视图绘制回顾

测量：确定大小

布局：确定位置

绘制：绘制视图

性能瓶颈

每个阶段耗时

自顶而下的遍历

触发多次

布局层级及复杂度

准则

减少View树层级

宽而浅，避免窄而深

不嵌套使用RelativeLayout

不在嵌套LinearLayout中使用weight

merge标签：减少一个层级，只能用于根View

ConstraintLayout

实现几乎完全扁平化布局

构建复杂布局性能更高

具有RelativeLayout和LinearLayout特性

过度绘制

一个像素最好只被绘制一次

调试GPU过度绘制

蓝色可接受

避免过度绘制方法

去掉多余背景色，减少复杂shape使用

避免层级叠加

自定义View使用clipRect屏蔽被遮盖View绘制

protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);

        // Don't draw anything until all the Asynctasks are done and all the DroidCards are ready.
        if (mDroids.length > 0 && mDroidCards.size() == mDroids.length) {
            // Loop over all the droids, except the last one.
            int i;
            for (i = 0; i < mDroidCards.size() - 1; i++) {

                mCardLeft = i * mCardSpacing;
                canvas.save();
                // 指定绘制区域

                canvas.clipRect(mCardLeft,0,mCardLeft+mCardSpacing,mDroidCards.get(i).getHeight());

                // Draw the card. Only the parts of the card that lie within the bounds defined by
                // the clipRect() get drawn.
                drawDroidCard(canvas, mDroidCards.get(i), mCardLeft, 0);

                canvas.restore();
            }

            // Draw the final card. This one doesn't get clipped.
            drawDroidCard(canvas, mDroidCards.get(mDroidCards.size() - 1),
                    mCardLeft + mCardSpacing, 0);
        }

        // Invalidate the whole view. Doing this calls onDraw() if the view is visible.
        invalidate();
    }

其他技巧

Viewstub:高效占位符、延迟初始化

onDraw中避免：创建大对象、耗时操作

TextView优化