Android View 的 measure 流程

Android View的主要工作流程包括:测量measure-->布局layout-->绘制draw,本篇主要讲解measure的过程。

在布局文件中,每个View都具有layout_widthlayout_height两个属性,用于指定View的尺寸。这两个属性的取值只有三种:固定数值wrap_contentmatch_parent。在measure的过程中,父容器会根据自己的限制信息和子View的尺寸属性算出子View的限制信息并传递给子View,子View再结合这些信息和自己的内容计算最终的尺寸。

这个限制信息就是MeasureSpec类。它实际上相当于一个32位的int,其中高两位代表父布局对子View的限制模式mode,剩下30位代表父布局为子View指定的尺寸size
mode有三种,分别是:
1、
EXACTLY
精确的,即子View不用管自己的内容,直接使用MeasureSpec中给的size。
2、AT_MOST最多的,即子View根据自己的内容计算一个合适的尺寸,但不能超过MeasureSpec中给的size。
3、UNSPECIFIED不限定,即子View想要多大就是多大,不用管MeasureSpec中给的size。

整个测量过程是从根节点的measure()方法开始的,虽然根节点是一个ViewGroup,但由于ViewGroup继承自View,而View中的measure()方法是final类型,不能被子类重写,因此还是执行的View中的measure()方法:

public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
    ...
    onMeasure(widthMeasureSpec, heightMeasureSpec)
    ...
}

measure()方法中会调用onMeasure()方法,onMeasure()可以被子类重写。一般来说,ViewGroup会在onMeasure()方法中对子View进行测量,然后根据父布局给出的限制结合子View的尺寸以及自己的布局规则得到自己的最终尺寸,并调用setMeasuredDimension()进行设置。

1、measureChildren()
遍历所有子View,跳过状态为Gone的,依次调用measureChild()进行测量

protected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {
    final int size = mChildrenCount;
    final View[] children = mChildren;
    for (int i = 0; i < size; ++i) {
        final View child = children[i];
        if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {
            measureChild(child, widthMeasureSpec, heightMeasureSpec);
        }
    }
}

2、measureChild()
根据自己的MeasureSpec和Padding以及子View指定的尺寸,调用getChildMeasureSpec()方法计算出子View的MeasureSpec,最后调用child.measure()将子View的MeasureSpec传给子View,由子View自己去计算自己的尺寸

protected void measureChild(View child, int parentWidthMeasureSpec,
        int parentHeightMeasureSpec) {
    final LayoutParams lp = child.getLayoutParams();

    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom, lp.height);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

3、measureChildWithMargins()
该方法与measureChild()类似,只不过计算时考虑了子View的margin

protected void measureChildWithMargins(View child,
        int parentWidthMeasureSpec, int widthUsed,
        int parentHeightMeasureSpec, int heightUsed) {
    final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();

    final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,
            mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin
                    + widthUsed, lp.width);
    final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,
            mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin
                    + heightUsed, lp.height);

    child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
}

4、getChildMeasureSpec()
根据父布局自己的MeasureSpec和子View的尺寸属性计算出子View的MeasureSpec

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
    int specMode = MeasureSpec.getMode(spec);
    int specSize = MeasureSpec.getSize(spec);

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

    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;
    }
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

上述代码简单的说就是:
(1)、如果子View的尺寸属性为具体数值,则子View的specMode为EXACTLY,specSize就用该数值。即子View指定了具体的数值,就精确的用指定的数值。
(2)、否则
size = 父布局的specSize - 父布局的padding
①、如果父布局的specMode为UNSPECIFIED未限定,那么子View的specMode也为UNSPECIFIED,specSize要么为0要么为size反正会被忽略。
②、如果父布局的specMode为EXACTLY精确的,并且子View的尺寸属性为match_parent,则子View的specMode为EXACTLY,specSize为size。即精确的与父布局同样大小。
③、其他情况,则子View的specMode为AT_MOST,specSize为size。即最多不超过父布局的大小。

5、计算了子View的MeasureSpec之后再调用child.measure(childWidthMeasureSpec, childHeightMeasureSpec)方法计算子View的真实尺寸,即回到了开头所说的View中的measure()方法,该方法中再调用子View的onMeasure()方法。如果子View又是一个ViewGroup,则重复上述过程,否则如果子View是一个View,那么onMeasure方法的实现逻辑为:

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int mode = MeasureSpec.getMode(measureSpec);
    int size = MeasureSpec.getSize(measureSpec);
    int viewSize = 0;
    swith (mode) {
        case MeasureSpec.EXACTLY:
            viewSize = size; //当前View尺寸设置为父布局尺寸
            break;
        case MeasureSpec.AT_MOST:
            viewSize = Math.min(size, getContentSize()); //当前View尺寸为内容尺寸和父布局尺寸当中的最小值
            break;
        case MeasureSpec.UNSPECIFIED:
            viewSize = getContentSize(); //内容有多大,就设置尺寸为多大
            break;
        default:
            break;
    }
    setMeasuredDimension(viewSize);
}

6、计算了子View的尺寸后,再根据父布局给出的限制结合子View的尺寸以及ViewGroup自身的布局规则得到最终尺寸
7、调用setMeasuredDimension(int measuredWidth, int measuredHeight)方法设置ViewGroup自身的最终尺寸

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