View测量流程与困惑
Posted 爱炒饭
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周末重新温习了一下《android开发艺术探索》的View测量部分,View的尺寸由父容器和控件本身的LayoutParams决定的。ViewGroup中测量测量子View的方法分别是measureChildren方法和measureChildWithMargins方法,二者的区别 是是否考虑子View的margins属性。 不同的ViewGroup实现会调用二者中的一个,比如FrameLayout和LinearLayout通过measureChildWithMargins方法测量子View, NumPadKey和AbsoluteLayout调用measureChildren方法测量子View。二者最终都是通过getChildMeasureSpec 方法获取子View的MeasureSpec,然后获取的MeasureSpec作为参数去调用子View的 measure方法,这里以measureChildWithMargins方法作为起点来剖析下。
下面的代码那是相当精炼和清楚,首先获取子View的MarginLayoutParams参数,然后将父容器的MeasureSpec,子View的上下左右参数Padding、Margin参数和父容器已经使用的宽高,子View的宽高作为参数带入到getChildMeasureSpec方法中。通过getChildMeasureSpec方法获取子View的MeasureSpec后带入到子View的measure方法中。
//ViewGroup.java
/**
* 触发子View去测量自身, 考虑父容器的MeasureSpec以及子View的padding和margins。
* 子View必须有MarginLayoutParams,重要实现在getChildMeasureSpec方法中。
* @param child 需要测量的子View
* @param parentWidthMeasureSpec 父容器的宽度MeasureSpec
* @param 父容器已经被使用的宽度
* @param parentHeightMeasureSpec 父容器的高度MeasureSpec
* @param heightUsed 父容器已经被使用的高度
*/
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);
getChildMeasureSpec方法根据父容器spec、间距padding以及子View的宽高参数来确定子View的mode和size最后封装成子View的MeasureSpec返回。
//ViewGroup.java
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);//父容器留给子View分配的剩余空间
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);
View的measure方法接着调用onMeasure方法,在onMeasure方法确定测量宽高后通过setMeasuredDimension方法设置宽高,整个测量流程是measure->onMeasure->setMeasuredDimension。
//View.java
public final void measure(int widthMeasureSpec, int heightMeasureSpec)
boolean optical = isLayoutModeOptical(this);
……
onMeasure(widthMeasureSpec, heightMeasureSpec);
……
……
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec)
setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),
getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));
……
针对getChildMeasureSpec方法, 刚哥总结了下面的表格:
针对View以及父容器都是EXACTLY的情况,这里View的尺寸就是子View设置的尺寸,这里我有个疑问,如果子View设置的尺寸大于父容器的尺寸怎么办呢?难道还是按照子View的尺寸吗?于是我做了个实验,xml中设置LinearLayout(父容器)的宽度是400px,TextView(子View)的宽度是800px,原谅我这里用了px而非dp,这是为了方便后面在activity中获取尺寸比较明显。
<LinearLayout
android:orientation="vertical"
android:background="#000"
android:layout_width="400px"
android:layout_height="wrap_content">
<TextView
android:onClick="go"
android:layout_marginTop="20dp"
android:background="#f00"
android:layout_width="800px"
android:layout_height="wrap_content"
android:text="Hello World!" />
</LinearLayout>
然后在activity的go方法中获取View的宽度以及其父容器的宽度
public void go(View view)
LogUtil.d("textview is clicked,textview width="+view.getWidth()+",,LinearLayout width="+((View)view.getParent()).getWidth());
log打印如下:
textview is clicked,textview width=800,,LinearLayout width=400
顿时我裂开了,子View宽度确实就是之前设置的800,并且比父容器宽度还大。但是这里虽然TextView宽度比较大,但是由于父容器就这么大,所以给TextView更多的字符串长度,还是会截断到400px,并且超出400px的区域TextView点击事件也不生效,没卵用。
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