Android开发学习之View测量的内置常用方法

最新推荐文章于 2022-09-28 22:19:26 发布

转载最新推荐文章于 2022-09-28 22:19:26 发布 · 489 阅读

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本文深入解析了Android中View的测量流程，重点介绍了measureChildWithMargins、getChildMeasureSpec等关键方法的作用及其实现细节，并解释了如何通过这些方法正确测量View的尺寸。

measureChildWithMargins

代码如下

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);
}

主要调用了getChildMeasureSpec()方法

getChildMeasureSpec

代码如下

public static int getChildMeasureSpec(int spec, int padding, int childDimension) {
// 以宽度为例，传入的参数分别是：父view的widthMeasureSpec、已用宽度(子view左右内外间距+已用的间距)、子view要求的宽度
int specMode = MeasureSpec.getMode(spec);
int specSize = MeasureSpec.getSize(spec);
int size = Math.max(0, specSize - padding); // 父宽度 - 子view的内外间距
// 也就是子view的最大宽度
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. sdk < 23时，sUseZeroUnspecifiedMeasureSpec为true
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);
}

combineMeasuredState

public static int combineMeasuredStates(int curState, int newState) {
return curState | newState;
}

没啥好说的，合并两个state

getMeasuredState

public final int getMeasuredState() {
return (mMeasuredWidth&MEASURED_STATE_MASK)
| ((mMeasuredHeight>>MEASURED_HEIGHT_STATE_SHIFT)
& (MEASURED_STATE_MASK>>MEASURED_HEIGHT_STATE_SHIFT));
}

通过位运算合并高度和宽度的state，结果就是第1个字节是宽度的state，第3个字节是高度的state

resolveSizeAndState

public static int resolveSizeAndState(int size, int measureSpec, int childMeasuredState) { // size是自己需要的尺寸，measureSpec是父view指定的尺寸
final int specMode = MeasureSpec.getMode(measureSpec);
final int specSize = MeasureSpec.getSize(measureSpec); // 父view指定的尺寸
final int result;
switch (specMode) {
case MeasureSpec.AT_MOST:
if (specSize < size) {
result = specSize | MEASURED_STATE_TOO_SMALL; // 设定标志位，表示父view得分配的再大一些
} else {
result = size;
}
break;
case MeasureSpec.EXACTLY:
result = specSize;
break;
case MeasureSpec.UNSPECIFIED:
default:
result = size;
}
return result | (childMeasuredState & MEASURED_STATE_MASK);
}

最后保存状态时，由于childMeasureState是第一个字节是宽度的状态，第三个字节是高度的状态，所以再测量宽度高度时调用此方法，要对childState进行位运算

setMeasuredDimension

这个是用来保存当前view尺寸的，当我们使用自定义view并且覆写了onMeasure()方法时，不在最后调用这个方法，直接就报错了

protected final void setMeasuredDimension(int measuredWidth, int measuredHeight) {
boolean optical = isLayoutModeOptical(this);
if (optical != isLayoutModeOptical(mParent)) { // layout_mode是LAYOUT_MODE_OPTICAL的情况很少出现，所以测量过程中涉及optical的，我们一般都可以直接忽略不计
Insets insets = getOpticalInsets();
int opticalWidth = insets.left + insets.right;
int opticalHeight = insets.top + insets.bottom;
measuredWidth += optical ? opticalWidth : -opticalWidth;
measuredHeight += optical ? opticalHeight : -opticalHeight;
}
setMeasuredDimensionRaw(measuredWidth, measuredHeight);
}

直接调用了setMeasuredDimensionRaw()方法

private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) {
mMeasuredWidth = measuredWidth;
mMeasuredHeight = measuredHeight;
mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;
}

嗯，就是这一步，保存了measuredHeight和measuredWidth.不过，这里要说明一下，measuredHeight/Width高八位是状态，低二十四位才是真正的尺寸，这也就是View.getMeasuredWidth/Height和View.getMeasuredWidth/HeightAndState()方法的区别所在