmask)
&& (getAbsAxisUsage(i, assumedClasses) & INPUT_DEVICE_CLASS_JOYSTICK)) {
device->classes = assumedClasses;
break;
}
}
}
...
}

2. 加载映射表

通过设备信息与设备类型，我们就能去加载正确的映射表了

status_t EventHub::openDeviceLocked(const char *devicePath) {
...
if (device->classes & (INPUT_DEVICE_CLASS_KEYBOARD | INPUT_DEVICE_CLASS_JOYSTICK)) {
// Load the keymap for the device.
keyMapStatus = loadKeyMapLocked(device);
}
...
}

status_t EventHub::loadKeyMapLocked(Device* device) {
return device->keyMap.load(device->identifier, device->configuration);
}

加载配置文件分为下面几个步骤

1. 通过设备的配置文件去加载配置文件内制定好的映射表

2. 如果1不成功则通过设备信息加载对应的映射表

3. 如果2不成功则加载通用映射表

4. 如果3不成功则加载虚拟映射表

status_t KeyMap::load(const InputDeviceIdentifier& deviceIdenfifier,
const PropertyMap* deviceConfiguration) {
// Use the configured key layout if available.
if (deviceConfiguration) {
String8 keyLayoutName;
if (deviceConfiguration->tryGetProperty(String8("keyboard.layout"),
keyLayoutName)) {
status_t status = loadKeyLayout(deviceIdenfifier, keyLayoutName);
if (status == NAME_NOT_FOUND) {
ALOGE("Configuration for keyboard device '%s' requested keyboard layout '%s' but "
"it was not found.",
deviceIdenfifier.name.string(), keyLayoutName.string());
}
}
String8 keyCharacterMapName;
if (deviceConfiguration->tryGetProperty(String8("keyboard.characterMap"),
keyCharacterMapName)) {
status_t status = loadKeyCharacterMap(deviceIdenfifier, keyCharacterMapName);
if (status == NAME_NOT_FOUND) {
ALOGE("Configuration for keyboard device '%s' requested keyboard character "
"map '%s' but it was not found.",
deviceIdenfifier.name.string(), keyLayoutName.string());
}
}
if (isComplete()) {
return OK;
}
}
// Try searching by device identifier.
if (probeKeyMap(deviceIdenfifier, String8::empty())) {
return OK;
}
// Fall back on the Generic key map.
// TODO Apply some additional heuristics here to figure out what kind of
//
generic key map to use (US English, etc.) for typical external keyboards.
if (probeKeyMap(deviceIdenfifier, String8("Generic"))) {
return OK;
}
// Try the Virtual key map as a last resort.
if (probeKeyMap(deviceIdenfifier, String8("Virtual"))) {
return OK;
}
// Give up!
ALOGE("Could not determine key map for device '%s' and no default key maps were found!",
deviceIdenfifier.name.string());
return NAME_NOT_FOUND;
}

一般的情况我们会走第2步，因此从probeKeyMap往下分析

bool KeyMap::probeKeyMap(const InputDeviceIdentifier& deviceIdentifier,
const String8& keyMapName) {
if (!haveKeyLayout()) {
loadKeyLayout(deviceIdentifier, keyMapName);
}
if (!haveKeyCharacterMap()) {
loadKeyCharacterMap(deviceIdentifier, keyMapName);
}
return isComplete();
}

对于按键，有键盘按键与自定义按键两种，两者加载的文件后缀不同。键盘按键的映射表后缀是.kcm，而自定义按键映射表后缀是.kl。另外两者映射表的格式也不同，我们这里以自定义按键映射表为例，其中有三个步骤：

获取映射表文件路径
加载映射表文件
如果加载映射表文件成功的话，设置该路径为当前设备的自定义映射文件路径。（否则会去解析Generic.kl或者virtual.kl）

status_t KeyMap::loadKeyLayout(const InputDeviceIdentifier& deviceIdentifier,
const String8& name) {
String8 path(getPath(deviceIdentifier, name,
INPUT_DEVICE_CONFIGURATION_FILE_TYPE_KEY_LAYOUT));
if (path.isEmpty()) {
return NAME_NOT_FOUND;
}
status_t status = KeyLayoutMap::load(path, &keyLayoutMap);
if (status) {
return status;
}
keyLayoutFile.setTo(path);
return OK;
}

1. 获取映射表文件路径

我们从加载映射表文件的步骤2进来，那传入的name为空，则调用到getInputDeviceConfigurationFilePathByDeviceIdentifier，即通过设备标识来产生路径

String8 KeyMap::getPath(const InputDeviceIdentifier& deviceIdentifier,
const String8& name, InputDeviceConfigurationFileType type) {
return name.isEmpty()
? getInputDeviceConfigurationFilePathByDeviceIdentifier(deviceIdentifier, type)
: getInputDeviceConfigurationFilePathByName(name, type);
}

如果设备标识中的vendor，product，version都不为0的话，表明可以通过这些信息来组合成一个字符串，这个字符串就是映射表文件的前缀，否则，会设备名称deviceIdentifier.name就是映射表文件的前缀。后缀通过type指定。

String8 getInputDeviceConfigurationFilePathByDeviceIdentifier(
const InputDeviceIdentifier& deviceIdentifier,
InputDeviceConfigurationFileType type) {
if (deviceIdentifier.vendor !=0 && deviceIdentifier.product != 0) {
if (deviceIdentifier.version != 0) {
// Try vendor product version.
String8 versionPath(getInputDeviceConfigurationFilePathByName(
String8::format("Vendor_%04x_Product_%04x_Version_%04x",
deviceIdentifier.vendor, deviceIdentifier.product,
deviceIdentifier.version),
type));
if (!versionPath.isEmpty()) {
return versionPath;
}
}
// Try vendor product.
String8 productPath(getInputDeviceConfigurationFilePathByName(
String8::format("Vendor_%04x_Product_%04x",
deviceIdentifier.vendor, deviceIdentifier.product),
type));
if (!productPath.isEmpty()) {
return productPath;
}
}
// Try device name.
return getInputDeviceConfigurationFilePathByName(deviceIdentifier.name, type);
}

假设当前设备的设备名称是input_ir，传入的type是INPUT_DEVICE_CONFIGURATION_FILE_TYPE_KEY_LAYOUT，则设备的文件名为input_ir.kl

2.加载映射表文件

加载映射表文件最终目的是解析该文件得到映射表，其中也分为三个步骤：

打开映射表文件
创建映射表
解析映射表文件并把映射项加入映射表

status_t KeyLayoutMap::load(const String8& filename, sp<KeyLayoutMap>* outMap) {
status_t status = Tokenizer::open(filename, &tokenizer);
sp<KeyLayoutMap> map = new KeyLayoutMap();
Parser parser(map.get(), tokenizer);
status = parser.parse();
}

我们直接看最重要的解析部分

parse函数是一个while循环，一行一行地解析映射表项

status_t KeyLayoutMap::Parser::parse() {
while (!mTokenizer->isEof()) {
mTokenizer->skipDelimiters(WHITESPACE);
if (!mTokenizer->isEol() && mTokenizer->peekChar() != '#') {
String8 keywordToken = mTokenizer->nextToken(WHITESPACE);
if (keywordToken == "key") {
mTokenizer->skipDelimiters(WHITESPACE);
status_t status = parseKey();
if (status) return status;
} else if (keywordToken == "axis") {
mTokenizer->skipDelimiters(WHITESPACE);
status_t status = parseAxis();
if (status) return status;
} else {
ALOGE("%s: Expected keyword, got '%s'.", mTokenizer->getLocation().string(),
keywordToken.string());
return BAD_VALUE;
}
mTokenizer->skipDelimiters(WHITESPACE);
if (!mTokenizer->isEol() && mTokenizer->peekChar() != '#') {
ALOGE("%s: Expected end of line or trailing comment, got '%s'.",
mTokenizer->getLocation().string(),
mTokenizer->peekRemainderOfLine().string());
return BAD_VALUE;
}
}
mTokenizer->nextLine();
}
return NO_ERROR;
}

每一行的解析步骤如下：

跳过行首的空格符
如果开头第一个字符是”#”，跳过当前行
如果开头的关键词是key，跳过空白分割符，调用parseKey解析，如果解析出错则返回错误
如果开头的关键词是axis，跳过空白分隔符，调用parseAxis解析，如果解析出错则返回错误
如果开头的关键词是其他的词，说明这个映射表文件有误，返回错误
跳过行末的空格符
如果行末还有”#”以外的字符，说明这个映射表文件有误，返回错误

下面以parseKey为例，分析它是怎么解析出scan code与key code的（由于我们没用到usage code，所以忽略usage，直接分析scan code流程）

status_t KeyLayoutMap::Parser::parseKey() {
String8 codeToken = mTokenizer->nextToken(WHITESPACE);
//scan code从字符串转换成数字
int32_t code = int32_t(strtol(codeToken.string(), &end, 0));
if (*end) {
return BAD_VALUE;
}
//我们用的是scan code
KeyedVector<int32_t, Key>& map =
mapUsage ? mMap->mKeysByUsageCode : mMap->mKeysByScanCode;
//如果有重复的scan code，会出错返回
if (map.indexOfKey(code) >= 0) {
ALOGE("%s: Duplicate entry for key %s '%s'.", mTokenizer->getLocation().string(),
mapUsage ? "usage" : "scan code", codeToken.string());
return BAD_VALUE;
}
mTokenizer->skipDelimiters(WHITESPACE);
String8 keyCodeToken = mTokenizer->nextToken(WHITESPACE);
//通过label获取key code
int32_t keyCode = getKeyCodeByLabel(keyCodeToken.string());
if (!keyCode) {
ALOGE("%s: Expected key code label, got '%s'.", mTokenizer->getLocation().string(),
keyCodeToken.string());
return BAD_VALUE;
}
//key label后可以接flag，flag从getKeyFlagByLabel解析
uint32_t flags = 0;
for (;;) {
mTokenizer->skipDelimiters(WHITESPACE);
if (mTokenizer->isEol() || mTokenizer->peekChar() == '#') break;
String8 flagToken = mTokenizer->nextToken(WHITESPACE);
uint32_t flag = getKeyFlagByLabel(flagToken.string());
if (!flag) {
ALOGE("%s: Expected key flag label, got '%s'.", mTokenizer->getLocation().string(),
flagToken.string());
return BAD_VALUE;
}
if (flags & flag) {
ALOGE("%s: Duplicate key flag '%s'.", mTokenizer->getLocation().string(),
flagToken.string());
return BAD_VALUE;
}
flags |= flag;
}
Key key;
key.keyCode = keyCode;
key.flags = flags;
map.add(code, key);
return NO_ERROR;
}

我们在前面说过，还有个从key label到key code的流程，该流程就是在getKeyCodeByLabel中实现的

int32_t getKeyCodeByLabel(const char* label) {
return int32_t(lookupValueByLabel(label, KEYCODES));
}

最终从KEYCODES这个列表内，根据label查找key code

static const KeycodeLabel KEYCODES[] = {
{ "SOFT_LEFT", 1 },
{ "SOFT_RIGHT", 2 },
{ "HOME", 3 },
{ "BACK", 4 },
{ "CALL", 5 },
{ "ENDCALL", 6 },
{ "0", 7 },
{ "1", 8 },
{ "2", 9 },
{ "3", 10 },
{ "4", 11 },
{ "5", 12 },
{ "6", 13 },
{ "7", 14 },
{ "8", 15 },
{ "9", 16 },
...
}

同理，在解析flag的时候也是从FLAGS这个列表内查找flag

uint32_t getKeyFlagByLabel(const char* label) {
return uint32_t(lookupValueByLabel(label, FLAGS));
}
// NOTE: If you edit these flags, also edit policy flags in Input.h.
static const KeycodeLabel FLAGS[] = {
{ "WAKE", 0x00000001 },
{ "WAKE_DROPPED", 0x00000002 },
{ "SHIFT", 0x00000004 },
{ "CAPS_LOCK", 0x00000008 },
{ "ALT", 0x00000010 },
{ "ALT_GR", 0x00000020 },
{ "MENU", 0x00000040 },
{ "LAUNCHER", 0x00000080 },
{ "VIRTUAL", 0x00000100 },
{ "FUNCTION", 0x00000200 },
{ NULL, 0 }
};