Android的View類是使用者介面的基礎構件，表示螢幕上的一塊矩形區域，負責這個區域的繪製和事件處理。自定義View的過程主要包括重寫onDraw及onMeasure方法 , 其中onMeasure方法的作用就是計算出自定義View的寬度和高度。這個計算的過程會參照父佈局給出的大小（widthMeasureSpec和heightMeasureSpec），以及自己特點算出結果 ；onDraw則根據onMeasure測量後的寬高進行介面的繪製。onDraw主要用到兩個類，Canvas和Paint。Canvas畫布，相當於現實中畫圖用的紙或布；Paint畫筆，相當於現實中的筆，基本方法有 drawLine 繪製直線 ，drawRect繪製矩形 ， drawCirlce繪製圓形。

自定義圓形進度條效果如下

完整程式碼：

package com.circleprogress;

import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.graphics.RectF;
import android.util.AttributeSet;
import android.view.View;

/**
 * Created by Administrator on 2017/6/22.
 */
 


public class CirlceView extends View {

    //定義第一支畫筆，畫背景的圓
    private Paint mPaintBackCircle;
    //定義第二支畫筆，畫前景色的圓（進度條的環）
    private Paint mPaintFrontCircle;

    //定義繪製文字的畫筆
    private Paint mPaintText;

    //定義環的寬度
    private float mStrokeWidth = 50 ;
    private float mhalfStrokeWidth = mStrokeWidth/2
 
;

    //定義圓心座標和半徑
    private float mX = 200 + mhalfStrokeWidth ;
    private float mY = 200 + mhalfStrokeWidth ;

    private float mRadius = 200 ;

    //定義矩形
    private RectF mRectF ;

    //開始進度
    private int mProgress = 0 ;
    //目標進度
    private int mTargetProgress = 70 ;
    //最大進度
    private int mMax = 100 ;

    //定義view的寬高
    private int mWidth ;
    private int mHeight ;

    //預設半徑
    private static final int DEFAULT_RADIUS = 200 ;
    private static final int DEFAULT_STROKE_WIDTH = 50 ;

    public CirlceView(Context context) {
        this(context,null);
    }

    public CirlceView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }

    public CirlceView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);

        //獲取自定義屬性
        if( attrs != null ){
            TypedArray array  = context.obtainStyledAttributes(attrs , R.styleable.CirlceView);

            mRadius = array.getDimensionPixelSize(R.styleable.CirlceView_radius , DEFAULT_RADIUS);
            mStrokeWidth = array.getDimensionPixelSize(R.styleable.CirlceView_stroke_width,DEFAULT_STROKE_WIDTH);

            array.recycle();
        }
        init();
    }

    private void initRect(){

        if(mRectF == null ){
            mRectF = new RectF();
            int viewSize = (int) (mRadius*2);

            int left = (mWidth - viewSize)/2 ;
            int top = (mHeight - viewSize)/2 ;
            int right = left + viewSize ;
            int bottom = top + viewSize ;
            mRectF.set(left,top,right,bottom);
        }
    }

    //初始化畫筆
    private void init(){
        mPaintBackCircle = new Paint();
        //設定顏色
        mPaintBackCircle.setColor(Color.WHITE);
        //設定防鋸齒
        mPaintBackCircle.setAntiAlias(true);
        //設定為空心
        mPaintBackCircle.setStyle(Paint.Style.STROKE);
        mPaintBackCircle.setStrokeWidth(mStrokeWidth);

        mPaintFrontCircle = new Paint();
        mPaintFrontCircle.setColor(0xFF66C796);
        mPaintFrontCircle.setAntiAlias(true);
        mPaintFrontCircle.setStyle(Paint.Style.STROKE);
        mPaintFrontCircle.setStrokeWidth(mStrokeWidth);

        mPaintText = new Paint();
        mPaintText.setColor(0xFF66C796);
        mPaintText.setAntiAlias(true);
        mPaintText.setTextSize(50);
        //設定文字居中
        mPaintText.setTextAlign(Paint.Align.CENTER);
    }

    @Override
    protected void onDraw(Canvas canvas) {

        initRect();
        //獲取進度百分比
        float angle = mProgress/(float)mMax * 360 ;

        canvas.drawCircle(mWidth/2 , mHeight/2 , mRadius , mPaintBackCircle );
        canvas.drawArc(mRectF , -90 , angle , false ,mPaintFrontCircle);
        canvas.drawText(mProgress+"%", mWidth/2  , mHeight/2 , mPaintText);

        if( mProgress < mTargetProgress) {
            //更新進度
            mProgress += 2;
            //通知重新繪製
            invalidate();
        }
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);

        //測量寬高
        mWidth = getRealSize(widthMeasureSpec);
        mHeight = getRealSize(heightMeasureSpec);

        //儲存測量的寬高
        setMeasuredDimension(mWidth,mHeight);
    }

    //測量View的真實尺寸
    public int getRealSize( int measureSpec ){

        int result = -1 ;

        int mode = MeasureSpec.getMode(measureSpec);
        int size = MeasureSpec.getSize(measureSpec);

        if( mode == MeasureSpec.AT_MOST || mode == MeasureSpec.UNSPECIFIED){
            //自己計算
            result = (int) (mRadius * 2 + mStrokeWidth);
        }else {
            result = size ;
        }

        return result;
    }
}

在values目錄下新建attrs.xml檔案 , 增加自定義屬性

<?xml version="1.0" encoding="utf-8"?>
<resources>
<declare-styleable name="CirlceView">
    <attr name="radius" format="dimension"></attr>
    <attr name="stroke_width" format="dimension"></attr>
    <attr name="back_circle_color" format="color"></attr>
    <attr name="front_arc_color" format="color"></attr>
    <attr name="text_visibility" format="boolean"></attr>
</declare-styleable>

</resources>

最後在佈局檔案中引入，設定自定義寬高和半徑

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context="com.circleprogress.MainActivity">

    <com.circleprogress.CirlceView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        app:radius="100dp"
        app:stroke_width="20dp"/>
</RelativeLayout>