Android图形学习--从view的invalidate认识刷新VSYNC

参考资料：

DJLZPP

AndroidQ 图形系统（11）UI刷新，SurfaceFlinger，Vsync机制总结_DJLZPP的博客-优快云博客

在以前的学习中，通过GraphicBuffer的传送和数据读写了解到数据的传送显示流程，没有涉及到画面的刷新，这里通过view的刷新来学习一下，

在GLSurfaceView中，相关onDrawFrame方法会随着屏幕刷新而调用起来。

在普通的view里面，通过调用invalidate方法来注册vsync信号，开始一帧画面的绘制，调用到onDraw方法。invalidate这个方法名字很奇怪，如果是pre-draw似乎好理解一些，也许是想表达当前画面失效的意思吧，invalidate是个理解的关键点，如同GraphicBuffer一样

DJLZPP同学的这个图很好，（借用一下，原文链接在上面）

没有做过app的可能对invalidate并不熟悉，PointerLocationView中绘制touch轨迹的时候，就使用了invalidate来进行刷新，手指滑动的时候调用，来刷新画面

527    public void onPointerEvent(MotionEvent event) {
528        final int action = event.getAction();
529        int NP = mPointers.size();
530
531        if (action == MotionEvent.ACTION_DOWN
532                || (action & MotionEvent.ACTION_MASK) == MotionEvent.ACTION_POINTER_DOWN) {
533            final int index = (action & MotionEvent.ACTION_POINTER_INDEX_MASK)
534                    >> MotionEvent.ACTION_POINTER_INDEX_SHIFT; // will be 0 for down
535            if (action == MotionEvent.ACTION_DOWN) {
536                for (int p=0; p<NP; p++) {
537                    final PointerState ps = mPointers.get(p);
538                    ps.clearTrace();
539                    ps.mCurDown = false;
540                }
541                mCurDown = true;
542                mCurNumPointers = 0;
543                mMaxNumPointers = 0;
544                mVelocity.clear();
545                if (mAltVelocity != null) {
546                    mAltVelocity.clear();
547                }
548            }
549
550            mCurNumPointers += 1;
551            if (mMaxNumPointers < mCurNumPointers) {
552                mMaxNumPointers = mCurNumPointers;
553            }
554
555            final int id = event.getPointerId(index);
556            while (NP <= id) {
557                PointerState ps = new PointerState();
558                mPointers.add(ps);
559                NP++;
560            }
561
562            if (mActivePointerId < 0 ||
563                    !mPointers.get(mActivePointerId).mCurDown) {
564                mActivePointerId = id;
565            }
566
567            final PointerState ps = mPointers.get(id);
568            ps.mCurDown = true;
569            InputDevice device = InputDevice.getDevice(event.getDeviceId());
570            ps.mHasBoundingBox = device != null &&
571                    device.getMotionRange(MotionEvent.AXIS_GENERIC_1) != null;
572        }
573
574        final int NI = event.getPointerCount();
575
576        mVelocity.addMovement(event);
577        mVelocity.computeCurrentVelocity(1);
578        if (mAltVelocity != null) {
579            mAltVelocity.addMovement(event);
580            mAltVelocity.computeCurrentVelocity(1);
581        }
582
583        final int N = event.getHistorySize();
584        for (int historyPos = 0; historyPos < N; historyPos++) {
585            for (int i = 0; i < NI; i++) {
586                final int id = event.getPointerId(i);
587                final PointerState ps = mCurDown ? mPointers.get(id) : null;
588                final PointerCoords coords = ps != null ? ps.mCoords : mTempCoords;
589                event.getHistoricalPointerCoords(i, historyPos, coords);
590                if (mPrintCoords) {
591                    logCoords("Pointer", action, i, coords, id, event);
592                }
593                if (ps != null) {
594                    ps.addTrace(coords.x, coords.y, false);
595                }
596            }
597        }
598        for (int i = 0; i < NI; i++) {
599            final int id = event.getPointerId(i);
600            final PointerState ps = mCurDown ? mPointers.get(id) : null;
601            final PointerCoords coords = ps != null ? ps.mCoords : mTempCoords;
602            event.getPointerCoords(i, coords);
603            if (mPrintCoords) {
604                logCoords("Pointer", action, i, coords, id, event);
605            }
606            if (ps != null) {
607                ps.addTrace(coords.x, coords.y, true);
608                ps.mXVelocity = mVelocity.getXVelocity(id);
609                ps.mYVelocity = mVelocity.getYVelocity(id);
610                mVelocity.getEstimator(id, ps.mEstimator);
611                if (mAltVelocity != null) {
612                    ps.mAltXVelocity = mAltVelocity.getXVelocity(id);
613                    ps.mAltYVelocity = mAltVelocity.getYVelocity(id);
614                    mAltVelocity.getEstimator(id, ps.mAltEstimator);
615                }
616                ps.mToolType = event.getToolType(i);
617
618                if (ps.mHasBoundingBox) {
619                    ps.mBoundingLeft = event.getAxisValue(MotionEvent.AXIS_GENERIC_1, i);
620                    ps.mBoundingTop = event.getAxisValue(MotionEvent.AXIS_GENERIC_2, i);
621                    ps.mBoundingRight = event.getAxisValue(MotionEvent.AXIS_GENERIC_3, i);
622                    ps.mBoundingBottom = event.getAxisValue(MotionEvent.AXIS_GENERIC_4, i);
623                }
624            }
625        }
626
627        if (action == MotionEvent.ACTION_UP
628                || action == MotionEvent.ACTION_CANCEL
629                || (action & MotionEvent.ACTION_MASK) == MotionEvent.ACTION_POINTER_UP) {
630            final int index = (action & MotionEvent.ACTION_POINTER_INDEX_MASK)
631                    >> MotionEvent.ACTION_POINTER_INDEX_SHIFT; // will be 0 for UP
632
633            final int id = event.getPointerId(index);
634            if (id >= NP) {
635                Slog.wtf(TAG, "Got pointer ID out of bounds: id=" + id + " arraysize="
636                        + NP + " pointerindex=" + index
637                        + " action=0x" + Integer.toHexString(action));
638                return;
639            }
640            final PointerState ps = mPointers.get(id);
641            ps.mCurDown = false;
642
643            if (action == MotionEvent.ACTION_UP
644                    || action == MotionEvent.ACTION_CANCEL) {
645                mCurDown = false;
646                mCurNumPointers = 0;
647            } else {
648                mCurNumPointers -= 1;
649                if (mActivePointerId == id) {
650                    mActivePointerId = event.getPointerId(index == 0 ? 1 : 0);
651                }
652                ps.addTrace(Float.NaN, Float.NaN, false);
653            }
654        }
655
656        invalidate();
657    }
658