自定义控件——视图的构建过程——视图的绘制方法

一时失言乱红尘 2024-04-07 12:37 32阅读 0赞

视图绘制有两种方法，分别是onDraw和dispatchDraw，它们的区别主要有下列两点：

(1)onDraw既可用于普通控件，也可用于布局类视图；而dispatchDraw专门用于布局类视图，像线性布局LinearLayout、相对布局RelativeLayout都属于布局类视图。

(2)onDraw方法先执行，dispatchDraw方法后执行，这两个方法中间再执行下级视图的绘制方法。

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xml布局：

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代码：

package com.example.myapplication;
    
    import androidx.appcompat.app.AppCompatActivity;
    import android.os.Bundle;
    import android.view.View;
    import android.widget.AdapterView;
    import android.widget.ArrayAdapter;
    import android.widget.Button;
    import android.widget.Spinner;
    import com.example.myapplication.widget.DrawRelativeLayout;
    
    public class MainActivity extends AppCompatActivity
    {
    
        private DrawRelativeLayout drl_content; // 声明一个绘画布局对象
        private Button btn_center;
    
        @Override
        protected void onCreate(Bundle savedInstanceState)
        {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
    
    
            // 从布局文件中获取名叫drl_content的绘画布局
            drl_content = findViewById(R.id.drl_content);
            btn_center = findViewById(R.id.btn_center);
    
            initTypeSpinner(); // 初始化绘图方式的下拉框
        }
    
        // 初始化绘图方式的下拉框
        private void initTypeSpinner()
        {
            ArrayAdapter<String> drawAdapter = new ArrayAdapter<String>(this, R.layout.item_select, descArray);
    
            Spinner sp_draw = findViewById(R.id.sp_draw);
    
            sp_draw.setPrompt("请选择绘图方式");
            sp_draw.setAdapter(drawAdapter);
            sp_draw.setOnItemSelectedListener(new DrawSelectedListener());
            sp_draw.setSelection(0);
        }
    
        private String[] descArray = {"不画图", "画矩形", "画圆角矩形", "画圆圈", "画椭圆", "onDraw画叉叉", "dispatchDraw画叉叉"};
    
        private int[] typeArray = {0, 1, 2, 3, 4, 5, 6};
    
        class DrawSelectedListener implements AdapterView.OnItemSelectedListener
        {
    
            public void onItemSelected(AdapterView<?> arg0, View arg1, int arg2, long arg3)
            {
                int type = typeArray[arg2];
    
                if (type == 5 || type == 6)
                {
                    btn_center.setVisibility(View.VISIBLE);
                }
                else
                {
                    btn_center.setVisibility(View.GONE);
                }
    
                drl_content.setDrawType(type); // 设置绘图布局的绘制类型
            }
    
            public void onNothingSelected(AdapterView<?> arg0) {}
        }
    
    }

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DrawRelativeLayout

package com.example.myapplication.widget;
    
    import android.content.Context;
    import android.graphics.Canvas;
    import android.graphics.Color;
    import android.graphics.Paint;
    import android.graphics.Paint.Style;
    import android.graphics.Rect;
    import android.graphics.RectF;
    import android.util.AttributeSet;
    import android.widget.RelativeLayout;
    
    public class DrawRelativeLayout extends RelativeLayout
    {
        private int mDrawType = 0; // 绘制类型
    
        private Paint mPaint = new Paint(); // 创建一个画笔对象
    
        private int mStrokeWidth = 3; // 线宽
    
        public DrawRelativeLayout(Context context)
        {
    
            this(context, null);
        }
    
        public DrawRelativeLayout(Context context, AttributeSet attrs)
        {
    
            super(context, attrs);
            mPaint.setAntiAlias(true); // 设置画笔为无锯齿
            mPaint.setDither(true); // 设置画笔为防抖动
            mPaint.setColor(Color.BLACK); // 设置画笔的颜色
            mPaint.setStrokeWidth(mStrokeWidth); // 设置画笔的线宽
            mPaint.setStyle(Style.STROKE); // 设置画笔的类型。STROKE表示空心，FILL表示实心
        }
    
        // onDraw方法在绘制下级视图之前调用
        @Override
        protected void onDraw(Canvas canvas)
        {
    
            super.onDraw(canvas);
    
            int width = getMeasuredWidth(); // 获得布局的实际宽度
            int height = getMeasuredHeight(); // 获得布局的实际高度
    
            if (width > 0 && height > 0) {
                if (mDrawType == 1) { // 绘制矩形
                    Rect rect = new Rect(0, 0, width, height);
                    canvas.drawRect(rect, mPaint); // 在画布上绘制矩形
                } else if (mDrawType == 2) { // 绘制圆角矩形
                    RectF rectF = new RectF(0, 0, width, height);
                    canvas.drawRoundRect(rectF, 30, 30, mPaint); // 在画布上绘制圆角矩形
                } else if (mDrawType == 3) { // 绘制圆圈
                    int radius = Math.min(width, height) / 2 - mStrokeWidth;
                    canvas.drawCircle(width / 2, height / 2, radius, mPaint); // 在画布上绘制圆圈
                } else if (mDrawType == 4) { // 绘制椭圆
                    RectF oval = new RectF(0, 0, width, height);
                    canvas.drawOval(oval, mPaint); // 在画布上绘制椭圆
                } else if (mDrawType == 5) { // 绘制矩形及其对角线
                    Rect rect = new Rect(0, 0, width, height);
                    canvas.drawRect(rect, mPaint); // 绘制矩形
                    canvas.drawLine(0, 0, width, height, mPaint); // 绘制左上角到右下角的线段
                    canvas.drawLine(0, height, width, 0, mPaint); // 绘制左下角到右上角的线段
                }
            }
        }
    
        // dispatchDraw方法在绘制下级视图之前调用
        @Override
        protected void dispatchDraw(Canvas canvas)
        {
            super.dispatchDraw(canvas);
    
            int width = getMeasuredWidth(); // 获得布局的实际宽度
            int height = getMeasuredHeight(); // 获得布局的实际高度
    
            if (width > 0 && height > 0)
            {
                if (mDrawType == 6) { // 绘制矩形及其对角线
                    Rect rect = new Rect(0, 0, width, height);
                    canvas.drawRect(rect, mPaint); // 绘制矩形
                    canvas.drawLine(0, 0, width, height, mPaint); // 绘制左上角到右下角的线段
                    canvas.drawLine(0, height, width, 0, mPaint); // 绘制左下角到右上角的线段
                }
            }
        }
    
        // 设置绘制类型
        public void setDrawType(int type)
        {
            setBackgroundColor(Color.WHITE); // 背景置为白色，目的是把画布擦干净
    
            mDrawType = type;
    
            invalidate(); // 立即重新绘图，此时会触发onDraw方法和dispatchDraw方法
        }
    }

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