自定义View之onLayout
2021-05-17 14:29 · 阅读时长6分钟
在自定义普通View时，会重写onDraw和onMeasure，不会重写onLayout，因为它主要是用于布局子View用的，只有在自定义ViewGroup时才会用到，本文涉及的所有代码都在文章末尾。
下面自定义一个FlexLayout，实现优先横行排列，当横行空间不够时可以换行的容器，宽高能够自适应，效果图如下
- 数量超过一行时
- 数量不超过一行时
实现大致可以分为以下几个步骤：
- 在onMeasure中计算出所有子View的大小，根据排版规则，计算出自身的测量大小。
- 在onLayout中遍历所有的子View，根据排版规则给子View设置位置。
注意：在计算的时候需要考虑自身的padding和子View的margin。
提示：默认情况下，ViewGroup中的onDraw不会调用，当给ViewGroup设置背景色之后，onDraw会被调用。
关于onMeasure可参考：自定义View之onMeasure
注释主要代码如下
1class CustomFlexLayout(context: Context, attrs: AttributeSet?) : ViewGroup(context, attrs) {
2
3 override fun onLayout(changed: Boolean, l: Int, t: Int, r: Int, b: Int) {
4 var x = paddingLeft
5 var y = paddingTop
6 var childMaxHeight = 0
7 for (i in 0 until childCount) {
8 val child = getChildAt(i)
9 val params = child.layoutParams as LayoutParams
10 var left = x + params.leftMargin
11 var top = y + params.topMargin
12 childMaxHeight = childMaxHeight.coerceAtLeast(
13 child.measuredHeight + params.topMargin + params.bottomMargin
14 )
15 if (left + child.measuredWidth > measuredWidth) {
16 left = paddingLeft + params.leftMargin
17 top += childMaxHeight
18 y += childMaxHeight
19 }
20 val bottom = top + child.measuredHeight
21 val right = left + child.measuredWidth
22 child.layout(left, top, right, bottom)
23 x = right + params.rightMargin
24 }
25 }
26
27 override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
28 measureChildren(widthMeasureSpec, heightMeasureSpec)
29 var x = paddingLeft
30 var y = paddingTop
31 var childMaxHeight = 0
32 var width = 0
33 var height = 0
34 for (i in 0 until childCount) {
35 val child = getChildAt(i)
36 val params = child.layoutParams as LayoutParams
37 width += child.measuredWidth + params.leftMargin + params.rightMargin
38 var left = x + params.leftMargin
39 var top = y + params.topMargin
40 childMaxHeight = childMaxHeight.coerceAtLeast(
41 child.measuredHeight + params.topMargin + params.bottomMargin
42 )
43 height = y + childMaxHeight
44 if (left + child.measuredWidth > measuredWidth) {
45 left = paddingLeft + params.leftMargin
46 top += childMaxHeight
47 y += childMaxHeight
48 }
49 x = left + child.measuredWidth + params.rightMargin
50 }
51 setMeasuredDimension(
52 resolveSize(width + paddingTop + paddingRight, widthMeasureSpec),
53 resolveSize(height + paddingBottom, heightMeasureSpec)
54 )
55 }
56
57 override fun generateLayoutParams(attrs: AttributeSet?): ViewGroup.LayoutParams {
58 return LayoutParams(context, attrs)
59 }
60
61 class LayoutParams(context: Context, attrs: AttributeSet?) : MarginLayoutParams(context, attrs)
62}项目完整代码如下
自定义viewonLayoutandroidview
1