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android中layout_weight的理解
来源:互联网   发布日期:2015-10-14 07:48:01   浏览:2727次  

导读:SDK中的解释 Indicates how much of the extra space in the LinearLayout will be allocated to the view associated with these LayoutParams. Specify 0 if the view should not be stretched. Otherwise the extra pixels will be pro-rated among all v...

SDK中的解释

Indicates how much of the extra space in the LinearLayout will be allocated to the view associated with these LayoutParams. Specify 0 if the view should not be stretched. Otherwise the extra pixels will be pro-rated among all views whose weight is greater than 0.

重点有两个,一个是layout_weight表示LinearLayout中额外空间的划分(可能扩展应用layout_weight前的大小也可能压缩),另一个是按比例.

以下说的都以 android:orientation="horizontal" 为例

看了一下源码,虽说不太懂,但了解了下大概意思,按照自己的理解总结一下,直接写一下简化的代码吧(下面的代码是LinearLayout源文件中一部分的精简,变量名称含义可能不准确,为叙述方便暂作此解释):

//Either expand children with weight to take up available space or

// shrink them if they extend beyond our current bounds

int delta = widthSize - mTotalLength;

if (delta != 0 && totalWeight > 0.0f) {

float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;

for (int i = 0; i < count; ++i) {

final View child = getVirtualChildAt(i);

if (child == null || child.getVisibility() == View.GONE) {

continue;

}

final LinearLayout.LayoutParams lp =

(LinearLayout.LayoutParams) child.getLayoutParams();

float childExtra = lp.weight;

if (childExtra > 0) {

int share = (int) (childExtra * delta / weightSum);

weightSum -= childExtra;

delta -= share;

int childWidth = child.getMeasuredWidth() + share;

if (childWidth < 0) {

childWidth = 0;

}

}

}

}

变量含义

widthSize: LinearLayout的宽度

mTotalLength:所有子View的宽度的和(还没用考虑layout_weight)

totalWeight:所有子View的layout_weight的和

mWeihtSUm: LinearLayout的android:weightSum属性

过程分析:

首先计算出额外空间(可以为负)如果额外空间不为0并且有子View的layout_weight不为0的话按layout_weight分配额外空间:

int delta = widthSize - mTotalLength;

if (delta != 0 && totalWeight > 0.0f) {

...

}

如果LinearLayout设置了weightSum则覆盖子View的layout_weight的和:

float weightSum = mWeightSum > 0.0f ? mWeightSum : totalWeight;

然后遍历LinearLayout的子元素,如果不为null且Visibility不为GONE的话,取得它的LayoutParams,如果它的layout_weight大于0,根据weightSum与它的weight计算出分配给它的额外空间

if (childExtra > 0) {

int share = (int) (childExtra * delta / weightSum);

weightSum -= childExtra;

delta -= share;

int childWidth = child.getMeasuredWidth() + share;

if (childWidth < 0) {

childWidth = 0;

}

}

网上有解释说layout_weight表示重要程度,表示划分额外空间的优先级,通过代码可以知道这种观点是错误的.layout_weight表示划分的比例,至于当View的layout_width为fill_parent时layout_weight比例相反的问题按我的理解可以作以下解释:

比如说如下XML:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="fill_parent"

android:layout_height="wrap_content"

android:background="#00ff00"

android:weightSum="0"

android:orientation="horizontal" >

<Button

android:id="@+id/imageViewLoginState"

android:layout_width="fill_parent"

android:layout_height="fill_parent"

android:layout_weight="1"

android:text="1" >

</Button>

<Button

android:id="@+id/imageViewLoginState1"

android:layout_width="fill_parent"

android:layout_height="fill_parent"

android:layout_weight="1"

android:text="2" >

</Button>

<Button

android:id="@+id/imageViewLoginState2"

android:layout_width="fill_parent"

android:layout_height="fill_parent"

android:layout_weight="2"

android:text="3" >

</Button>

</LinearLayout>

按一般理解,3个Button的比例应该为1:1:2,但实际情况是这样的:

\

按我的理解,系统是这样设置按钮的大小的,变量名按前面代码的意义:

假设Container即LinearLayout的宽度为PARENT_WIDTH

三个按钮的宽度都是FILL_PARENT,所以在应用layout_width之前,三个按钮的宽度都为PARENT_WIDTH

所以额外空间 delta = PARENT_WIDTH - 3 * PARENT_WIDTH = -2 * PARENT

因为LinearLayout没有设置android:weightSum(默认为0,设置为0就当没设置吧),所以 mWeightSum = 1 + 1 +2 =4

所以:

第一个按钮的宽度为PARENT_WIDTH + share = PARENT_WIDTH + (layout_weight * delta / mWeightSum) = PARENT_WIDTH + (1 * (-2 * PARENT_WIDTH) /4) = 1 /2 *PARENT_WIDTH

weightSum -= childExtra;(=3)

delta -= share;(=-3/2 * PARENT_WIDTH)

第二个按钮的宽度为PARENT_WIDTH + share = PARENT_WIDTH + (layout_weight * delta / mWeightSum) = PARENT_WIDTH + (1 * (-3 / 2 * PARENT_WIDTH) /3) = 1 /2 *PARENT_WIDTH

weightSum -= childExtra;(=2)

delta -= share;(=-PARENT_WIDTH)

第三个按钮的宽度为PARENT_WIDTH + share = PARENT_WIDTH + (layout_weight * delta / mWeightSum) = PARENT_WIDTH + (2 * (- PARENT_WIDTH) /2) = 0

所以最终的而已就是前两个按钮平分LinearLayout,第三个按钮消失了.

大致过程是这样,但不全对,比如如果上例中LinearLayout的weightSum设置为2的话,前两个按钮的宽度为0,但当计算第三个按钮的宽度是mWeightSum = 0,但layout_weight * delta / mWeightSum无法计算,不知道系统怎么处理的,在我的能力之外了,weightSum为2时的效果图:\

weightSum为3时的效果图:

SDK中说明的是,layout_weight表示额外空间怎么划分,要注意额外2字,要有额外的空间才可以将按比例将其分配给设置了layout_weight的子View,所以,如果LinearLayout设置为WRAP_CONTENT的话是没有额外的空间的,layout_weight就没有用处,所只要layout_width不设置为WRAP_CONTENT就行,也可以设置为具体的值,如果值太小的话,额外空间为负,可能压缩子控件,使其大小比XML文件中定义的小,例如:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="100dp"

android:layout_height="wrap_content"

android:background="#00ff00"

android:orientation="horizontal" >

<Button

android:id="@+id/button1"

android:layout_width="60dp"

android:layout_height="fill_parent"

android:layout_weight="1"

android:text="1" >

</Button>

<Button

android:id="@+id/button2"

android:layout_width="60dp"

android:layout_height="fill_parent"

android:layout_weight="1"

android:text="2" >

</Button>

<Button

android:id="@+id/button3"

android:layout_width="60dp"

android:layout_height="fill_parent"

android:layout_weight="2"

android:text="3" >

</Button>

</LinearLayout>

额外空间 delta = 100- 3 * 60 = -80

mWeightSum = 1 + 1 +2 =4

所以:

第一个按钮的宽度为60+ share = 60 + (layout_weight * delta / mWeightSum) = 60 + (1 * (-80) /4) = 40

weightSum -= childExtra;(=3)

delta -= share;(=-60)

第二个按钮的宽度为60 + share = 60 + (layout_weight * delta / mWeightSum) = 60 + (1 * (-60) /3) = 40

weightSum -= childExtra;(=2)

delta -= share;(=-40)

第三个按钮的宽度为60 + share = 60 + (layout_weight * delta / mWeightSum) = 60 + (2 * (-40) /2) = 20

效果图:

\

以下代码也说明了layout_weight表示额外空间的分配:

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="200dp"

android:layout_height="wrap_content"

android:background="#00ff00"

android:orientation="horizontal" >

<Button

android:id="@+id/button1"

android:layout_width="60dp"

android:layout_height="fill_parent"

android:layout_weight="1"

android:text="1" >

</Button>

<Button

android:id="@+id/button2"

android:layout_width="40dp"

android:layout_height="fill_parent"

android:layout_weight="1"

android:text="2" >

</Button>

</LinearLayout>

额外空间为100,所以Button2的宽度为60+100/2=100,Button2的宽度为40+100/2=90

作者:AngelDevil

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