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2022-09-03
How to use the attributes :layout_weight
LinearLayout supports assigning a weight to individual children with the android:layout_weight attribute. The value assigned to a view in terms of how much space it should occupy on the screen.Default weight is zero!
android:layout_weight indicates that the individual view expands equally to fill the space remaining after all views are measured I want to show you how to calculate the space of individual view.There are some following steps:
Step1: To measured all views,so each view has their own size.No matter what value the android:layout_widthorandroid:layout_height is.After all views are measured,their size is confirmed.i.e. if the android:layout_width is 0dp,so the width of the view is 0dp,if the android:layout_width is match_parent,so its width is as large as its parent layout.If the android:layout_width is wrap_content,so its width is as large as its content. Here we know,the method onMeasure of each view will be called.Step2: To calculate the space remaining using the size of the parent layout of these views minuses the total size from all these sub views. i.e. To sum up all the width of these views,so that we get a total width size of these views. Then to use the width of their parent layout minuses the total width size.So that we get the width remaining.Step3: To calculate how much each view with the attribute android:layout_weight should occupies the space remaining. i.e. There are three fields,their weights are 1,2,3. The first view should occupy space = 1/(1+2+3)*space remaining,and so on.Step4: At this point,each view with the attribute android:layout_weight should add their occupied space remaining as their their final size. Their onMeasure will be called again. As you see. the method onMeasure of each view with android:layout_weight will be called at least twice.
Summary: When we setup an individual view in LinearLayout with android:layout_weight,Its final size including two parts:one is its original size,the other is the space remaining occupied according to its value of weight.These behavior will trigger the invoke to onMeasure twice.The space remaining will be distributed among the views with the value of android:layout_weight is bigger than zero.The space remaining equals the size of parent minuses the total size of children views.
Here are examples:
Equal distribution
To create a linear layout in which each child uses the same amount of space on the screen,set the android:layout_width of each view to 0dp or the android:layout_height of each view to 0dp. Then set the android:layout_weight to 1.
Example 1:
Unequal distribution
Example 2:
In this example,their weights are both 1, so that the total weight is 1+1 = 2. The first component is smaller than the second one. Because the width of the first one pluses 1/2 of space remaining is smaller than the second one.If their attribute android:layout_weight are both 0dp,so that they can allocate the space remaining equally.
Example 3:
In this example,the total weight is 1. The Button will occupy all the space remaining.
Example 4: I think this one is the most exciting example.
Their widths are both as large as their parent layout with android:layout_width="match_parent".Keep in mind:the final size of a component with android:layout_weight includes its original size and part of the space remaining . Let 's calculate the space remaining. **space remaining = parent layout width - 3*(parent layout width) ** so space remaining equals -2*(parent layout width).space remaining = match_parent - 3 * match_parent = -2 * match_parent so,
the width of tv1 = 1*match_parent + 2/5 * (-2*match_parent) = 1/5*match_parentthe width of button1 = 1*match_parent + 1/5 * (-2*match_parent) = 3/5*match_parentthe width of tv2 = 1*match_parent + 2/5 * (-2*match_parent) = 1/5*match_parent
If I change the weight of tv1 to 3.Now it is changed from 2 to 3. What would happen?
space remaining = match_parent - 3 * match_parent = -2 * match_parent so,
the width of tv1 = 1*match_parent + 3/6 * (-2*match_parent) = 0the width of button1 = 1*match_parent + 1/6 * (-2*match_parent) = 2/3*match_parentthe width of tv2 = 1*match_parent + 2/6 * (-2*match_parent) = 1/3*match_parent
As you see,tv1 is gone.
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