android中layout_weight的理解

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

  1. layout_weight表示LinearLayout中额外空间的划分(可能扩大应用layout_weight前的大小也可能缩小)。
  2. 按比例(layout_weight大小的比例)

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_weight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

然后更新weightSum与delta:
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

更新weightSum与delta:

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时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

然后:

weightSum -= childExtra;(=3)
delta  -= share;(=-60)

第二个按钮的宽度为:

60 + share = 60 + (layout_weight * delta / mWeightSum)

                = 60 + (2 * (-60) /3)

                = 40

然后:

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Button1的宽度为60+100/2=110,Button2的宽度为40+100/2=90。

posted @ 2012-04-08 16:44 AngelDevil 阅读(...) 评论(...) 编辑 收藏