android studio 用canvas画图drawline 不显示

Question

class CustomView extends View{
    Paint paint;
    public CustomView(Context context) {
        super(context);
        paint = new Paint(); 
        paint.setColor(Color.YELLOW);
        paint.setStrokeJoin(Paint.Join.ROUND);
        paint.setStrokeCap(Paint.Cap.ROUND);
        paint.setStrokeWidth(10f);
    }
   
    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawColor(Color.RED);   
        canvas.drawLine(10, 10, 100, 100, paint);}
}

为什么代码可以运行的 但是画的线没显示

Answer 1

public class CustomView extends View {
    Paint paint;

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

    public void init() {
        paint = new Paint();
        paint.setColor(Color.YELLOW);
        paint.setStrokeJoin(Paint.Join.ROUND);
        paint.setStrokeCap(Paint.Cap.ROUND);
        paint.setStrokeWidth(10f);
    }

    public CustomView(Context context, @Nullable AttributeSet attrs) {
        super(context, attrs);
        init();

    }

    public CustomView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawColor(Color.RED);
        canvas.drawLine(10, 10, 100, 100, paint);
    }
}

这是自定义view,
然后去你的layout文件夹下的资源文件,比如activity_main.xml,加入你的View
比如:

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

    <你项目的包名.CustomView
        android:layout_width="match_parent"
        android:layout_height="match_parent" />
</androidx.constraintlayout.widget.ConstraintLayout>