ViewGroup

生命周期

每次onMeasure，onLayout用到的值都不能在构造方法里面初始化。

座标系

2021-12-05_4.02.40_coordinate

getLeft()直接调用是0 ? 需要自己设置 chidView.layout(left, top,right,bottom),

没调用chidView.layout 就获取不到 getLeft()的值。

left这些参数的座标系 是基于ViewGroup.

getMeasuredWidth getWidth

getMeasuredWidth

  1. 在measure()过程结束后就可以获取到对应的值。

  2. 通过setMeasuredDimension()方法来进行设置。

getWidth
  1. 在view.layout()过程结束后才能获取到；
  2. 通过视图右边的坐标-左边的左边计算出来的
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final int childHeightMeasureSpec;
if (lp.height == LayoutParams.MATCH_PARENT) {
    final int height = Math.max(0, getMeasuredHeight()
            - getPaddingTopWithForeground() - getPaddingBottomWithForeground()
            - lp.topMargin - lp.bottomMargin);
    childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
            height, MeasureSpec.EXACTLY);
} else {
    childHeightMeasureSpec = getChildMeasureSpec(heightMeasureSpec,
            getPaddingTopWithForeground() + getPaddingBottomWithForeground() +
            lp.topMargin + lp.bottomMargin,
            lp.height);
}

child.measure(childWidthMeasureSpec, childHeightMeasureSpec);

Frame layout 子节点重新绘制，为什么

Framelayout边界怎么算的

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switch (absoluteGravity & Gravity.HORIZONTAL_GRAVITY_MASK) {
    case Gravity.CENTER_HORIZONTAL:
        childLeft = parentLeft + (parentRight - parentLeft - width) / 2 +
        lp.leftMargin - lp.rightMargin;
        break;
    case Gravity.RIGHT:
        if (!forceLeftGravity) {
            childLeft = parentRight - width - lp.rightMargin;
            break;
        }
    case Gravity.LEFT:
    default:
        childLeft = parentLeft + lp.leftMargin;
}

LayoutParams MeasureSpec

的关系

ViewGroup测量

先度量child View,再度量自己的大小。

先度量 child View,child view度量完成，如果是 wrap_content，就是all childview的size.

​ // parent size child want size

​ 

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public static int getChildMeasureSpec(int spec, int padding, int childDimension) {// padding是parent view的
    int specMode = MeasureSpec.getMode(spec);
    int specSize = MeasureSpec.getSize(spec);

    int size = Math.max(0, specSize - padding);// parent size

    int resultSize = 0;
    int resultMode = 0;

    switch (specMode) {
    // Parent has imposed an exact size on us
    case MeasureSpec.EXACTLY:
        if (childDimension >= 0) {
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size. So be it.
            resultSize = size;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size. It can't be
            // bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        }
        break;

    // Parent has imposed a maximum size on us
    case MeasureSpec.AT_MOST:
        if (childDimension >= 0) {
            // Child wants a specific size... so be it
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size, but our size is not fixed.
            // Constrain child to not be bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size. It can't be
            // bigger than us.
            resultSize = size;
            resultMode = MeasureSpec.AT_MOST;
        }
        break;

    // Parent asked to see how big we want to be
    case MeasureSpec.UNSPECIFIED:
        if (childDimension >= 0) {
            // Child wants a specific size... let him have it
            resultSize = childDimension;
            resultMode = MeasureSpec.EXACTLY;
        } else if (childDimension == LayoutParams.MATCH_PARENT) {
            // Child wants to be our size... find out how big it should
            // be
            resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
            resultMode = MeasureSpec.UNSPECIFIED;
        } else if (childDimension == LayoutParams.WRAP_CONTENT) {
            // Child wants to determine its own size.... find out how
            // big it should be
            resultSize = View.sUseZeroUnspecifiedMeasureSpec ? 0 : size;
            resultMode = MeasureSpec.UNSPECIFIED;
        }
        break;
    }
    //noinspection ResourceType
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}
MeasureSpec创建规则

  1. 子View为精确宽高，无论父容器的MeasureSpec，子View的MeasureSpec都为精确值且遵循LayoutParams中的值。
  2. 子View为match_parent时，如果父容器是精确模式，则子View也为精确模式且为父容器的剩余空间大小；如果父容器是最大模式，则子View也是最大模式且不会超过父容器的剩余空间。
  3. 子View为wrap_content时，无论父View是精确还是最大模式，子View的模式总是最大模式，且不会超过父容器的剩余空间。

艺术探索

https://www.jianshu.com/p/cb529ed03597

https://juejin.cn/post/6844904094205739016

MeasureSpec表示
  1. MeasureSpec用int表示,总共32位，高2位代表SpecMode，低30位代表SpecSize,SpecMode是指测量模式，而SpecSize是指在某种测量模式下的规格大小。MeasureSpec通过将SpecMode和SpecSize打包成一个int值来避免过多的对象内存分配.打包makeMeasureSpec。
  2. 高2位 00 , 01, 10, 分别对应UNSPECIFIED, EXACTLY,AT_MOST。

  • UNSPECIFIED

    父容器不对View有任何限制，要多大给多大，这种情况一般用于系统内部，表示一种测量的状态. 开发者用不了.

  • EXACTLY

    父容器已经检测出View所需要的精确大小，这个时候View的最终大小就是SpecSize所指定的值。它对应于LayoutParams中的match_parent和具体的数值这两种模式。

  • AT_MOST

    父容器指定了一个可用大小即SpecSize,View的大小不能大于这个值

​

Inflate解析过程

https://www.bilibili.com/video/BV1Ka4y1j7HA?p=4&spm_id_from=pageDriver

https://www.youtube.com/watch?v=aOb4Hvqbeu4

https://www.bilibili.com/video/BV1M54y1y79P?from=search&seid=18028227823566625572&spm_id_from=333.337.0.0

自定义View Margin

https://blog.csdn.net/qiantanlong/article/details/82347850

VIEW
ViewGroup
https://noteforme.github.io/2020/08/13/ViewGroup/
Author
Jon
Posted on
August 13, 2020
Licensed under