自定义View 入门

自定义View

attrs

values 文件夹中定义attr属性文件

attr的format取值类型( 属性定义时可以指定多种类型 )

typeName	Description
reference	参考某一资源id
color	颜色值
flag	可用于位或运算
enum	枚举值
fraction	百分数
boolean	布尔值
dimension	尺寸值
float	浮点数
integer	整数
string	字符串

xml中使用自定义的attr属性值

代码中获取自定义的attr属性值

context.obtainStyledAttributes(attrs, R.styleable.CircleView)?.apply {
          paint.color = getColor(R.styleable.CircleView_color_circle, Color.GREEN)
          recycle()
      }

onMeasure

View只要对自身的MeasureSpec进行处理即可，而ViewGroup中的onMeasure除了对本身的
MeasureSpec要进行处理，还要调用ChildView的测量的相关方法,
如:
measureChildren(widthMeasureSpec, heightMeasureSpec)

measureSpecMode

availableSize指父容器中剩余可用的全部空间

parentLayoutParams	parentSpecMode	childLayoutParams	childSpecMode	childSpecSize
match_parent	EXACTLY	match_parent	EXACTLY	childSize
match_parent	EXACTLY	wrap_content	AT_MOST	availableSize
match_parent	EXACTLY	dp	EXACTLY	childSize
wrap_parent	AT_MOST	wrap_content	AT_MOST	availableSize
wrap_parent	AT_MOST	match_parent	AT_MOST	availableSize
wrap_parent	AT_MOST	dp	EXACTLY	childSize
dp	EXACTLY	match_parent	EXACTLY	childSize
dp	EXACTLY	dp	EXACTLY	childSize
dp	EXACTLY	wrap_content	AT_MOST	availableSize
	UNSPECIFIED	match_parent	UNSPECIFIED	UNSPECIFIED
	UNSPECIFIED	wrap_content	UNSPECIFIED	UNSPECIFIED
	UNSPECIFIED	dp	EXACTLY	childSize

example(只对Height进行了处理)

据表可以看到在子View的 SpecMode 为 AT_MOST的时候，SpecSize都为 availableSize,
即使LayoutParams设置为wrap_content(AT_MOST)。所以在自定义View在重写onMeasure时，
对AT_MOST要进行特殊处理来支持wrap_content.可以设置一个默认的SIZE，或者也可以通过计算得到SIZE

 override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
        val heightSpecMode = MeasureSpec.getMode(heightMeasureSpec)
        val widthSize = MeasureSpec.getSize(widthMeasureSpec)
        val heightSize = MeasureSpec.getSize(heightMeasureSpec)
        if (heightSpecMode == MeasureSpec.AT_MOST) {
            val mHeight = TypedValue.applyDimension(
                TypedValue.COMPLEX_UNIT_DIP,
                DEFAULT_HEIGHT,
                Resources.getSystem().displayMetrics
            ).toInt()
            setMeasuredDimension(widthSize, mHeight)
        } else {
            setMeasuredDimension(widthSize, heightSize)
        }
    }

onLayout

一般自定义View时，ViewGroup需要重写onLayout()方法，经过自定义处理后在其中调用childView的layout方法。
View一般则不需要重写onLayout()

protected void onLayout(boolean changed, int left, int top, int right, int bottom) { }
`

paramName	Description
changed	见View.java的 `protected boolean setFrame(int left, int top, int right, int bottom)`
left	View左边界距父布局的左边界距离
top	View上边界距父布局的上边界距离
right	View右边界距父布局的右边界距离
bottom	View底边界距父布局的底边界距离

onDraw

自定义View时，一般ViewGroup不需要重写onDraw()方法。
View则需要重写onDraw()方法，画出相应的内容

ViewGroup扩展

ViewGroup的子项是否延迟按下状态，通常像ListView这种可以滚动的布局返回是 true
属于ViewGroup中的方法
public boolean shouldDelayChildPressedState() { return true; }

自定义ViewGroup中的LayoutParams属性,使其在childView中使用, 如LinearLayout中的layout_weight

定义attr属性文件

 
    
    自定义LayoutParams(内部类),获取属性值 
   
   inner class LayoutParams : ViewGroup.MarginLayoutParams {
        var position: Int = 0

        constructor(context: Context, attrs: AttributeSet?) : super(context, attrs) {
            context.obtainStyledAttributes(attrs, R.styleable.VerticalLinearLayout_Layout)?.run {
                position = getInteger(R.styleable.VerticalLinearLayout_Layout_layout_position, position)
                recycle()
            }
        }

        constructor(width: Int, height: Int) : super(width, height) {}

        constructor(p: ViewGroup.LayoutParams) : super(p)

        constructor(source: ViewGroup.MarginLayoutParams) : super(source)

        constructor(source: LayoutParams) : super(source)

    }
 
    
    重写ViewGroup中的相关方法 
   
  override fun generateLayoutParams(attrs: AttributeSet?): LayoutParams {
        return LayoutParams(context, attrs)
    }

    override fun generateLayoutParams(p: ViewGroup.LayoutParams?): ViewGroup.LayoutParams {
        return LayoutParams(p)
    }

    override fun generateDefaultLayoutParams(): LayoutParams {
        return LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT)
    }

    override fun checkLayoutParams(p: ViewGroup.LayoutParams?): Boolean {
        return p is VerticalLinearLayout.LayoutParams
    }

 
 资源 
 Android开发艺术探索 - 任玉刚
 例子