【Android】 自定義View那點事（一）基礎篇

前言

在Android開發中常常會遇到特殊的需求要求開發者實現獨特的視圖UI。因此對於開發者來講自定義View能夠實現多種多樣的需求效果。那此次咱們就來講說關於自定義View的實現。

View繪製原理

View的繪製是在方法onDraw中實現的。經過獲取View的畫布canvas，使用Panit在canvas上繪製內容。

public class CustomerView extends View {
  @Override
  protected void onDraw(Canvas canvas) {
  
  }
}

畫筆Paint

就和咱們實際畫畫同樣，繪製須要一支畫筆才行。Paint就是咱們繪製所須要的畫筆。實例化Paint並設置繪製所須要的參數就能繪製出本身所須要的內容。Android開發文檔中Paint實現方法不少，這裏咱們介紹幾個經常使用基本的方法。

setAlpha(bookean a) //設置抗鋸齒
setColor(int color) //設置畫筆顏色
setShader(Shader shader)//設置畫筆漸變顏色
setStyle(Paint.Style style)//設置畫筆模式（例如實心/空心）
setXfermode(Xfermode xfermode)//設置畫筆繪製模式
setStrokeWidth(float width) //設置畫筆粗細

畫布Canvas

有了畫筆，如今就是要用畫筆在畫布上畫畫了。想要繪製怎麼樣的圖形是經過Canvas的方法實現的。咱們也介紹幾個經常使用基本的繪製方法。只要理科學的好人人都是現代派畫家O(∩_∩)O~。

drawARGB(int a, int r, int g, int b)//設置畫布背景顏色
drawColor(int color)//設置畫布背景顏色
drawRect(float left, float top, float right, float bottom, Paint paint)//繪製矩形
drawPath(Path path, Paint paint)//繪製自定義圖形
drawCircle(float cx, float cy, float radius, Paint paint)//繪製原型
drawArc(float left, float top, float right, float bottom, float startAngle, float sweepAngle, boolean useCenter, Paint paint)//繪製弧以及圓
drawLine(float startX, float startY, float stopX, float stopY, Paint paint)//繪製線條
drawText(String text, int start, int end, float x, float y, Paint paint)//繪製文字
drawPoint(float x, float y, Paint paint)//繪製點
drawRoundRect(float left, float top, float right, float bottom, float rx, float ry, Paint paint)//繪製圓角矩形
drawOval(RectF oval, Paint paint)//繪製圓
drawBitmap(Bitmap bitmap, float left, float top, Paint paint) //繪製圖片

畫起來

知道了畫筆Paint和畫布Canavs的基本方法，最後咱們實踐畫一畫爲成爲畫家邁出第一步。

public class NormalView extends BaseView {
    Path triangle2;
    Path all;
    private float[] mTickRightStartPosition = new float[2];
    public NormalView(Context context) {
        super(context);
    }

    public NormalView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

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

    void initView(){
        mPaint = new Paint();
        mPaint.setColor(Color.BLACK);
        mPaint.setStrokeWidth(5);
        mPaint.setAntiAlias(true);
    }
    
    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        mCanVas = canvas;
        //繪製正方形
        mCanVas.drawRect(20,40,60,80,mPaint);
        //繪製矩形
        mCanVas.drawRect(20,100,60,160,mPaint);
        //繪製三角形
        Path path = new Path();
        path.moveTo(60, 200);// 此點爲多邊形的起點
        path.lineTo(40, 250);
        path.lineTo(80, 250);
        path.close(); // 使這些點構成封閉的多邊形
        mCanVas.drawPath(path,mPaint);
        //畫圓
        mCanVas.drawCircle(40,400,40,mPaint);
        //畫文字
        mPaint.setTextSize(50);
        mCanVas.drawText("Hello world!",0,12,40,500,mPaint);
        //畫圖片
        Bitmap bitmap = BitmapFactory.decodeResource(this.getResources(), R.mipmap.ic_launcher);
        mCanVas.drawBitmap(bitmap,40,600,mPaint);
        //畫背景色
        mCanVas.drawARGB(100,100,100,100);
        //畫弧以及畫橢圓
        RectF rectF = new RectF();
        Rect rect = new Rect(40,750,150,800);
        rectF.set(rect);
        mCanVas.drawArc(rectF,0,-180,true,mPaint);
        //畫線
        mCanVas.drawLine(40,850,100,900,mPaint);
        //畫點
        mCanVas.drawPoint(40,950,mPaint);
        //畫圓角矩形
        RectF roundRectF = new RectF();
        Rect roundRect = new Rect(40,1000,400,1050);
        roundRectF.set(roundRect);
        mCanVas.drawRoundRect(roundRectF,20,5,mPaint);
        //畫圓
        RectF OvalRectF = new RectF();
        Rect OvalRect = new Rect(40,1100,60,1200);
        OvalRectF.set(OvalRect);
        mCanVas.drawOval(OvalRectF,mPaint);

        Path hexagon = new Path();
        hexagon.moveTo(40,1400);
        hexagon.lineTo(90,1350);
        hexagon.lineTo(140,1350);
        hexagon.lineTo(190,1400);
        hexagon.lineTo(140,1450);
        hexagon.lineTo(90,1450);
        hexagon.lineTo(40,1400);
        hexagon.close();
        mCanVas.drawPath(hexagon,mPaint);

        Path triangle1 = new Path();
        triangle1.moveTo(500,100);
        triangle1.lineTo(500,300);
        triangle1.lineTo(700,300);
        triangle1.lineTo(500,100);
        triangle1.close();

        triangle2 = new Path();
        triangle2.moveTo(700,100);
        triangle2.lineTo(900,100);
        triangle2.lineTo(900,300);
        triangle2.lineTo(700,100);
        triangle2.close();

        all = new Path();
        all.addPath(triangle1);
        all.addPath(triangle2);
        mCanVas.drawPath(all,mPaint);
    }
}

實踐成功！咱們接下來還要繼續努力，成爲畫家這仍是剛開始呢。下一次咱們繼續學習關於View的內容。Fighting！