java - 如何在 View 上绘制 Canvas

Question

我想使用 Canvas 在 View 上绘制文本。

这是我尝试执行此操作的一些代码。

public class MainActivity extends AppCompatActivity {


View view;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    view=findViewById(R.id.viewOfText);


    Canvas canvas = new Canvas();
    Paint paint = new Paint();
    paint.setColor(Color.BLACK);
    paint.setStyle(Paint.Style.FILL);
    paint.setTextSize(40f);
    canvas.drawText("Text", 10, 25, paint);

    view.draw(canvas);
    view.invalidate();



}
}

但我没有看到任何结果

Answer 1

为 Canvas 创建扩展 View 的新类的更常用方法

例如

DrawText.java

package com.test.canvas;

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

public class DrawText extends View {
    Paint paint = new Paint();

    private void init() {
        paint.setColor(Color.BLACK);
    }

    public DrawText(Context context) {
        super(context);
        init();
    }

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

    public DrawText(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
        init();
    }

    @Override
    public void onDraw(Canvas canvas) {
        paint.setColor(Color.BLACK);
        paint.setStyle(Paint.Style.FILL);
        paint.setTextSize(40f);
        canvas.drawText("Text", 100, 100, paint);
    }

}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns: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:layout_height="match_parent"
    tools:context=".MainActivity">


    <com.test.canvas.DrawText
        android:id="@+id/DrawText"
        android:layout_width="match_parent"
        android:layout_height="match_parent">

    </com.test.canvas.DrawText>
</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.java

package com.test.canvas;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        setContentView(R.layout.activity_main);
    }
}