废话不多说,直接看过程吧
在android中由WindowManagerService这个系统服务来循环读取窗口获取的消息(包括按下,弹起,双击,单击等)然后分发到各个类接收,在这个过程中有一个类会进行消息过滤处理,就是PhoneWindowManager了,PhoneWindowManager中有两个方法interceptKeyBeforeDispatching和interceptKeyBeforeQueueing,其中包括了几乎所有按键的处理,interceptKeyBeforeDispatching主要处理Home键、Menu键、Search键等,
interceptKeyBeforeQueueing主要处理音量键、电源键、耳机键等。
截屏功能的代码就是在interceptKeyBeforeQueueing方法中,看两段代码
case KeyEvent.KEYCODE_VOLUME_DOWN: case KeyEvent.KEYCODE_VOLUME_UP: case KeyEvent.KEYCODE_VOLUME_MUTE: { if (keyCode == KeyEvent.KEYCODE_VOLUME_DOWN) { if (down) { if (isScreenOn && !mVolumeDownKeyTriggered && (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) { mVolumeDownKeyTriggered = true; mVolumeDownKeyTime = event.getDownTime(); mVolumeDownKeyConsumedByScreenshotChord = false; cancelPendingPowerKeyAction(); interceptScreenshotChord(); } } else { mVolumeDownKeyTriggered = false; cancelPendingScreenshotChordAction(); } ...... case KeyEvent.KEYCODE_POWER: { result &= ~ACTION_PASS_TO_USER; if (down) { if (isScreenOn && !mPowerKeyTriggered && (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) { mPowerKeyTriggered = true; mPowerKeyTime = event.getDownTime(); interceptScreenshotChord(); } ......
可以看到正是这里(响应down事件)捕获是否按了音量下键和电源键,而且两个地方都会进入函数interceptScreenshotChord()中,接下来看看这个函数做了什么操作:
private void interceptScreenshotChord() { if (mVolumeDownKeyTriggered && mPowerKeyTriggered && !mVolumeUpKeyTriggered) { final long now = SystemClock.uptimeMillis(); if (now <= mVolumeDownKeyTime + SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS && now <= mPowerKeyTime + SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS) { mVolumeDownKeyConsumedByScreenshotChord = true; cancelPendingPowerKeyAction(); mHandler.postDelayed(mScreenshotChordLongPress, ViewConfiguration.getGlobalActionKeyTimeout()); } } }
在这个函数中,用两个布尔变量判断是否同时按了音量下键和电源键后,再计算两个按键响应Down事件之间的时间差不超过150毫秒,也就认为是同时按了这两个键后,算是真正的捕获到屏幕截屏的组合键。
调用函数interceptScreenshotChord,
private void interceptScreenshotChord() { if (mScreenshotChordEnabled && mVolumeDownKeyTriggered && mPowerKeyTriggered && !mVolumeUpKeyTriggered) { final long now = SystemClock.uptimeMillis(); if (now <= mVolumeDownKeyTime + SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS && now <= mPowerKeyTime + SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS) { mVolumeDownKeyConsumedByScreenshotChord = true; cancelPendingPowerKeyAction(); mHandler.postDelayed(mScreenshotChordLongPress, getScreenshotChordLongPressDelay()); } } }
发送消息mScreenshotChordLongPress到消息队列里面,点进去可以看到
private final Runnable mScreenshotChordLongPress = new Runnable() { public void run() { takeScreenshot(); } };
这个takeScreenshot一看就知道是实现截图功能的函数了,在takeScreenshot里面主要是绑定一个TakeScreenshotService的服务,还有一些返回的消息处理,再往下就是涉及到JNI调用底层服务的东西,在这就不作介绍了,也没仔细研究过