1. MeasureSpec类

MeasureSpec用来计算子视图的大小，有三种类型，UNSPECIFIED、EXACTLY和AT_MOST。

UNSPECIFIED表示未定义，即父控件未做限制，可以为任何值，一般设置为0。
EXACTLY表示实际值，即父容器已经指定了具体的值。
AT_MOST表示父容器提供了最大值，但子控件可以选择自己的范围。

使用静态方法来获取实际的mode和size

public static int getMode(int measureSpec)
public static int getSize(int measureSpec)

2. View的measure方法

(1) measure方法

measure()方法计算高宽，调用onMeasure()方法计算或setMeasuredDimensionRaw()方法设置。

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

    int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
    if (cacheIndex < 0 || sIgnoreMeasureCache) {
        // measure ourselves, this should set the measured dimension flag back
        onMeasure(widthMeasureSpec, heightMeasureSpec);
        mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
    } else {
        long value = mMeasureCache.valueAt(cacheIndex);
        // Casting a long to int drops the high 32 bits, no mask needed
        setMeasuredDimensionRaw((int) (value >> 32), (int) value);
        mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
    }

    ... ...
}

(2) onMeasure方法

调用setMeasuredDimension()方法来设置实际的宽和高，getDefaultSize()方法获取默认的宽高。

protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
            getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
}

(3) getDefaultSize方法

在getDefaultSize方法中，使用MeasureSpec来获取mode和size，并返回计算后的值。
当mode为UNSPECIFIED时，返回默认值，否则返回建议值。

public static int getDefaultSize(int size, int measureSpec) {
    int result = size;
    int specMode = MeasureSpec.getMode(measureSpec);
    int specSize = MeasureSpec.getSize(measureSpec);

    switch (specMode) {
    case MeasureSpec.UNSPECIFIED:
        result = size;
        break;
    case MeasureSpec.AT_MOST:
    case MeasureSpec.EXACTLY:
        result = specSize;
        break;
    }
    return result;
}

(4) setMeasuredDimension方法

同样调用setMeasuredDimensionRaw()方法设置宽高

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
    ... ...

    setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}

3. ViewGroup的measureChild方法

ViewGroup除了需要计算自身的宽高以外，还要计算子控件的宽高，系统提供了measureChildren、measureChild和getChildMeasureSpec来支持一般的操作。

(1) measureChildren方法

只要child的Visibility不是GONE，就计算child的宽高，调用measureChild方法

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方法

使用getChildMeasureSpec方法获得宽高，最后调用View.measure方法设置。

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) getChildMeasureSpec方法

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;
    }
    //noinspection ResourceType
    return MeasureSpec.makeMeasureSpec(resultSize, resultMode);
}

Android measure方法详解

1. MeasureSpec类

2. View的measure方法

3. ViewGroup的measureChild方法

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