Android中invalidate过程详解

04 July 2012

invalidate()函数的主要作用是请求View树进行重绘，该函数可以由应用程序调用，或者由系统函数间接调用，例如setEnable(), setSelected(), setVisiblity()都会间接调用到invalidate()来请求View树重绘，更新View树的显示。

注：requestLayout()和requestFocus()函数也会引起视图重绘

下面我们通过源码来了解invalidate()函数的工作原理，首先我们来看View类中invalidate()的实现过程：

/**
 * Invalidate the whole view. If the view is visible,
 * {@link #onDraw(android.graphics.Canvas)} will be called at some point in
 * the future. This must be called from a UI thread. To call from a non-UI thread,
 * call {@link #postInvalidate()}.
 */
public void invalidate() {
    invalidate(true); 
}

invalidate()函数会转而调用invalidate(true)，继续往下看：

/**
 * This is where the invalidate() work actually happens. A full invalidate()
 * causes the drawing cache to be invalidated, but this function can be called with
 * invalidateCache set to false to skip that invalidation step for cases that do not
 * need it (for example, a component that remains at the same dimensions with the same
 * content).
 *
 * @param invalidateCache Whether the drawing cache for this view should be invalidated as
 * well. This is usually true for a full invalidate, but may be set to false if the
 * View's contents or dimensions have not changed.
 */
void invalidate(boolean invalidateCache) {
    if (ViewDebug.TRACE_HIERARCHY) {
        ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE);
    }

    if (skipInvalidate()) {
        return;
    }
    if ((mPrivateFlags & (DRAWN | HAS_BOUNDS)) == (DRAWN | HAS_BOUNDS) ||
            (invalidateCache && (mPrivateFlags & DRAWING_CACHE_VALID) == DRAWING_CACHE_VALID) ||
            (mPrivateFlags & INVALIDATED) != INVALIDATED || isOpaque() != mLastIsOpaque) {
        mLastIsOpaque = isOpaque();
        mPrivateFlags &= ~DRAWN;
        mPrivateFlags |= DIRTY;
        if (invalidateCache) {
            mPrivateFlags |= INVALIDATED;
            mPrivateFlags &= ~DRAWING_CACHE_VALID;
        }
        final AttachInfo ai = mAttachInfo;
        final ViewParent p = mParent;
        //noinspection PointlessBooleanExpression,ConstantConditions
        if (!HardwareRenderer.RENDER_DIRTY_REGIONS) {
            if (p != null && ai != null && ai.mHardwareAccelerated) {
                // fast-track for GL-enabled applications; just invalidate the whole hierarchy
                // with a null dirty rect, which tells the ViewAncestor to redraw everything
                p.invalidateChild(this, null);
                return;
            }
        }

        if (p != null && ai != null) {
            final Rect r = ai.mTmpInvalRect;
            r.set(0, 0, mRight - mLeft, mBottom - mTop);
            // Don't call invalidate -- we don't want to internally scroll
            // our own bounds
            p.invalidateChild(this, r);
        }
    }
}

下面我们来具体进行分析invalidate(true)函数的执行流程：

首先调用skipInvalidate()，该函数主要判断该View是否不需要重绘，如果不许要重绘则直接返回，不需要重绘的条件是该View不可见并且未进行动画
接下来的if语句是来进一步判断View是否需要绘制，其中表达式(mPrivateFlags & (DRAWN | HAS_BOUNDS)) == (DRAWN | HAS_BOUNDS)的意思指的是如果View需要重绘并且其大小不为0，其余几个本人也未完全理解，还望高手指点～～如果需要重绘，则处理相关标志位
对于开启硬件加速的应用程序，则调用父视图的invalidateChild函数绘制整个区域，否则只绘制dirty区域（r变量所指的区域），这是一个向上回溯的过程，每一层的父View都将自己的显示区域与传入的刷新Rect做交集。

接下来看invalidateChild()的实现过程：

public final void invalidateChild(View child, final Rect dirty) {
    if (ViewDebug.TRACE_HIERARCHY) {
        ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE_CHILD);
    }

    ViewParent parent = this;

    final AttachInfo attachInfo = mAttachInfo;
    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 & DRAW_ANIMATION) == DRAW_ANIMATION;

        if (dirty == null) {
            if (child.mLayerType != LAYER_TYPE_NONE) {
                mPrivateFlags |= INVALIDATED;
                mPrivateFlags &= ~DRAWING_CACHE_VALID;
                child.mLocalDirtyRect.setEmpty();
            }
            do {
                View view = null;
                if (parent instanceof View) {
                    view = (View) parent;
                    if (view.mLayerType != LAYER_TYPE_NONE) {
                        view.mLocalDirtyRect.setEmpty();
                        if (view.getParent() instanceof View) {
                            final View grandParent = (View) view.getParent();
                            grandParent.mPrivateFlags |= INVALIDATED;
                            grandParent.mPrivateFlags &= ~DRAWING_CACHE_VALID;
                        }
                    }
                    if ((view.mPrivateFlags & DIRTY_MASK) != 0) {
                        // already marked dirty - we're done
                        break;
                    }
                }

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

                if (parent instanceof ViewRootImpl) {
                    ((ViewRootImpl) parent).invalidate();
                    parent = null;
                } else if (view != null) {
                    if ((view.mPrivateFlags & DRAWN) == DRAWN ||
                            (view.mPrivateFlags & DRAWING_CACHE_VALID) == DRAWING_CACHE_VALID) {
                        view.mPrivateFlags &= ~DRAWING_CACHE_VALID;
                        view.mPrivateFlags |= DIRTY;
                        parent = view.mParent;
                    } else {
                        parent = null;
                    }
                }
            } while (parent != null);
        } else {
            // Check whether the child that requests the invalidate is fully opaque
            final boolean isOpaque = child.isOpaque() && !drawAnimation &&
                    child.getAnimation() == null;
            // Mark the child as dirty, using the appropriate flag
            // Make sure we do not set both flags at the same time
            int opaqueFlag = isOpaque ? DIRTY_OPAQUE : DIRTY;

            if (child.mLayerType != LAYER_TYPE_NONE) {
                mPrivateFlags |= INVALIDATED;
                mPrivateFlags &= ~DRAWING_CACHE_VALID;
                child.mLocalDirtyRect.union(dirty);
            }

            final int[] location = attachInfo.mInvalidateChildLocation;
            location[CHILD_LEFT_INDEX] = child.mLeft;
            location[CHILD_TOP_INDEX] = child.mTop;
            Matrix childMatrix = child.getMatrix();
            if (!childMatrix.isIdentity()) {
                RectF boundingRect = attachInfo.mTmpTransformRect;
                boundingRect.set(dirty);
                //boundingRect.inset(-0.5f, -0.5f);
                childMatrix.mapRect(boundingRect);
                dirty.set((int) (boundingRect.left - 0.5f),
                        (int) (boundingRect.top - 0.5f),
                        (int) (boundingRect.right + 0.5f),
                        (int) (boundingRect.bottom + 0.5f));
            }

            do {
                View view = null;
                if (parent instanceof View) {
                    view = (View) parent;
                    if (view.mLayerType != LAYER_TYPE_NONE &&
                            view.getParent() instanceof View) {
                        final View grandParent = (View) view.getParent();
                        grandParent.mPrivateFlags |= INVALIDATED;
                        grandParent.mPrivateFlags &= ~DRAWING_CACHE_VALID;
                    }
                }

                if (drawAnimation) {
                    if (view != null) {
                        view.mPrivateFlags |= 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.mViewFlags & FADING_EDGE_MASK) != 0 &&
                            view.getSolidColor() == 0) {
                        opaqueFlag = DIRTY;
                    }
                    if ((view.mPrivateFlags & DIRTY_MASK) != DIRTY) {
                        view.mPrivateFlags = (view.mPrivateFlags & ~DIRTY_MASK) | opaqueFlag;
                    }
                }

                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) boundingRect.left, (int) boundingRect.top,
                                (int) (boundingRect.right + 0.5f),
                                (int) (boundingRect.bottom + 0.5f));
                    }
                }
            } while (parent != null);
        }
    }
}

大概流程如下，我们主要关注dirty区域不是null（非硬件加速）的情况：

判断子视图是否是不透明的（不透明的条件是isOpaque()返回true，视图未进行动画以及child.getAnimation() == null），并将判断结果保存到变量isOpaque中，如果不透明则将变量opaqueFlag设置为DIRTY_OPAQUE，否则设置为DIRTY。
定义location保存子视图的左上角坐标
如果子视图正在动画，那么父视图也要添加动画标志，如果父视图是ViewGroup，那么给mPrivateFlags添加DRAW_ANIMATION标识，如果父视图是ViewRoot，则给其内部变量mIsAnimating赋值为true
设置dirty标识，如果子视图是不透明的，则父视图设置为DIRTY_OPAQUE，否则设置为DIRTY
调用parent.invalidateChildInparent()，这里的parent有可能是ViewGroup，也有可能是ViewRoot（最后一次while循环），首先来看ViewGroup, ViewGroup中该函数的主要作用是对dirty区域进行计算

以上过程的主体是一个do{}while{}循环，不断的将子视图的dirty区域与父视图做运算来确定最终要重绘的dirty区域，最终循环到ViewRoot（ViewRoot的parent为null）为止，并将dirty区域保存到ViewRoot的mDirty变量中

/**
 * Don't call or override this method. It is used for the implementation of
 * the view hierarchy.
 *
 * This implementation returns null if this ViewGroup does not have a parent,
 * if this ViewGroup is already fully invalidated or if the dirty rectangle
 * does not intersect with this ViewGroup's bounds.
 */
public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
    if (ViewDebug.TRACE_HIERARCHY) {
        ViewDebug.trace(this, ViewDebug.HierarchyTraceType.INVALIDATE_CHILD_IN_PARENT);
    }

    if ((mPrivateFlags & DRAWN) == DRAWN ||
            (mPrivateFlags & DRAWING_CACHE_VALID) == DRAWING_CACHE_VALID) {
        if ((mGroupFlags & (FLAG_OPTIMIZE_INVALIDATE | FLAG_ANIMATION_DONE)) !=
                    FLAG_OPTIMIZE_INVALIDATE) {
            dirty.offset(location[CHILD_LEFT_INDEX] - mScrollX,
                    location[CHILD_TOP_INDEX] - mScrollY);

            final int left = mLeft;
            final int top = mTop;

            if ((mGroupFlags & FLAG_CLIP_CHILDREN) != FLAG_CLIP_CHILDREN ||
                    dirty.intersect(0, 0, mRight - left, mBottom - top) ||
                    (mPrivateFlags & DRAW_ANIMATION) == DRAW_ANIMATION) {
                mPrivateFlags &= ~DRAWING_CACHE_VALID;

                location[CHILD_LEFT_INDEX] = left;
                location[CHILD_TOP_INDEX] = top;

                if (mLayerType != LAYER_TYPE_NONE) {
                    mLocalDirtyRect.union(dirty);
                }

                return mParent;
            }
        } else {
            mPrivateFlags &= ~DRAWN & ~DRAWING_CACHE_VALID;

            location[CHILD_LEFT_INDEX] = mLeft;
            location[CHILD_TOP_INDEX] = mTop;
            if ((mGroupFlags & FLAG_CLIP_CHILDREN) == FLAG_CLIP_CHILDREN) {
                dirty.set(0, 0, mRight - mLeft, mBottom - mTop);
            } else {
                // in case the dirty rect extends outside the bounds of this container
                dirty.union(0, 0, mRight - mLeft, mBottom - mTop);
            }

            if (mLayerType != LAYER_TYPE_NONE) {
                mLocalDirtyRect.union(dirty);
            }

            return mParent;
        }
    }

    return null;
}

该函数首先调用offset将子视图的坐标位置转换为在父视图（当前视图）的显示位置，这里主要考虑scroll后导致子视图在父视图中的显示区域会发生变化，接着调用union函数求得当前视图与子视图的交集,求得的交集必定是小于dirty的范围，因为子视图的dirty区域有可能超出其父视图（当前视图）的范围，最后返回当前视图的父视图。

再来看ViewRoot中invalidateChildInparent的执行过程：

public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
        invalidateChild(null, dirty);
        return null;
}

该函数仅仅调用了ViewRoot的invalidateChild，下面继续看invalidateChild的源码：

public void invalidateChild(View child, Rect dirty) {
    checkThread();
    if (DEBUG_DRAW) Log.v(TAG, "Invalidate child: " + dirty);
    if (dirty == null) {
        // Fast invalidation for GL-enabled applications; GL must redraw everything
        invalidate();
        return;
    }
    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);
        }
    }
    if (!mDirty.isEmpty() && !mDirty.contains(dirty)) {
        mAttachInfo.mSetIgnoreDirtyState = true;
        mAttachInfo.mIgnoreDirtyState = true;
    }
    mDirty.union(dirty);
    if (!mWillDrawSoon) {
        scheduleTraversals();
    }
}

具体分析如下：

判断此次调用是否在UI线程中进行
将dirty的坐标位置转换为ViewRoot的屏幕显示区域
更新mDirty变量，并调用scheduleTraversals发起重绘请求

至此一次invalidate()就结束了。

总结

invalidate主要给需要重绘的视图添加DIRTY标记，并通过和父视图的矩形运算求得真正需要绘制的区域，并保存在ViewRoot中的mDirty变量中，最后调用scheduleTraversals发起重绘请求，scheduleTraversals会发送一个异步消息，最终调用performTraversals()执行重绘，performTraversals()的具体过程以后再分析。

以上所有代码基于Android 4.0.4，并结合《Android内核剖析》分析总结而成，源码中涉及到的部分细节本人也未完全理解，还望高手指点~~