前言

涉及到的源码有

frameworks\base\services\core\java\com\android\server\policy\PhoneWindowManager.java

vendor\mediatek\proprietary\packages\apps\SystemUI\src\com\android\systemui\screenshot\TakeScreenshotService.java
vendor\mediatek\proprietary\packages\apps\SystemUI\src\com\android\systemui\screenshot\GlobalScreenshot.java

按键处理都是在 PhoneWindowManager 中，真正截屏的功能实现在 GlobalScreenshot 中， PhoneWindowManager 和 systemui 通过 bind TakeScreenshotService 来实现截屏功能

流程

一般未经过特殊定制的 Android 系统，截屏都是通过同时按住音量下键和电源键来截屏，后来我们使用的一些华为、oppo等厂商的系统你会发现可以通过三指滑动来截屏，下一篇我们会定制此功能，而且截屏显示风格类似 iphone 在左下角显示截屏缩略图，点击可跳转放大查看，3s 无操作后向左自动滑动消失。

好了，现在我们先来理一下系统截屏的流程

    system_process D/WindowManager: interceptKeyTi keyCode=25 down=true repeatCount=0 keyguardOn=false mHomePressed=false canceled=false metaState:0
    system_process D/WindowManager: interceptKeyTq keycode=25 interactive=true keyguardActive=false policyFlags=22000000 down =false canceled = false isWakeKey=false mVolumeDownKeyTriggered =true result = 1 useHapticFeedback = false isInjected = false
    system_process D/WindowManager: interceptKeyTi keyCode=25 down=false repeatCount=0 keyguardOn=false mHomePressed=false canceled=false metaState:0
    system_process D/WindowManager: interceptKeyTq keycode=26 interactive=true keyguardActive=false policyFlags=22000000 down =false canceled = false isWakeKey=false mVolumeDownKeyTriggered =false result = 1 useHapticFeedback = false isInjected = false

上面是按下音量下键和电源键的日志，音量下键对应 keyCode=25 ，电源键对应 keyCode=26，来看到 PhoneWindowManager 中的 interceptKeyBeforeQueueing() 方法，在此处处理按键操作

    /** {@inheritDoc} */
    @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;
        }

        .....

        if (DEBUG_INPUT) {
            Log.d(TAG, "interceptKeyTq keycode=" + keyCode
                    + " interactive=" + interactive + " keyguardActive=" + keyguardActive
                    + " policyFlags=" + Integer.toHexString(policyFlags));
        }

      .....

        // Handle special keys.
        switch (keyCode) {
            .......

            case KeyEvent.KEYCODE_VOLUME_DOWN:
            case KeyEvent.KEYCODE_VOLUME_UP:
            case KeyEvent.KEYCODE_VOLUME_MUTE: {
                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();
                            interceptAccessibilityShortcutChord();
                        }
                    } else {
                        mScreenshotChordVolumeDownKeyTriggered = false;
                        cancelPendingScreenshotChordAction();
                        cancelPendingAccessibilityShortcutAction();
                    }
                } 
        ....
    }

看到 KEYCODE_VOLUME_DOWN 中，记录当前按下音量下键的时间 mScreenshotChordVolumeDownKeyTime，cancelPendingPowerKeyAction() 移除电源键长按消息 MSG_POWER_LONG_PRESS，来看下核心方法 interceptScreenshotChord()

// Time to volume and power must be pressed within this interval of each other.
private static final long SCREENSHOT_CHORD_DEBOUNCE_DELAY_MILLIS = 150;

private void interceptScreenshotChord() {
        if (mScreenshotChordEnabled
                && mScreenshotChordVolumeDownKeyTriggered && mScreensho