View测量、布局及绘制原理

View绘制的流程

View的绘制是从上往下一层层迭代下来的。
DecorView -> ViewGroup (-->ViewGroup) ->View。还记得我是事件分发的总结中说过，布局更像一个盒子结构设计，所以外层盒子都没绘制，怎么放里面的东西呢

Measure流程

如果有了widthMeasureSpec, heightMeasureSpec，通过一定的处理(可以重写，自定义处理步骤)，从中获取View的宽/高，调用setMeasuredDimension()方法，指定View的宽高，完成测量工作

1、日常使用代码示例：

    @Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {

    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    
    
    // 获取宽-测量规则的模式和大小
    int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    int widthSize = MeasureSpec.getSize(widthMeasureSpec);
    
    // 获取高-测量规则的模式和大小
    int heightMode = MeasureSpec.getMode(heightMeasureSpec);
    int heightSize = MeasureSpec.getSize(heightMeasureSpec);
    
    // 设置wrap_content的默认宽 / 高值
    // 默认宽/高的设定并无固定依据,根据需要灵活设置
    // 类似TextView,ImageView等针对wrap_content均在onMeasure()对设置默认宽 / 高值有特殊处理,具体读者可以自行查看
    int mWidth = 400;
    int mHeight = 400;
    
      // 当布局参数设置为wrap_content时，设置默认值
    if (getLayoutParams().width == ViewGroup.LayoutParams.WRAP_CONTENT && getLayoutParams().height == ViewGroup.LayoutParams.WRAP_CONTENT) {
        setMeasuredDimension(mWidth, mHeight);
    // 宽 / 高任意一个布局参数为= wrap_content时，都设置默认值
    } else if (getLayoutParams().width == ViewGroup.LayoutParams.WRAP_CONTENT) {
        setMeasuredDimension(mWidth, heightSize);
    } else if (getLayoutParams().height == ViewGroup.LayoutParams.WRAP_CONTENT) {
        setMeasuredDimension(widthSize, mHeight);
}

2、MeasureSpec是如何确定的

• 对于DecorView,其确定是通过屏幕的大小，和自身的布局参数LayoutParams
• 对于其他View(包括ViewGroup),其确定是通过父布局的MeasureSpec和自身的布局参数LayoutParams（在这你就可以想想布局中的match_parent和wrap_content设置了，是不是你想要的效果）

Layout流程

测量完View大小后，就需要将View布局在Window中，View的布局主要通过确定上下左右四个点来确定的。

其中布局也是自上而下，不同的是ViewGroup先在layout()中确定自己的布局，然后在onLayout()方法中再调用子View的layout()方法，让子View布局。在Measure过程中，ViewGroup一般是先测量子View的大小，然后再确定自身的大小。

ViewGroup中有用，View中不用复写

日常示例使用代码：

 @Override
protected void onLayout(boolean changed, int l, int t, int r, int b) {
    int mPaddingLeft = getPaddingLeft();
    int mPaddingRight = getPaddingRight();
    int mPaddingTop = getPaddingTop();

    int lineX = mPaddingLeft;
    int lineY = mPaddingTop;
    int lineWidth = r - l;
    usefulWidth = lineWidth - mPaddingLeft - mPaddingRight;
    int lineUsed = mPaddingLeft + mPaddingRight;
    int lineHeight = 0;
    int lineNum = 0;

    lineNumList.clear();
    for (int i = 0; i < this.getChildCount(); i++) {
        View child = this.getChildAt(i);
        if (child.getVisibility() == GONE) {
            continue;
        }
        int spaceWidth = 0;
        int spaceHeight = 0;
        int left = 0;
        int top = 0;
        int right = 0;
        int bottom = 0;
        int childWidth = child.getMeasuredWidth();
        int childHeight = child.getMeasuredHeight();

        LayoutParams childLp = child.getLayoutParams();
        if (childLp instanceof MarginLayoutParams) {
            MarginLayoutParams mlp = (MarginLayoutParams) childLp;
            spaceWidth = mlp.leftMargin + mlp.rightMargin;
            spaceHeight = mlp.topMargin + mlp.bottomMargin;
            left = lineX + mlp.leftMargin;
            top = lineY + mlp.topMargin;
            right = lineX + mlp.leftMargin + childWidth;
            bottom = lineY + mlp.topMargin + childHeight;
        } else {
            left = lineX;
            top = lineY;
            right = lineX + childWidth;
            bottom = lineY + childHeight;
        }
        spaceWidth += childWidth;
        spaceHeight += childHeight;

        if (lineUsed + spaceWidth > lineWidth) {
            //approach the limit of width and move to next line
            lineNumList.add(lineNum);
            lineY += lineHeight + lineSpacing;
            lineUsed = mPaddingLeft + mPaddingRight;
            lineX = mPaddingLeft;
            lineHeight = 0;
            lineNum = 0;
            if (childLp instanceof MarginLayoutParams) {
                MarginLayoutParams mlp = (MarginLayoutParams) childLp;
                left = lineX + mlp.leftMargin;
                top = lineY + mlp.topMargin;
                right = lineX + mlp.leftMargin + childWidth;
                bottom = lineY + mlp.topMargin + childHeight;
            } else {
                left = lineX;
                top = lineY;
                right = lineX + childWidth;
                bottom = lineY + childHeight;
            }
        }
        child.layout(left, top, right, bottom);
        lineNum ++;
        if (spaceHeight > lineHeight) {
            lineHeight = spaceHeight;
        }
        lineUsed += spaceWidth;
        lineX += spaceWidth;
    }
    // add the num of last line
    lineNumList.add(lineNum);
}

可能这个示例代码不是很具有代表性，毕竟需要重写onLayout的操作还是少，关键是遍历View child = this.getChildAt(i) ，child.layout(left, top, right, bottom);

Draw过程

无论单一View,或者ViewGroup都需要实现该方法，各凭本事了