在android组件中主要分为两种:容器(LinearLayout....)和子View(TextView......),但是这些现有的组件往往不能满足app的开发。比如实现一个流式标签，炫酷的进度条显示呢。都需要我们探寻源码，分析和改造成我们想要的效果。这次的主题先从View的自定义入手。

概述：

针对View的自定义主要从 onMeasure（）和onDraw（）这两个方法入手。
    onMeasure（）：测量View的大小
        测量view只要针对我们在xml中wrap_content和match_parent的两种属性。而在onMeasure（）对      应的字段是AT_MOST 和EXACTLY。Android默认的实现了EXACTLY的测量也就是精准测量(对应xml属性    match_parent和具体值)，对于AT_MOST默认填充父容器，如果要实现包裹那就需要我们自己动手丰衣      足食了。

onDraw（）：绘制内容，比如形状，图片啊都在这里实现。
        主要用到类有paint（画笔），canvas（画板）由这两个类，我们就可以随心所欲的画画l咯。

实践出真理：

“纸上得来终觉浅,绝知此事要躬行“，现在我们就一步一步的用代码来分析自定义View的实践。想了好久我要用啥一个相对简单但大家都熟悉的view作为本文的的入门demo呢？最后决定我们来实现android自带的TextView。

入门demo：

创建一个class类，名为MyTextView  继承View。

package huangzhibo.com.learndemo.view;import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.support.annotation.Nullable;
import android.util.AttributeSet;
import android.view.View;import huangzhibo.com.learndemo.R;/*** Created by HuangZhiBo on 2017/8/20/020.*/public class MyTextView extends View {private String text;private int paintColor = Color.BLACK;public MyTextView(Context context) {super(context);}public MyTextView(Context context, @Nullable AttributeSet attrs) {super(context, attrs);}@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {super.onMeasure(widthMeasureSpec, heightMeasureSpec);/**实现AT_MOST的测量*/}public void setText(String info) {text = info;}public void setTextColor(int color) {paintColor = color;}@Overrideprotected void onDraw(Canvas canvas) {if (text == null) {  //文本为空return;}Paint paint = new Paint();paint.setColor(paintColor);paint.setAntiAlias(true); //抗锯齿paint.setTextSize(30);  //文本大小canvas.drawText(text,this.getWidth()/2-getTextWidth(paint,text)/2,this.getHeight()/2,paint);}/*** 测量文字宽度* @param paint* @param str* @return*/public static int getTextWidth(Paint paint, String str) {int w= 0;if (str != null && str.length() > 0) {int len = str.length();float[] widths = new float[len];paint.getTextWidths(str, widths);for (int j = 0; j < len; j++) {w+= (int) Math.ceil(widths[j]);}}return w;}
}

xml布局

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayoutxmlns:android="http://schemas.android.com/apk/res/android"xmlns:app="http://schemas.android.com/apk/res-auto"xmlns:tools="http://schemas.android.com/tools"android:layout_width="match_parent"android:orientation="vertical"android:layout_height="match_parent"tools:context="huangzhibo.com.learndemo.activity.CustomView"><huangzhibo.com.learndemo.view.MyTextViewandroid:background="@color/colorAccent"android:id="@+id/myTextView"android:layout_width="wrap_content"android:layout_height="wrap_content"/>
</RelativeLayout>

代码都有相应的注释，这里就不在废话，这里对于onMeasure并没有处理，采用默认。注意看layout_width和layout_height的属性值为wrap_content。上文我说道，如果对于onMearsure并未处理，那么view将填充父容器。为了看清楚边界，view设置了一个背景。结果不出所料，那么接下我们就来实现Android对于wrap_content的测量。

效果图：

在对于onMeasure的AT_MOST，我查了下网上很多资料都是直接给个固定值，然后通过比较返回最大值，这样根本就是治标不治本，我们要的效果是完全的TextView效果。那么问题来了，要实现包裹效果，也就需要确定宽和高。而这个宽和高就是文本的宽和高加上内边距(pading)。思路有了那就好办了。

package huangzhibo.com.learndemo.view;import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.support.annotation.Nullable;
import android.util.AttributeSet;
import android.view.View;import huangzhibo.com.learndemo.R;import static android.os.Build.VERSION_CODES.M;/*** Created by HuangZhiBo on 2017/8/20/020.*/public class MyTextView extends View {private String text;private int paintColor = Color.BLACK;private Paint mPaint;public MyTextView(Context context) {super(context);}public MyTextView(Context context, @Nullable AttributeSet attrs) {super(context, attrs);mPaint=new Paint();mPaint.setColor(paintColor);mPaint.setAntiAlias(true); //抗锯齿mPaint.setTextSize(30);  //文本大小}@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {super.onMeasure(widthMeasureSpec, heightMeasureSpec);int result = 200;/**实现AT_MOST的测量*/setMeasuredDimension(meaSureWidth(widthMeasureSpec),meaSureHeight(heightMeasureSpec));}/*** 测量宽度* @param measureSpec* @return*/private int meaSureWidth(int measureSpec){int result=0;int specMode = MeasureSpec.getMode(measureSpec);int specSize = MeasureSpec.getSize(measureSpec);if (specMode==MeasureSpec.AT_MOST){/**文本宽度+左右内边距*/int v = (int)mPaint.measureText(text) + getPaddingLeft() + getPaddingRight();result= Math.min(v, specSize);}return result;}/*** 测量宽度* @param measureSpec* @return*/private int meaSureHeight(int measureSpec){int result=0;int specMode = MeasureSpec.getMode(measureSpec);int specSize = MeasureSpec.getSize(measureSpec);if (specMode==MeasureSpec.AT_MOST){/**文本高度+上下内边距*/int v = (int) (-mPaint.ascent() + mPaint.descent())  + getPaddingTop() + getPaddingBottom();result= Math.min(v, specSize);}return result;}public void setText(String info) {text = info;}public void setTextColor(int color) {paintColor = color;}@Overrideprotected void onDraw(Canvas canvas) {if (text == null) {  //文本为空return;}/**注意 drawText中的x,y分别指的是文字的左边位置,文字baseLine的位置*/canvas.drawText(text,getPaddingLeft(),this.this.getHeight()-mPaint.descent()-getPaddingBottom(),mPaint);}/*** 测量文字宽度* @param paint* @param str* @return*/public static int getTextWidth(Paint paint, String str) {int w= 0;if (str != null && str.length() > 0) {int len = str.length();float[] widths = new float[len];paint.getTextWidths(str, widths);for (int j = 0; j < len; j++) {w+= (int) Math.ceil(widths[j]);}}return w;}
}

效果图：

到这里，我们已经完全解决了AT_MOST的测量了。不过对于以上的代码还需要解释下：

• (-mPaint.ascent() + mPaint.descent()) 这个为啥能得到文字高度？原理可以看下这篇文章
  http://blog.csdn.net/mq2856992713/article/details/52327938（这个必须掌握）

基本的自定义view，我相信到这里你已经差不多掌握了，那么如何达到炉火纯青，信手拈来的境界呢？没有捷径，就是熟能生巧，接下来实现几个效果View。

仿华为记步View和常见的验证码View:

华为记步View代码如下:

package huangzhibo.com.learndemo.view;import android.animation.ValueAnimator;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.graphics.RectF;
import android.support.annotation.Nullable;
import android.util.AttributeSet;
import android.util.Log;
import android.view.View;import huangzhibo.com.learndemo.R;/*** Created by HuangZhiBo on 2017/7/31/031.*/public class StepView extends View {/*圆弧宽度*/private float borderWidth = 38f;/* 画步数的数值的字体大小*/private float numberTextSize = 0;/*** 开始绘制圆弧的角度*/private float startAngle = 135;/*** 终点对应的角度和起始点对应的角度的夹角*/private float angleLength = 270;/*** 所要绘制的当前步数的红色圆弧终点到起点的夹角*/private float currentAngleLength = 0;private String stepNumber;/*** 动画时长*/private int animationLength = 3000;public StepView(Context context) {super(context);}public StepView(Context context, @Nullable AttributeSet attrs) {super(context, attrs);}public StepView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);}@Overrideprotected void onDraw(Canvas canvas) {super.onDraw(canvas);/*中心点坐标*/float centerX = (getWidth()) / 2;/*指定圆弧的外轮廓矩形区域*/RectF rectF = new RectF(0 + borderWidth, borderWidth, 2 * centerX - borderWidth, 2 * centerX - borderWidth);/*绘制红色圆弧*/drawArcYellow(canvas, rectF);/*绘制蓝色走过步数*/drawArcRed(canvas, rectF);/*文字*/drawTex(canvas, rectF);}private void drawTex(Canvas canvas, RectF rectF) {Paint paint = new Paint();paint.setColor(getResources().getColor(R.color.colorAccent));paint.setAntiAlias(true);paint.setTextSize(32);int length = String.valueOf(currentAngleLength).length();canvas.drawText(currentAngleLength + "", rectF.centerX() - 70, getHeight() / 2, paint);}/*** 1.绘制总步数的黄色圆弧** @param canvas 画笔* @param rectF  参考的矩形*/private void drawArcYellow(Canvas canvas, RectF rectF) {Paint paint = new Paint();paint.setColor(getResources().getColor(R.color.colorAccent));/** 结合处为圆弧*/paint.setStrokeJoin(Paint.Join.ROUND);/** 设置画笔的样式 Paint.Cap.Round ,Cap.SQUARE等分别为圆形、方形*/paint.setStrokeCap(Paint.Cap.ROUND);/** 设置画笔的填充样式 Paint.Style.FILL  :填充内部;Paint.Style.FILL_AND_STROKE  ：填充内部和描边;  Paint.Style.STROKE  ：仅描边*/paint.setStyle(Paint.Style.STROKE);/**抗锯齿功能*/paint.setAntiAlias(true);/**设置画笔宽度*/paint.setStrokeWidth(38f);canvas.drawArc(rectF, startAngle, angleLength, false, paint);}/*** 2.绘制当前步数的蓝色圆弧** @param canvas* @param rectF*/private void drawArcRed(Canvas canvas, RectF rectF) {Paint paint = new Paint();/**设置结合处的样子，Miter:结合处为锐角， Round:结合处为圆弧：BEVEL：结合处为直线。*/paint.setStrokeJoin(Paint.Join.ROUND);paint.setStyle(Paint.Style.STROKE);//设置填充样式/*** 当画笔样式为STROKE或FILL_OR_STROKE时，设置笔刷的图形样式，如圆形样式Cap.ROUND,或方形样式Cap.SQUARE   */paint.setStrokeCap(Paint.Cap.ROUND);paint.setAntiAlias(true);//抗锯齿功能paint.setStrokeWidth(borderWidth);//设置画笔宽度paint.setColor(getResources().getColor(R.color.colorPrimary));canvas.drawArc(rectF, startAngle, currentAngleLength, false, paint);}/*** 所走的步数进度** @param totalStepNum  设置的步数* @param currentCounts 所走步数*/public void setCurrentCount(int totalStepNum, int currentCounts) {stepNumber = currentCounts + "";
/**如果当前走的步数超过总步数则圆弧还是270度，不能成为园*/if (currentCounts > totalStepNum) {currentCounts = totalStepNum;}
/**所走步数占用总共步数的百分比*/float scale = (float) currentCounts / totalStepNum;
/**换算成弧度最后要到达的角度的长度-->弧长*/float currentAngleLength = scale * angleLength;
/**开始执行动画*/setAnimation(0, currentAngleLength, animationLength);}/*** 为进度设置动画* ValueAnimator是整个属性动画机制当中最核心的一个类，属性动画的运行机制是通过不断地对值进行操作来实现的，* 而初始值和结束值之间的动画过渡就是由ValueAnimator这个类来负责计算的。* 它的内部使用一种时间循环的机制来计算值与值之间的动画过渡，* 我们只需要将初始值和结束值提供给ValueAnimator，并且告诉它动画所需运行的时长，* 那么ValueAnimator就会自动帮我们完成从初始值平滑地过渡到结束值这样的效果。** @param last* @param current*/private void setAnimation(float last, float current, int length) {ValueAnimator progressAnimator = ValueAnimator.ofFloat(last, current);progressAnimator.setDuration(length);progressAnimator.setTarget(currentAngleLength);progressAnimator.addUpdateListener(new ValueAnimator.AnimatorUpdateListener() {@Overridepublic void onAnimationUpdate(ValueAnimator animation) {currentAngleLength = (float) animation.getAnimatedValue();invalidate();}});progressAnimator.start();}
}

代码里面也注释很多了,这里就不在过多解释

常见的验证码View代码:

package huangzhibo.com.learndemo.utils.commonedite;import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.support.annotation.ColorRes;
import android.support.v4.content.ContextCompat;
import android.support.v7.widget.AppCompatEditText;
import android.text.Editable;
import android.text.TextPaint;
import android.text.TextWatcher;
import android.util.AttributeSet;
import android.util.DisplayMetrics;
import android.util.TypedValue;
import android.view.WindowManager;import huangzhibo.com.learndemo.R;/*** Created by HuangZhiBo on 2017/7/11/011.*/public class VerificationCodeEditText extends AppCompatEditText implements VerificationAction, TextWatcher {private int mFigures;private int mVerCodeMargin;private int mBottomSelectedColor; //底部选种颜色private int mBottomNormalColor;  //未选中颜色private float mBottomLineHeigth; //底部高度private int mSeleceBackgroundColor; //选中的背景颜色private OnVerificationCodeChangedListener onCodeChangeListener;private int mCurrentPosition = 0;private int mEachRectLength = 0;private Paint mSelectBackGroundPaint;private Paint mNormalBackGroundPaint;private Paint mBottomSelectdPaint;private Paint mBottomNormalPaint;public VerificationCodeEditText(Context context) {this(context, null);}public VerificationCodeEditText(Context context, AttributeSet attrs) {this(context, null,0);}public VerificationCodeEditText(Context context, AttributeSet attrs, int defStyleAttr) {super(context, attrs, defStyleAttr);initAttrs(attrs);setBackgroundColor(ContextCompat.getColor(context, android.R.color.transparent)); //防止出现下划线initPaint();setFocusableInTouchMode(true);super.addTextChangedListener(this);}/*** 初始化pain*/private void initPaint() {mSelectBackGroundPaint = new Paint();mSelectBackGroundPaint.setColor(mSeleceBackgroundColor);mNormalBackGroundPaint = new Paint();mNormalBackGroundPaint.setColor(getColor(android.R.color.transparent));mBottomSelectdPaint = new Paint();mBottomSelectdPaint.setColor(mBottomSelectedColor);mBottomNormalPaint = new Paint();mBottomNormalPaint.setColor(mBottomNormalColor);mBottomSelectdPaint.setStrokeWidth(mBottomLineHeigth);mBottomNormalPaint.setStrokeWidth(mBottomLineHeigth);}private void initAttrs(AttributeSet attrs) {TypedArray ta = getContext().obtainStyledAttributes(attrs, R.styleable.EditText);mFigures = ta.getInteger(R.styleable.EditText_figures, 4);mVerCodeMargin = (int) ta.getDimension(R.styleable.EditText_verCodeMargin, 10);mBottomSelectedColor = ta.getColor(R.styleable.EditText_bottomLineSelectedColor, getCurrentTextColor());mBottomNormalColor = ta.getColor(R.styleable.EditText_bottomLineNormalColor, getColor(android.R.color.holo_red_dark));mBottomLineHeigth = ta.getDimension(R.styleable.EditText_bottomLineHeight, dp2px(5));mSeleceBackgroundColor = ta.getColor(R.styleable.EditText_selectedBackgroundColor, getColor(android.R.color.holo_red_dark));ta.recycle();}@Overrideprotected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {int widthResult = 0, heighResult = 0;//最终宽度int widthMode = MeasureSpec.getMode(widthMeasureSpec);int widthSize = MeasureSpec.getSize(widthMeasureSpec);if (widthMode == MeasureSpec.EXACTLY) {widthResult = widthSize;} else {widthResult = getScreenWidth(getContext());}//每个矩形的宽度mEachRectLength = (widthResult - (mVerCodeMargin * (mFigures - 1))) / mFigures;//最终高度int heightMode = MeasureSpec.getMode(heightMeasureSpec);int heightSize = MeasureSpec.getSize(heightMeasureSpec);if (heightMode == MeasureSpec.EXACTLY) {heighResult = heightSize;} else {heighResult = mEachRectLength;}setMeasuredDimension(widthResult, heighResult);}@Overrideprotected void onDraw(Canvas canvas) {mCurrentPosition = getText().length();int width = mEachRectLength - getPaddingLeft() - getPaddingRight(); //每个矩形宽度int height = getMeasuredHeight() - getPaddingTop() - getPaddingBottom(); //整体高度//绘制每个矩形for (int i = 0; i < mFigures; i++) {canvas.save();int start = width * i + i * mVerCodeMargin;int end = width + start;//画一个矩形if (i == mCurrentPosition) {//选中canvas.drawRect(start, 0, end, height, mSelectBackGroundPaint);} else {canvas.drawRect(start, 0, end, height, mNormalBackGroundPaint);}canvas.restore();}//绘制文字String value = getText().toString();for (int i = 0; i < value.length(); i++) {canvas.save();int start = width * i + i * mVerCodeMargin;float x = start + width / 2;TextPaint paint = getPaint();paint.setTextAlign(Paint.Align.CENTER);paint.setColor(getCurrentTextColor());Paint.FontMetrics fontMetrics = paint.getFontMetrics();float baseLine = (height - fontMetrics.bottom + fontMetrics.top) / 2 - fontMetrics.top;//top 是个负数canvas.drawText(String.valueOf(value.charAt(i)), x, baseLine, paint);canvas.restore();}//绘制底线for (int i = 0; i < mFigures; i++) {canvas.save();float lineY = height - mBottomLineHeigth / 2;int start = width * i + i * mVerCodeMargin;int end = width + start;if (i < mCurrentPosition) {canvas.drawLine(start, lineY, end, lineY, mSelectBackGroundPaint);} else {canvas.drawLine(start, lineY, end, lineY, mBottomNormalPaint);}canvas.restore();}}/*** 获取手机屏幕的宽度*/static int getScreenWidth(Context context) {DisplayMetrics metrics = new DisplayMetrics();WindowManager wm = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);wm.getDefaultDisplay().getMetrics(metrics);return metrics.widthPixels;}@Overridefinal public void setCursorVisible(boolean visible) {super.setCursorVisible(false);//隐藏光标的显示}@Overridepublic void beforeTextChanged(CharSequence s, int start, int count, int after) {mCurrentPosition = getText().length();postInvalidate();}@Overridepublic void onTextChanged(CharSequence s, int start, int before, int count) {mCurrentPosition = getText().length();postInvalidate();if (onCodeChangeListener != null) {onCodeChangeListener.onVerCodeChanged(getText(), start, before, count);}}@Overridepublic void afterTextChanged(Editable s) {mCurrentPosition = getText().length();postInvalidate();if (getText().length() == mFigures) {if (onCodeChangeListener != null) {onCodeChangeListener.onInputCompleted(getText());}} else if (getText().length() > mFigures) {getText().delete(mFigures, getText().length());}}@Overridepublic void setFigures(int figures) {mFigures = figures;postInvalidate();}@Overridepublic void setVerCodeMargin(int margin) {mVerCodeMargin = margin;postInvalidate();}@Overridepublic void setBottomSelectedColor(@ColorRes int bottomSelectedColor) {mBottomSelectedColor = bottomSelectedColor;postInvalidate();}@Overridepublic void setBottomNormalColor(@ColorRes int bottomNormalColor) {mBottomNormalColor = bottomNormalColor;postInvalidate();}@Overridepublic void setSelectedBackgroundColor(@ColorRes int selectedBackground) {mSeleceBackgroundColor = selectedBackground;postInvalidate();}@Overridepublic void setBottomLineHeight(int bottomLineHeight) {mBottomLineHeigth=bottomLineHeight;postInvalidate();}@Overridepublic void setOnVerificationCodeChangedListener(OnVerificationCodeChangedListener listener) {this.onCodeChangeListener=listener;}/*** 返回颜色*/private int getColor(@ColorRes int color) {return ContextCompat.getColor(getContext(), color);}/*** dp转px*/private int dp2px(int dp) {return (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, dp,getResources().getDisplayMetrics());}}

对于自定义验证码的View,需要考虑超出文字字数处理,文字变化监听处理,相对于这个demo来说还是有一点难度,需要大家慢慢的去理解和消化。这里把几个要点总结下，以便更好的去理解这个实现逻辑

• 初始界面，绘画一个矩形（分为选中和未选中），底部横线，绘制文字
• 文字输入监听，重新绘制
• 文字超出处理

以上的demo已经放在网上仓库，需要的话可以点击https://git.oschina.net/huagnzhibo123/LearnDemo

这个demo有我最近在整理的项目常用的工具类和框架，会不断完善。