android 截屏源码分析,android 截图功能源码解析

一般没有修改rom的android原生系统截图功能的组合键是音量减+开机键；今天我们从源码角度来分析截图功能是如何在源码中实现的。

在android系统中，由于我们的每一个Android界面都是一个Activity，而界面的显示都是通过Window对象实现的，每个Window对象实际上都是PhoneWindow的实例，而每个PhoneWindow对象都对应一个PhoneWindowManager对象，当我们在Activity界面执行按键操作的时候，在将按键的处理操作分发到App之前，首先会回调PhoneWindowManager中的dispatchUnhandledKey方法，该方法主要用于执行当前App处理按键之前的操作，我们具体看一下该方法的实现。

/** {@inheritDoc} */

@Override

public KeyEvent dispatchUnhandledKey(WindowState win, KeyEvent event, int policyFlags) {

...

KeyEvent fallbackEvent = null;

if ((event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {

final KeyCharacterMap kcm = event.getKeyCharacterMap();

final int keyCode = event.getKeyCode();

final int metaState = event.getMetaState();

final boolean initialDown = event.getAction() == KeyEvent.ACTION_DOWN

&& event.getRepeatCount() == 0;

// Check for fallback actions specified by the key character map.

final FallbackAction fallbackAction;

if (initialDown) {

fallbackAction = kcm.getFallbackAction(keyCode, metaState);

} else {

fallbackAction = mFallbackActions.get(keyCode);

}

if (fallbackAction != null) {

...

final int flags = event.getFlags() | KeyEvent.FLAG_FALLBACK;

fallbackEvent = KeyEvent.obtain(

event.getDownTime(), event.getEventTime(),

event.getAction(), fallbackAction.keyCode,

event.getRepeatCount(), fallbackAction.metaState,

event.getDeviceId(), event.getScanCode(),

flags, event.getSource(), null);

if (!interceptFallback(win, fallbackEvent, policyFlags)) {

fallbackEvent.recycle();

fallbackEvent = null;

}

if (initialDown) {

mFallbackActions.put(keyCode, fallbackAction);

} else if (event.getAction() == KeyEvent.ACTION_UP) {

mFallbackActions.remove(keyCode);

fallbackAction.recycle();

}

}

}

...

return fallbackEvent;

}

这里我们关注一下方法体中调用的：interceptFallback方法，通过调用该方法将处理按键的操作下发到该方法中，我们继续看一下该方法的实现逻辑。

private boolean interceptFallback(WindowState win, KeyEvent fallbackEvent, int policyFlags) {

int actions = interceptKeyBeforeQueueing(fallbackEvent, policyFlags);

if ((actions & ACTION_PASS_TO_USER) != 0) {

long delayMillis = interceptKeyBeforeDispatching(

win, fallbackEvent, policyFlags);

if (delayMillis == 0) {

return true;

}

}

return false;

}

然后我们看到在interceptFallback方法中我们调用了interceptKeyBeforeQueueing方法，通过阅读我们我们知道该方法主要实现了对截屏按键的处理流程，这样我们继续看一下interceptKeyBeforeWueueing方法的处理：

@Override

public int interceptKeyBeforeQueueing(KeyEvent event, int policyFlags) {

if (!mSystemBooted) {

// If we have not yet booted, don't let key events do anything.

return 0;

}

...

// Handle special keys.

switch (keyCode) {

case KeyEvent.KEYCODE_VOLUME_DOWN:

case KeyEvent.KEYCODE_VOLUME_UP:

case KeyEvent.KEYCODE_VOLUME_MUTE: {

if (mUseTvRouting) {

// On TVs volume keys never go to the foreground app

result &= ~ACTION_PASS_TO_USER;

}

if (keyCode == KeyEvent.KEYCODE_VOLUME_DOWN) {

if (down) {

if (interactive && !mScreenshotChordVolumeDownKeyTriggered

&& (event.getFlags() & KeyEvent.FLAG_FALLBACK) == 0) {

mScreenshotChordVolumeDownKeyTriggered = true;

mScreenshotChordVolumeDownKeyTime = event.getDownTime();

mScreenshotChordVolumeDownKeyConsumed = false;

cancelPendingPowerKeyAction();

interceptScreenshotChord();

}

} else {

mScreenshotChordVolumeDownKeyTriggered = false;

cancelPendingScreenshotChordAction();

}

}

...

return result;

}

可以发现这里首先判断当前系统是否已经boot完毕，若尚未启动完毕，则所有的按键操作都将失效，若启动完成，则执行后续的操作，这里我们只是关注音量减少按键和电源按键组