Android invalidate和postInvalidat

在自定义View中，我们常常会用到invalidate和postInvalidate去更新UI，其中，前者是在用在主线程，后者用在子线程中。

public void invalidate() {
    invalidate(true);
}

public void invalidate(boolean invalidateCache) {
    invalidateInternal(0, 0, mRight - mLeft, mBottom - mTop, invalidateCache, true);
}

void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache, boolean fullInvalidate) {
    //如果这个View拥有GhostView，则让GhostView去更新界面
    //GhostView顾名思义，就类似于一个幽灵一样，当它不可见时，View才会绘制出来，否则它将替代View进行显示。
    if (mGhostView != null) {
        mGhostView.invalidate(true);
        return;
    }
    //如果当前View不可见并且无动画执行
    if (skipInvalidate()) {
        return;
    }

    // Reset content capture caches
    mCachedContentCaptureSession = null;

    if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)) == (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)
            || (invalidateCache && (mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == PFLAG_DRAWING_CACHE_VALID)
            || (mPrivateFlags & PFLAG_INVALIDATED) != PFLAG_INVALIDATED
            || (fullInvalidate && isOpaque() != mLastIsOpaque)) {
        if (fullInvalidate) {
            mLastIsOpaque = isOpaque();
            mPrivateFlags &= ~PFLAG_DRAWN;
        }

        mPrivateFlags |= PFLAG_DIRTY;

        if (invalidateCache) {
            mPrivateFlags |= PFLAG_INVALIDATED;
            mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
        }

        // Propagate the damage rectangle to the parent view.
        final AttachInfo ai = mAttachInfo;
        final ViewParent p = mParent;
        if (p != null && ai != null && l < r && t < b) {
            final Rect damage = ai.mTmpInvalRect;
            //此处将调用ViewGroup的invalidateChild方法，更新当前View所在区域
            damage.set(l, t, r, b);
            p.invalidateChild(this, damage);
        }

        // Damage the entire projection receiver, if necessary.
        if (mBackground != null && mBackground.isProjected()) {
            final View receiver = getProjectionReceiver();
            if (receiver != null) {
                receiver.damageInParent();
            }
        }
    }
}

private boolean skipInvalidate() {
    return (mViewFlags & VISIBILITY_MASK) != VISIBLE && mCurrentAnimation == null &&
            (!(mParent instanceof ViewGroup) ||
                    !((ViewGroup) mParent).isViewTransitioning(this));
}

从invalidateInternal()方法中，我们可以清楚的发现，调用view的invalidate()方法，会调用父ViewGroup的invalidateChild()方法去更新此View的区域。接下来我们看ViewGroup中该方法：

public final void invalidateChild(View child, final Rect dirty) {
    final AttachInfo attachInfo = mAttachInfo;
    if (attachInfo != null && attachInfo.mHardwareAccelerated) {
        // HW accelerated fast path
        onDescendantInvalidated(child, child);
        return;
    }

    ViewParent parent = this;
    if (attachInfo != null) {
        // If the child is drawing an animation, we want to copy this flag onto
        // ourselves and the parent to make sure the invalidate request goes
        // through
        final boolean drawAnimation = (child.mPrivateFlags & PFLAG_DRAW_ANIMATION) != 0;

        // Check whether the child that requests the invalidate is fully opaque
        // Views being animated or transformed are not considered opaque because we may
        // be invalidating their old position and need the parent to paint behind them.
        Matrix childMatrix = child.getMatrix();
        // Mark the child as dirty, using the appropriate flag
        // Make sure we do not set both flags at the same time

        if (child.mLayerType != LAYER_TYPE_NONE) {
            mPrivateFlags |= PFLAG_INVALIDATED;
            mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
        }

        final int[] location = attachInfo.mInvalidateChildLocation;
        location[CHILD_LEFT_INDEX] = child.mLeft;
        location[CHILD_TOP_INDEX] = child.mTop;
        if (!childMatrix.isIdentity() ||
                (mGroupFlags & ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS) != 0) {
            RectF boundingRect = attachInfo.mTmpTransformRect;
            boundingRect.set(dirty);
            Matrix transformMatrix;
            if ((mGroupFlags & ViewGroup.FLAG_SUPPORT_STATIC_TRANSFORMATIONS) != 0) {
                Transformation t = attachInfo.mTmpTransformation;
                boolean transformed = getChildStaticTransformation(child, t);
                if (transformed) {
                    transformMatrix = attachInfo.mTmpMatrix;
                    transformMatrix.set(t.getMatrix());
                    if (!childMatrix.isIdentity()) {
                        transformMatrix.preConcat(childMatrix);
                    }
                } else {
                    transformMatrix = childMatrix;
                }
            } else {
                transformMatrix = childMatrix;
            }
            transformMatrix.mapRect(boundingRect);
            dirty.set((int) Math.floor(boundingRect.left),
                    (int) Math.floor(boundingRect.top),
                    (int) Math.ceil(boundingRect.right),
                    (int) Math.ceil(boundingRect.bottom));
        }

        do {
            View view = null;
            if (parent instanceof View) {
                view = (View) parent;
            }

            if (drawAnimation) {
                if (view != null) {
                    view.mPrivateFlags |= PFLAG_DRAW_ANIMATION;
                } else if (parent instanceof ViewRootImpl) {
                    ((ViewRootImpl) parent).mIsAnimating = true;
                }
            }

            // If the parent is dirty opaque or not dirty, mark it dirty with the opaque
            // flag coming from the child that initiated the invalidate
            if (view != null) {
                if ((view.mPrivateFlags & PFLAG_DIRTY_MASK) != PFLAG_DIRTY) {
                    view.mPrivateFlags = (view.mPrivateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DIRTY;
                }
            }

            parent = parent.invalidateChildInParent(location, dirty);
            if (view != null) {
                // Account for transform on current parent
                Matrix m = view.getMatrix();
                if (!m.isIdentity()) {
                    RectF boundingRect = attachInfo.mTmpTransformRect;
                    boundingRect.set(dirty);
                    m.mapRect(boundingRect);
                    dirty.set((int) Math.floor(boundingRect.left),
                            (int) Math.floor(boundingRect.top),
                            (int) Math.ceil(boundingRect.right),
                            (int) Math.ceil(boundingRect.bottom));
                }
            }
        } while (parent != null);
    }
}

其中核心部分是

do{
    ......
    parent = parent.invalidateChildInParent(location, dirty);
    if (view != null) {
        // Account for transform on current parent
        Matrix m = view.getMatrix();
        if (!m.isIdentity()) {
            RectF boundingRect = attachInfo.mTmpTransformRect;
            boundingRect.set(dirty);
            m.mapRect(boundingRect);
            dirty.set((int) Math.floor(boundingRect.left),
                    (int) Math.floor(boundingRect.top),
                    (int) Math.ceil(boundingRect.right),
                    (int) Math.ceil(boundingRect.bottom));
        }
    }
} while (parent != null);

通过do-while循环，不断调用invalidateChildInParent方法，并返回一个parent。在ViewGroup方法中，该方法只是为了矫正参数。直到循环到最外层的ViewRootImpl对象。

    @Override
    public void invalidateChild(View child, Rect dirty) {
        invalidateChildInParent(null, dirty);
    }

    @Override
    public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
//只允许主线程更新UI
        checkThread();
        if (DEBUG_DRAW) Log.v(TAG, "Invalidate child: " + dirty);

        if (dirty == null) {
            invalidate();
            return null;
        } else if (dirty.isEmpty() && !mIsAnimating) {
            return null;
        }

        if (mCurScrollY != 0 || mTranslator != null) {
            mTempRect.set(dirty);
            dirty = mTempRect;
            if (mCurScrollY != 0) {
                dirty.offset(0, -mCurScrollY);
            }
            if (mTranslator != null) {
                mTranslator.translateRectInAppWindowToScreen(dirty);
            }
            if (mAttachInfo.mScalingRequired) {
                dirty.inset(-1, -1);
            }
        }

        final Rect localDirty = mDirty;
        if (!localDirty.isEmpty() && !localDirty.contains(dirty)) {
            mAttachInfo.mSetIgnoreDirtyState = true;
            mAttachInfo.mIgnoreDirtyState = true;
        }

        // Add the new dirty rect to the current one
        localDirty.union(dirty.left, dirty.top, dirty.right, dirty.bottom);
        // Intersect with the bounds of the window to skip
        // updates that lie outside of the visible region
        final float appScale = mAttachInfo.mApplicationScale;
        final boolean intersected = localDirty.intersect(0, 0,
                (int) (mWidth * appScale + 0.5f), (int) (mHeight * appScale + 0.5f));
        if (!intersected) {
            localDirty.setEmpty();
        }
        if (!mWillDrawSoon && (intersected || mIsAnimating)) {
            scheduleTraversals();
        }

        return null;
    }

这里，当dirty是空，即不指定刷新区域，就直接刷新这个界面，否则，将dirty并入localDirty

   void invalidate() {
       mDirty.set(0, 0, mWidth, mHeight);
       if (!mWillDrawSoon) {
           scheduleTraversals();
       }
   }

   void scheduleTraversals() {
       if (!mTraversalScheduled) {
           mTraversalScheduled = true;
           mTraversalBarrier = mHandler.getLooper().postSyncBarrier();
           mChoreographer.postCallback(
                   Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
           if (!mUnbufferedInputDispatch) {
               scheduleConsumeBatchedInput();
           }
           notifyRendererOfFramePending();
       }
   }

   final TraversalRunnable mTraversalRunnable = new TraversalRunnable();

   final class TraversalRunnable implements Runnable {
       @Override
       public void run() {
           doTraversal();
       }
   }

   void doTraversal() {
       if (mTraversalScheduled) {
           mTraversalScheduled = false;
           mHandler.getLooper().removeSyncBarrier(mTraversalBarrier);

           if (mProfile) {
               Debug.startMethodTracing("ViewAncestor");
           }

           Trace.traceBegin(Trace.TRACE_TAG_VIEW, "performTraversals");
           try {
               performTraversals();
           } finally {
               Trace.traceEnd(Trace.TRACE_TAG_VIEW);
           }

           if (mProfile) {
               Debug.stopMethodTracing();
               mProfile = false;
           }
       }

自此，调用performTraversals()方法，开始重绘UI。