6.4 Android绘图技巧(Primary：四大方法&Layer)-白红宇

1.Canvas的四大金刚

Canvas.save()
这个方法从字面上可以理解为保存画布，作用就是将之前的所有已绘制的图像保存起来。让后续的操作就好像在一个新的图层上操作一样，这一点与Photoshop中的图层理解基本一致。

Canvas.restore()
可以理解为Photoshop中的合并图层操作，作用是将我们在save()之后绘制的所有图像与save()之前的图像进行合并。

Canvas.translate()
Android默认绘图坐标零点位于屏幕左上角，那么在调用translate()之后，则将零点(0,0)移动到了(x,y)。之后所有绘图操作都将以(x,y)为原点执行。

Canvas.rotate()
与translate()同理，旋转坐标系一个一定的角度。

2.Demo：仪表盘

2.1.画外圆

2.2.画刻度和刻度值

2.3.画指针

2.4.全代码和运行结果

Clock.java:

package com.yishengxu.myapplication;import android.content.Context;import android.graphics.Canvas;import android.graphics.Paint;import android.util.AttributeSet;import android.view.View;public class Clock extends View {    private int mHeight, mWidth;    public Clock(Context context) {        super(context);    }    public Clock(Context context, AttributeSet attrs) {        super(context, attrs);    }    public Clock(Context context, AttributeSet attrs, int defStyleAttr) {        super(context, attrs, defStyleAttr);    }    @Override    protected void onDraw(Canvas canvas) {        // 获取宽高参数        mWidth = getMeasuredWidth();        mHeight = getMeasuredHeight();        // 画外圆        Paint paintCircle = new Paint();        paintCircle.setStyle(Paint.Style.STROKE);        paintCircle.setAntiAlias(true);        paintCircle.setStrokeWidth(5);        canvas.drawCircle(mWidth / 2,                mHeight / 2, mWidth / 2, paintCircle);        // 画刻度        Paint painDegree = new Paint();        paintCircle.setStrokeWidth(3);        for (int i = 0; i < 24; i++) {            // 区分整点与非整点            if (i == 0 || i == 6 || i == 12 || i == 18) {                painDegree.setStrokeWidth(5);                painDegree.setTextSize(30);                canvas.drawLine(mWidth / 2, mHeight / 2 - mWidth / 2,//基线起点x                        mWidth / 2, mHeight / 2 - mWidth / 2 + 60,//基线起点y                        painDegree);                String degree = String.valueOf(i);//Integer.toString(i)                canvas.drawText(degree,                        mWidth / 2 - painDegree.measureText(degree) / 2,//measureText()在画布上输出文本之前，检查字体的宽度：                        mHeight / 2 - mWidth / 2 + 90,                        painDegree);            } else {                painDegree.setStrokeWidth(3);                painDegree.setTextSize(15);                canvas.drawLine(mWidth / 2, mHeight / 2 - mWidth / 2,                        mWidth / 2, mHeight / 2 - mWidth / 2 + 30,                        painDegree);                String degree = String.valueOf(i);                canvas.drawText(degree,                        mWidth / 2 - painDegree.measureText(degree) / 2,                        mHeight / 2 - mWidth / 2 + 60,                        painDegree);            }            // 通过旋转画布简化坐标运算            canvas.rotate(15, mWidth / 2, mHeight / 2);//二三参数为枢轴点的xy，枢轴点即旋转中心        }        // 画圆心        Paint paintPointer = new Paint();        paintPointer.setStrokeWidth(30);        canvas.drawPoint(mWidth / 2, mHeight / 2, paintPointer);        // 画指针        Paint paintHour = new Paint();        paintHour.setStrokeWidth(20);        Paint paintMinute = new Paint();        paintMinute.setStrokeWidth(10);        canvas.save();//只是保存“缓冲区”绘制的内容        canvas.translate(mWidth / 2, mHeight / 2);        canvas.drawLine(0, 0, 100, 100, paintHour);        canvas.drawLine(0, 0, 100, 200, paintMinute);        canvas.restore();//将“缓冲区”绘制的内容和已经save()的内容一同合并并保存起来，这里跟上边的save注意区分开来    }}

MainActivity.java:

package com.yishengxu.myapplication;import android.app.Activity;import android.os.Bundle;public class MainActivity extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(new Clock(this));    }}

效果图：

3.Layer图层

创建一个新的Layer到“栈”中，可以使用saveLayer(), savaLayerAlpha(), 从“栈”中推出一个Layer，可以使用restore(),restoreToCount()。但Layer入栈时，后续的DrawXXX操作都发生在这个Layer上，而Layer退栈时，就会把本层绘制的图像“绘制”到上层或是Canvas上，在复制Layer到Canvas上时，可以指定Layer的透明度

透明度：

127，半透明

255，完全不透明

0，完全透明
实例如Demo下图：

上Demo：

package com.imooc.myapplication;import android.app.Activity;import android.content.Context;import android.graphics.Canvas;import android.graphics.Color;import android.graphics.Paint;import android.os.Bundle;import android.view.View;public class MainActivity extends Activity {    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(new MyLayer(this));    }    public class MyLayer extends View {        private Paint mPaint;        private static final int LAYER_FLAGS =                        Canvas.MATRIX_SAVE_FLAG |                        Canvas.CLIP_SAVE_FLAG |                        Canvas.HAS_ALPHA_LAYER_SAVE_FLAG |                        Canvas.FULL_COLOR_LAYER_SAVE_FLAG |                        Canvas.CLIP_TO_LAYER_SAVE_FLAG;//此乃API定义的常量，ctrl+E 进入文档查看便知晓其含义        public MyLayer(Context context) {            super(context);            mPaint = new Paint(Paint.ANTI_ALIAS_FLAG);        }        @Override        protected void onDraw(Canvas canvas) {            canvas.drawColor(Color.WHITE);//背景            mPaint.setColor(Color.BLUE);            canvas.drawCircle(150, 150, 100, mPaint);//“零图层”            canvas.saveLayerAlpha(0, 0, 400, 400, 127, LAYER_FLAGS);            mPaint.setColor(Color.RED);            canvas.drawCircle(200, 200, 100, mPaint);            canvas.restore();        }    }}

半透明：

完全不透明： canvas.saveLayerAlpha(0, 0, 400, 400, 255, LAYER_FLAGS);

image.png

完全透明： canvas.saveLayerAlpha(0, 0, 400, 400, 0, LAYER_FLAGS);