Android View 繪制刷新流程分析

Android中對View的更新有很多種方式，使用時要區分不同的應用場合。
1.不使用多線程和雙緩沖
     這種情況最簡單，一般只是希望在View發生改變時對UI進行重繪。你只需顯式地調用View對象中的invalidate(){關於invalidate的解釋:當調用線程處於空閒狀態時，會調用onDraw，刷新界面，也就是說，該函數僅是標記當前界面過期，並不直接負責刷新界面；}方法即可。系統會自動調用View的onDraw()方法。
2.使用多線程但不使用雙緩沖
     這種情況需要開啟新的線程，新開的線程就不好訪問View對象了。強行訪問的話會報：android.view.ViewRoot$CalledFromWrongThreadException：Only the original thread that created a view hierarchy can touch its views.
     這時候你需要創建一個繼承了android.os.Handler的子類，並重寫handleMessage(Message msg)方法。android.os.Handler是能發送和處理消息的，你需要在Activity中發出更新UI的消息，然後在Handler（可以使用匿名內部類）中處理消息（因為匿名內部類可以訪問父類變量， 你可以直接調用View對象中的invalidate()方法 ）。也就是說：在新線程創建並發送一個Message，然後再主線程中捕獲、處理該消息。
3.使用多線程和雙緩沖
    Android中SurfaceView是View的子類，她同時也實現了雙緩沖。可以定義一個她的子類並實現SurfaceHolder.Callback接口。由於實現SurfaceHolder.Callback接口，新線程就不需要android.os.Handler幫忙了。SurfaceHolder中lockCanvas()方法可以鎖定畫布，繪制完新的圖像後調用unlockCanvasAndPost(canvas)解鎖（顯示）
先看看源代碼對SurfaceHolder接口的描述

/**
 * 允許你控制surface view的大小、樣式，編輯像素或監視surface的改變,典型的運用於SurfaceView中，需要注意 * lockCanvas方法和Callback.surfaceCreated方法*/再看SurfaceHolder.Callback的描述

    /**
     * A client may implement this interface to receive information about
     * changes to the surface.  When used with a {@link SurfaceView}, the
     * Surface being held is only available between calls to
     * {@link #surfaceCreated(SurfaceHolder)} and
     * {@link #surfaceDestroyed(SurfaceHolder)}.  The Callback is set with
     * {@link SurfaceHolder#addCallback SurfaceHolder.addCallback} method.
     */下面是一個繼承自SurfaceView並實現SurfaceHolder.Callback接口的類

public class MySurfaceView extends SurfaceView implements SurfaceHolder.Callback {
    private SurfaceHolder holder;

    public MySurfaceView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    public MySurfaceView(Context context) {
        super(context);
        holder = this.getHolder();
        holder.addCallback(this);
        this.setLongClickable(true);// 不設置將無法捕捉onFling()事件
        setFocusable(true);// 設置鍵盤焦點
        setFocusableInTouchMode(true);// 設置觸摸屏焦點
    }

    protected void paintView(Canvas canvas) { // 自定義方法，類似於onDraw
    }public void rePaint() { // 自定義類似於invalidate方法，調用此方法刷新View
        Canvas c = null;
        try {
            c = holder.lockCanvas();
            paintView(c);
        } finally {
            if (c != null) {
                holder.unlockCanvasAndPost(c);
            }
        }
    }

    @Override
    public void surfaceCreated(SurfaceHolder holder) {
        Canvas canvas = holder.lockCanvas(null);// 獲取畫布
        canvas.drawColor(Color.WHITE);// 設置畫布背景
        holder.unlockCanvasAndPost(canvas);// 解鎖畫布，提交畫好的圖像
    }

    @Override
    public void surfaceChanged(SurfaceHolder holder, int format, int width,
            int height) {
    }

    @Override
    public void surfaceDestroyed(SurfaceHolder holder) {
    }
}------------------------------------------------------View的繪制流程-----------------------------------------------------

View的繪制繪制流程:父View負責刷新、布局、顯示子View；而當子View需要刷新時，則是通知父View來完成。下面通過查看原代碼來驗證
1.子類調用invalidate方法()

    /**
     * 使當前View無效. 如果View可見，onDraw方法將會在之後某個時間點被調用，這個方法的調用必須在UI線程中，如果在非UI線程中調用需要使用postInvalidate()方法*/
    public void invalidate() {
        invalidate(true);
    }

    /**
     * invalidate實際上是調用這個方法.drawing的緩存被設置為無效之後一個完整的invalidate將會發生.但是這個功能可以通過設置invalidateCachefalse來跳過無效的步驟當並不需要重新繪制View的時候(例如，一個組件保持著同樣的尺寸和內容)
     * @param invalidateCache 這個View的緩存是否應該被設置為無效,通常是false表示要進行全部繪制,但是可能設置為true當View的Content和dimension都沒有改變時.
     */
    void invalidate(boolean invalidateCache) {
        if (skipInvalidate()) {
            return;
        }
        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 || isOpaque() != mLastIsOpaque) {
            // ......final AttachInfo ai = mAttachInfo; // 獲取匹配
            final ViewParent p = mParent; // 獲取父類對象
            // noinspection PointlessBooleanExpression,ConstantConditions
            if (!HardwareRenderer.RENDER_DIRTY_REGIONS) {
                if (p != null && ai != null && ai.mHardwareAccelerated) {
                    p.invalidateChild(this, null);
                    return;
                }
            }

            if (p != null && ai != null) {
                final Rect r = ai.mTmpInvalRect;
                r.set(0, 0, mRight - mLeft, mBottom - mTop); // 設置View的尺寸
                p.invalidateChild(this, r); // 調用parent對象讓parent對象重繪制child
            }
        }
    }>>2.child View調用invalidate時，首先找到自己父View(View的成員變量mParent記錄自己的父View)，然後將AttachInfo中保存的信息告訴父View刷新自己，父View調用invalidateChild函數刷新child View
下面查看ViewGroup中的invalidateChild方法的實現

    /**
     * 不要調用或重寫此方法,這個方法是用於實現View的繪制層次
     */
    public final void invalidateChild(View child, final Rect dirty) {
        ViewParent parent = this;

        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            // 如果child view繪制的是動畫，我們希望child的mPrivateFlags拷貝到ViewGroup之上
            // 並且讓parent確保無效的請求通過
            final boolean drawAnimation = (child.mPrivateFlags & PFLAG_DRAW_ANIMATION)
                    == PFLAG_DRAW_ANIMATION;

            // ...final int[] location = attachInfo.mInvalidateChildLocation;
            location[CHILD_LEFT_INDEX] = child.mLeft;
            location[CHILD_TOP_INDEX] = child.mTop;
            // ...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.mViewFlags & FADING_EDGE_MASK) != 0 &&
                            view.getSolidColor() == 0) {
                        opaqueFlag = PFLAG_DIRTY;
                    }
                    if ((view.mPrivateFlags & PFLAG_DIRTY_MASK) != PFLAG_DIRTY) {
                        view.mPrivateFlags = (view.mPrivateFlags & ~PFLAG_DIRTY_MASK) | opaqueFlag;
                    }
                }

                parent = parent.invalidateChildInParent(location, dirty); // 轉到第三步，調用此方法層層刷新View
                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 - 0.5f),
                                (int) (boundingRect.top - 0.5f),
                                (int) (boundingRect.right + 0.5f),
                                (int) (boundingRect.bottom + 0.5f));
                    }
                }
            } while (parent != null);
        }
    }3>>.調用invalidateChildInParent函數依次層層刷新

    /**
     * 這個方法返回null如果ViewGroup已經沒有父View了，
     * 或者如果這個ViewGrop已經全部被設置為無效，或者當前View的需要刷新的rectangle區域與ViewGroup不相交
     */
    public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
        if ((mPrivateFlags & PFLAG_DRAWN) == PFLAG_DRAWN ||
                (mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == PFLAG_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); // 根據父View的位置，偏移刷新區域

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

                if ((mGroupFlags & FLAG_CLIP_CHILDREN) == FLAG_CLIP_CHILDREN) {
                    if (!dirty.intersect(0, 0, mRight - left, mBottom - top)) { // 計算實際可刷新區域
                        dirty.setEmpty();
                    }
                }
                mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;

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

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

                return mParent;

            } else {
                mPrivateFlags &= ~PFLAG_DRAWN & ~PFLAG_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) {
                    mPrivateFlags |= PFLAG_INVALIDATED;
                    mLocalDirtyRect.union(dirty);
                }
                return mParent;
            }
        }
        return null;
    }