0.4 View 工作流程

自定义 View 核心要点总结与代码注释

一、核心知识点总结

1. View 的工作流程

• measure：确定 View 的测量宽高（onMeasure）
• layout：确定 View 的最终位置（onLayout，对单一 View 无作用）
• draw：将内容绘制到屏幕上（onDraw）

2. MeasureSpec 说明

• EXACTLY：对应 match_parent 或具体数值
• AT_MOST：对应 wrap_content
• UNSPECIFIED：父容器不对子 View 限制大小（多用于系统内部）

3. 自定义 View 注意事项

• 必须处理 wrap_content：否则其行为等同于 match_parent
• 必须手动处理 padding：在 onDraw 中考虑 padding 值
• 背景影响最小尺寸：getSuggestedMinimumWidth() 会结合 minWidth 与背景 drawable 的最小宽度
• 推荐使用 resolveSize()：统一处理 MeasureSpec 逻辑

4. 自定义 View 分类

• 继承 View：需自行处理 wrap_content 和 padding
• 继承已有控件（如 TextView）：无需处理测量和 padding
• 继承 ViewGroup：需实现子 View 的测量与布局
• 组合控件：继承 ViewGroup 封装多个子 View

---

二、代码文件（带完整注释）

CircleView.kt

package com.shakespace.artofandroid.chapter04import android.content.Context
import android.graphics.Canvas
import android.graphics.Color
import android.graphics.Paint
import android.util.AttributeSet
import android.view.View
import com.shakespace.artofandroid.R/*** 自定义 CircleView，演示如何正确处理：* - 自定义属性（circle_color）* - wrap_content 的默认尺寸* - padding 的绘制偏移*/
class CircleView @JvmOverloads constructor(context: Context?,private val attrs: AttributeSet? = null,defStyleAttr: Int = 0
) : View(context, attrs, defStyleAttr) {// 默认颜色为红色var color = Color.RED// 开启抗锯齿的画笔val paint = Paint(Paint.ANTI_ALIAS_FLAG)init {initView()}/*** 初始化自定义属性*/private fun initView() {// 从 XML 中读取自定义属性val typedArray = context.obtainStyledAttributes(attrs, R.styleable.CircleView)color = typedArray.getColor(R.styleable.CircleView_circle_color, Color.RED)// radius 属性被注释，实际使用 view 宽高决定paint.color = colortypedArray.recycle() // 必须回收}/*** 重写 onMeasure 以支持 wrap_content* 若不重写，wrap_content 行为等同于 match_parent*/override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {// 调用父类无实际作用，但保留良好习惯super.onMeasure(widthMeasureSpec, heightMeasureSpec)// 使用 resolveSize 处理 AT_MOST (wrap_content) 情况// 默认期望大小为 200pxval measuredWidth = Math.max(this.suggestedMinimumWidth,resolveSize(200, widthMeasureSpec))val measuredHeight = Math.max(this.suggestedMinimumHeight,resolveSize(200, heightMeasureSpec))// 设置最终测量尺寸this.setMeasuredDimension(measuredWidth, measuredHeight)}/*** 绘制圆形，正确处理 padding*/override fun onDraw(canvas: Canvas?) {super.onDraw(canvas)// 减去 padding 得到可绘制区域val width = width - paddingLeft - paddingRightval height = height - paddingTop - paddingBottom// 取可绘制区域的内切圆半径val radius = Math.min(width, height) / 2// 圆心位置需加上 paddingLeft / paddingTopval centerX = (paddingLeft + width / 2).toFloat()val centerY = (paddingTop + height / 2).toFloat()canvas?.drawCircle(centerX, centerY, radius.toFloat(), paint)}
}

三、 .txt 内容

z_memo01_view.txt

1. ViewRoot -- ViewRootImplstart with performTraversalsperformMeasure -- measure -- onMeasure ----- (View) measureperformLayout -- layout --onLayout     -----(View ) layoutperformDraw -- draw -- onDraw          ----- (View ) draw2. when measure finished , can use getMeasurdWidth/getMeasureHeight to get width/height (usually it is  equivalent to final width/height)3. when layout finished , can use getTop/getBottom/getLeft/getRight to get top/bottom/left/right position4. MeasureSpecUNSPECIFIED     usually use in system interiorEXACTLY         final width/height == SpecSize , match to match_parent or exact value in xml.AT_MOST         width/height <= SpecSize , match to wrap_content5. ViewRootImpl.javaprivate static int getRootMeasureSpec(int windowSize, int rootDimension) {int measureSpec;switch (rootDimension) {case ViewGroup.LayoutParams.MATCH_PARENT:// Window can't resize. Force root view to be windowSize.measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.EXACTLY);break;case ViewGroup.LayoutParams.WRAP_CONTENT:// Window can resize. Set max size for root view.measureSpec = MeasureSpec.makeMeasureSpec(windowSize, MeasureSpec.AT_MOST);break;default:// Window wants to be an exact size. Force root view to be that size.measureSpec = MeasureSpec.makeMeasureSpec(rootDimension, MeasureSpec.EXACTLY);break;}return measureSpec;}6. for View , it's MeasureSpec depends on Parental MeasureSpec and child's LayoutParamsexact value  -- > EXACTLYmatch_parent ---> parentSize  , if parent is UNSPECIFIED  child is 0wrap_content --->EXACTLY/AT_MOST --> AT_MOST and not more than parentSizeUNSPECIFIED  ----> child is 07.protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {setMeasuredDimension(getDefaultSize(getSuggestedMinimumWidth(), widthMeasureSpec),getDefaultSize(getSuggestedMinimumHeight(), heightMeasureSpec));}public static int getDefaultSize(int size, int measureSpec) {int result = size;int specMode = MeasureSpec.getMode(measureSpec);int specSize = MeasureSpec.getSize(measureSpec);switch (specMode) {case MeasureSpec.UNSPECIFIED:result = size;break;case MeasureSpec.AT_MOST:case MeasureSpec.EXACTLY:result = specSize;getChildMeasureSpecbreak;}return result;}So , Both AT_MOST and EXACTLY , will return specSize in MeasureSpec.When UNSPECIFIED , size would be getSuggestedMinimumWidth() or getSuggestdMinimumHeight()protected int getSuggestedMinimumWidth() {return (mBackground == null) ? mMinWidth : max(mMinWidth, mBackground.getMinimumWidth());}public int getMinimumWidth() {final int intrinsicWidth = getIntrinsicWidth();return intrinsicWidth > 0 ? intrinsicWidth : 0;}如果view没有背景， 那么宽度是mMinWidth ， 对应属性 android：minWidth 的值，如果没有设置，则为0 。如果设置了背景，宽度为max(mMinWidth，mBackground.getMinimumWidth())getMinimumWidth 返回的是drawable的原始宽度，如果没有，返回0。 （ShapeDrawable没有原始宽度，BitmapDrawable有原始宽度）note : 直接继承View， 需要重写onMeasure处理wrap_content时的自身大小(通常是一个默认size)， 否则wrap_content和match_parent相同solution: set a width/height when wrap_contente.g:protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {super.onMeasure(widthMeasureSpec, heightMeasureSpec);int size = this.dp2px(150.0F);this.setMeasuredDimension(Math.max(this.getSuggestedMinimumWidth(),resolveSize(size, widthMeasureSpec)),Math.max(this.getSuggestedMinimumHeight(),resolveSize(size, heightMeasureSpec)));}8.

z_memo02_viewgroup.txt

ViewGroup1. ViewGroup is a abstract class , it will call measureChildren to measure child viewprotected void measureChildren(int widthMeasureSpec, int heightMeasureSpec) {final int size = mChildrenCount;final View[] children = mChildren;for (int i = 0; i < size; ++i) {final View child = children[i];if ((child.mViewFlags & VISIBILITY_MASK) != GONE) {measureChild(child, widthMeasureSpec, heightMeasureSpec);}}}protected void measureChild(View child, int parentWidthMeasureSpec,int parentHeightMeasureSpec) {final LayoutParams lp = child.getLayoutParams();final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,mPaddingLeft + mPaddingRight, lp.width);final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,mPaddingTop + mPaddingBottom, lp.height);child.measure(childWidthMeasureSpec, childHeightMeasureSpec);}create childWidthMeasureSpec by getLayoutParams and parentWidthMeasureSpecgetChildMeasureSpec2. LinearLayout@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {if (mOrientation == VERTICAL) {measureVertical(widthMeasureSpec, heightMeasureSpec);} else {measureHorizontal(widthMeasureSpec, heightMeasureSpec);}}measureChildBeforeLayout(child, i, widthMeasureSpec, 0,heightMeasureSpec, usedHeight);will call child.measure(childWidthMeasureSpec, childHeightMeasureSpec);use mTotalLength to store the preliminary height// Add in our paddingmTotalLength += mPaddingTop + mPaddingBottom;int heightSize = mTotalLength;// Check against our minimum heightheightSize = Math.max(heightSize, getSuggestedMinimumHeight());// Reconcile our calculated size with the heightMeasureSpecint heightSizeAndState = resolveSizeAndState(heightSize, heightMeasureSpec, 0);heightSize = heightSizeAndState & MEASURED_SIZE_MASK;LinearLayout will measure itself once children's measurement finished在Vertical模式下, 如果是match_parent， 则高度为specSize，如果是wrap_conent，则高度是所有子元素高度总和，但不能超过父容器剩余空间3. 通常在measure完成之后就可以通过getMeasuredWidth/Height来获取测量的宽高，但是某些情况下，系统可能需要多次measure，所以最好在onLayout中再去获取测量的宽高。4. 在onCreate\onStart\onResume 都无法正确获得测量的宽高，可以使用以下四种方式Activity/View -- onWindowFocusChanged (会被调用多次)view.post(runnable)ViewTreeObserver . onGlobalLayoutListener   (调用多次)view.measure(int widthMeasureSpec,int heightMeasureSpec)需要根据LayoutParams判断，如果是match_parent 无法测量。如果是具体值：View.MeasureSpec.makeMeasureSpec(100,View.MeasureSpec.EXACTLY) 得到spec如果是wrap_content：通过 View.MeasureSpec.makeMeasureSpec((1 shl 30)-1,View.MeasureSpec.AT_MOST) 得到spec* 1 shl 30 是 1 << 30 的Kotlin写法5. two incorrect wayView.MeasureSpec.makeMeasureSpec(-1,View.MeasureSpec.UNSPECIFIED)view.measure(WindowManager.LayoutParams.WRAP_CONTENT,indowManager.LayoutParams.WRAP_CONTEN)6. Layout通过setFrame 设定view的四个顶点，初始化mLeft、mRight、mTop、mBottom再通过onLayout确定子View的位置在LinearLayout中的VerticalLayout方法childTop += lp.topMargin;setChildFrame(child, childLeft, childTop + getLocationOffset(child),childWidth, childHeight);childTop会不断增加。private void setChildFrame(View child, int left, int top, int width, int height) {child.layout(left, top, left + width, top + height);}其中的width和height ，就是子元素的测量宽高。7. getMeasuredWidth 和 getWidth 区别1.在默认实现中， 两者是相同的2.两者的赋值实际不同。3. 如果重写layout方法public void layout(int l , int t , int r, int b ){super.layout(l,t,r+100,b+100)}没有实际意义，但是会导致最终宽高比测量宽高大100px4. 此外，通常测量会进行多次，前几次测量的结果可能不准确。8. onDraw()1. draw background      background.draw(canvas)2. draw itself          onDraw()3. draw children        dispatchDraw4. draw decoration      onDrawScrollBarscomments in ViewGroup.java/** Draw traversal performs several drawing steps which must be executed* in the appropriate order:**      1. Draw the background*      2. If necessary, save the canvas' layers to prepare for fading*      3. Draw view's content*      4. Draw children*      5. If necessary, draw the fading edges and restore layers*      6. Draw decorations (scrollbars for instance)*/9. View has a special  method "setWillNotDraw" , if a view not need draw anything , set true.in default , this flag in View is false , and is true in ViewGroup , so if need draw something in ViewGroupneed set this in false .public void setWillNotDraw(boolean willNotDraw) {setFlags(willNotDraw ? WILL_NOT_DRAW : 0, DRAW_MASK);}

z_memo03_custom_view.txt

自定义view分类
1. 继承View重写onDraw()通常用于实现一些不规则的效果，通过绘制的方式实现。需要自己支持 wrap_content和padding
2. 继承ViewGroup 派生特殊的Layout需要处理ViewGroup的测量和布局过程，并同时处理子元素的测量和布局过程。
3. 继承特定的View用于扩展已有View的功能， 例如TextView， 不需要自己支持wrap_content和padding
4. 继承特定的ViewGroup当需要将某几种View组合在一起时常用，可以将多个view放在一个ViewGroup中。，不需要处理ViewGroup的测量和布局过程。自定义View须知1、支持wrap_content需要在onMeasure时对wrap_content做处理，给一个最小宽高。2、如果有必要，需要支持padding如果不在onDraw时计算padding，那么padding将不起作用。ViewGroup的measure和layout中也需要考虑padding的影响。3、 尽量不要在View中使用handler，使用post完全可以替代handler4、 有动画和线程需要及时停止，可以在onDetachedFromWindow中操作此方法对应onAttachedToWindow当View不可见时也需要及时处理线程和动画。5、 带有滑动嵌套时，需要处理滑动冲突。