優化ViewPager 左右滑動卡頓問題

優化ViewPager 左右滑動卡頓問題

在項目中，使用到ViewPager控件，突然發現左右滑動到某個頁面出現卡頓現象，
ViewPager 已使用緩存策略，切換卡片也不會重新刷新界面，應該不是主動刷新View引起，查看日誌打印也印證了這點，沒辦法只能硬着頭皮查找View刷新邏輯，發現ViewPager 在左右滑動會輪尋計算子View，會做計算和比較，
可能是這裏耗時，回過頭再對比同一個頁面顯示不同的View，卡頓效果不同，只有某個ViewGroup 顯示卡頓明顯，細看xml發現該ViewGroup嵌套較深，重新修改嵌套邏輯再次測試發現有效果，就這樣重新提交代碼。

突然領導半夜發微信，說卡頓現象任然存在，心想不可能啊，自己都已經使用真機測試，優化效果很明顯，那爲什麼領導還會覺得卡頓呢。也只能帶着疑問睡覺了。

第二天，使用復現問題機器對照測試發現卡頓真的還存在，只是沒有先前那麼明顯，回來細看代碼發現沒有問題，整個佈局嵌套也已優化到兩層，按理說不會存在卡頓現象，沒辦法，只好再次查看Android源碼View刷新流程，希望能找到點蛛絲馬跡，果不其然，在一個刷新方法裏面注意到了這一個邏輯

 /**
     * Manually render this view (and all of its children) to the given Canvas.
     * The view must have already done a full layout before this function is
     * called.  When implementing a view, implement
     * {@link #onDraw(android.graphics.Canvas)} instead of overriding this method.
     * If you do need to override this method, call the superclass version.
     *
     * @param canvas The Canvas to which the View is rendered.
     */
    @CallSuper
    public void draw(Canvas canvas) {
        final int privateFlags = mPrivateFlags;
        final boolean dirtyOpaque = (privateFlags & PFLAG_DIRTY_MASK) == PFLAG_DIRTY_OPAQUE &&
                (mAttachInfo == null || !mAttachInfo.mIgnoreDirtyState);
        mPrivateFlags = (privateFlags & ~PFLAG_DIRTY_MASK) | PFLAG_DRAWN;

        /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

        // Step 1, draw the background, if needed
        int saveCount;

        if (!dirtyOpaque) {
            drawBackground(canvas);
        }

        // skip step 2 & 5 if possible (common case)
        final int viewFlags = mViewFlags;
        boolean horizontalEdges = (viewFlags & FADING_EDGE_HORIZONTAL) != 0;
        boolean verticalEdges = (viewFlags & FADING_EDGE_VERTICAL) != 0;
        if (!verticalEdges && !horizontalEdges) {
            // Step 3, draw the content
            if (!dirtyOpaque) onDraw(canvas);

            // Step 4, draw the children
            dispatchDraw(canvas);

            drawAutofilledHighlight(canvas);

            // Overlay is part of the content and draws beneath Foreground
            if (mOverlay != null && !mOverlay.isEmpty()) {
                mOverlay.getOverlayView().dispatchDraw(canvas);
            }

            // Step 6, draw decorations (foreground, scrollbars)
            onDrawForeground(canvas);

            // Step 7, draw the default focus highlight
            drawDefaultFocusHighlight(canvas);

            if (debugDraw()) {
                debugDrawFocus(canvas);
            }

            // we're done...
            return;
        }
       ..................................
       ..................................
    }

 /*
         * Draw traversal performs several drawing steps which must be executed
         * in the appropriate order:
         *
         *      1. Draw the background
         *      2. If necessary, save the canvas' layers to prepare for fading
         *      3. Draw view's content
         *      4. Draw children
         *      5. If necessary, draw the fading edges and restore layers
         *      6. Draw decorations (scrollbars for instance)
         */

分析draw 方法，發現繪製的第一步就是繪製背景，可能是這裏耗時，經過查看xml發現該佈局View背景都有設置，那我是不是可以在這個點優化一下呢，於是把View 背景設置去掉，去掉冗餘大背景，修改ImageView 背景改成src 設置，儘快修改，測試發現去掉xml的背景，滑動頁面不卡頓了。

切記，xml佈局嵌套不要太多，同一個佈局，子View 背景不要設置太多