本文共 8863 字,大约阅读时间需要 29 分钟。
在看LayoutParams之前,得要先了解MeasureSpec这个类,view的最终测量就是基于这个类,对于view树来说,view的尺寸的解析也是基于这个类的:
public static class MeasureSpec { private static final int MODE_SHIFT = 30; private static final int MODE_MASK = 0x3 << MODE_SHIFT; /** @hide */ @IntDef({UNSPECIFIED, EXACTLY, AT_MOST}) @Retention(RetentionPolicy.SOURCE) public @interface MeasureSpecMode {} public static final int UNSPECIFIED = 0 << MODE_SHIFT; public static final int EXACTLY = 1 << MODE_SHIFT; public static final int AT_MOST = 2 << MODE_SHIFT; public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size, @MeasureSpecMode int mode) { if (sUseBrokenMakeMeasureSpec) { return size + mode; } else { return (size & ~MODE_MASK) | (mode & MODE_MASK); } } public static int makeSafeMeasureSpec(int size, int mode) { if (sUseZeroUnspecifiedMeasureSpec && mode == UNSPECIFIED) { return 0; } return makeMeasureSpec(size, mode); } @MeasureSpecMode public static int getMode(int measureSpec) { //noinspection ResourceType return (measureSpec & MODE_MASK); } public static int getSize(int measureSpec) { return (measureSpec & ~MODE_MASK); }}
首先,对于view之间尺寸的传递是通过传递一个32位的int数,这个数的高两位存放的是对子view尺寸的限制,剩下的30位存放的父view给子view的大小,这个数的组装和解析就是交由MeasureSpec来做的,对于view尺寸的限制有以下三种模式:
对子view没有任何限制,子view要多大就给多大,自定义view中用的很少;
子view的大小不能超过父view给定的大小,"wrap_content"就是这个模式;
子view直接使用父view给定的大小,具体尺寸和“match_parent”就是这个模式;
这个类应该不难理解,就是将size和mode进行“与 或”运算以达到拼装和解析。
这里我们看一下TextView中onMeasure()的伪代码:
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { int widthMode = MeasureSpec.getMode(widthMeasureSpec); int heightMode = MeasureSpec.getMode(heightMeasureSpec); int widthSize = MeasureSpec.getSize(widthMeasureSpec); int heightSize = MeasureSpec.getSize(heightMeasureSpec); int width; int height; if (widthMode == MeasureSpec.EXACTLY) { width = widthSize; } else { width = desired; } if (heightMode == MeasureSpec.EXACTLY) { height = heightSize; } else { height = desired; } setMeasuredDimension(width, height);}
这里我们看到对于传进来的关于宽高的两个数就是通过MeasureSpec去解析,拿到宽高的限制模式和尺寸,然后通过对应的限制模式来获取对应的尺寸,最后通过setMeasuredDimension()将获得的宽高设置到view中,setMeasuredDimension()调用的就是下面这个方法:
private void setMeasuredDimensionRaw(int measuredWidth, int measuredHeight) { mMeasuredWidth = measuredWidth; mMeasuredHeight = measuredHeight; mPrivateFlags |= PFLAG_MEASURED_DIMENSION_SET;}
至此view的测量就结束了。现在我们回过来想一想,那这个mode是如何设置的呢?我们知道,子view的上一层肯定是ViewGroup,那measure中的数肯定就是从ViewGroup中传递下来的了,可以发现ViewGroup中有一个measureChild()的方法,顾名思义应该就是测量自view的了啊:
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);}
注意,这里就获取到了child的LayoutParams,对于LayoutParams是在哪里设置的呢,如果还不知道,可以看一下我的,上一篇在讲到递归生成view的时候,里面有一个方法:
viewGroup.addView(view, params);
这个方法里面就会将params设置到view中去,如果params为null,那么就会使用默认的params,默认的params如下:
protected LayoutParams generateDefaultLayoutParams() { return new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);}
可以看到用到的属性都是“wrap_content”。
说到这,你可能对LayoutParams还不太明白,那我们就先来看看LayoutParams这个类:
public static class LayoutParams { @SuppressWarnings({"UnusedDeclaration"}) @Deprecated public static final int FILL_PARENT = -1; public static final int MATCH_PARENT = -1; public static final int WRAP_CONTENT = -2; @ViewDebug.ExportedProperty(category = "layout", mapping = { @ViewDebug.IntToString(from = MATCH_PARENT, to = "MATCH_PARENT"), @ViewDebug.IntToString(from = WRAP_CONTENT, to = "WRAP_CONTENT") }) public int width; @ViewDebug.ExportedProperty(category = "layout", mapping = { @ViewDebug.IntToString(from = MATCH_PARENT, to = "MATCH_PARENT"), @ViewDebug.IntToString(from = WRAP_CONTENT, to = "WRAP_CONTENT") }) public int height; public LayoutAnimationController.AnimationParameters layoutAnimationParameters; public LayoutParams(Context c, AttributeSet attrs) { TypedArray a = c.obtainStyledAttributes(attrs, R.styleable.ViewGroup_Layout); setBaseAttributes(a, R.styleable.ViewGroup_Layout_layout_width, R.styleable.ViewGroup_Layout_layout_height); a.recycle(); } public LayoutParams(int width, int height) { this.width = width; this.height = height; } public LayoutParams(LayoutParams source) { this.width = source.width; this.height = source.height; } LayoutParams() { } protected void setBaseAttributes(TypedArray a, int widthAttr, int heightAttr) { width = a.getLayoutDimension(widthAttr, "layout_width"); height = a.getLayoutDimension(heightAttr, "layout_height"); }}
就维护了两个宽高属性,还记得我们一般在写layout的xml文件时,里面的view都会有layout_width和layout_parent这里两个属性,这两个属性就是作用在这里了,这里在多说一句,LayoutParams还有好多子类作用于不同的view,感兴趣的可以自己去看看。
讲完LayoutParams后,就让我们回到上面的measureChild(),为了拿到子类的模式和尺寸,这里进入到getChildMeasureSpec()这个方法里面:
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);}
别看这个方法挺多的,其实挺简单的,这里盗张图:
这里再用文字总结一下(由于UNSPECIFIED一般系统内部使用,这里就把他排除):
1、子view的childDimension大于0时,返回的mode是MeasureSpc.EXACTLY,size是chilDimension;
2、子view的childDimension是LayoutParams.MATCH_PARENT和父view是EXACTLY时,返回的mode是MeasureSpec.EXACTLY,size是父view给的size;
3、其他的情况返回的mode是MeasureSpec.AT_MOST,size是父view的尺寸;
这里我在说一下我们在实际中使用的的几种情况,都是与上面对应的:
1、在layout的xml文件中,父view使用的是match_parent,子view使用的也是match_parent,那么子view的模式就是MeasureSpec.EXACTLY;
2、在layout的xml文件中,父view使用的是match_parent,子view使用的是wrap_content,那么子view的模式就是MeasureSpec.AT_MOST;
3、在layout的xml文件中,子view使用的也是具体的尺寸,那么不管父view使用的是什么,子view的模式都是MeasureSpec.EXACTLY;
对于上面mode和size自己可以好好体会下,在这个地方刚开始看的时候是很迷糊的,后来多看了几遍才慢慢理解了,主要是一开始看的时候对这个LayoutParams也不是很懂(不知道是怎么设置的)。