java画布矩形位置

Question

我是Android上画布的新手。我试图在屏幕中央画一个正方形，我想在单击按钮时增大或减小正方形的大小。问题是我不明白正方形的位置。 当我添加这样的代码时，它的位置正确（5左5顶）：canvas.drawRect(5, 5, sideLength, sideLength, mPaint); 但是我想在屏幕中央画这个正方形，就像我之前提到的。 我已经试过了，但是方块没有出现在屏幕上：canvas.drawRect(getWidth()/2, getHeight()/2, sideLength, sideLength, mPaint);

如果你能帮我找到解决办法，那就太好了

主要活动。xml

<安卓.support.constraint.ConstraintLayout 
xmlns:安卓="http://schemas.安卓.com/apk/res/安卓"
xmlns:app="http://schemas.安卓.com/apk/res-auto"
xmlns:tools="http://schemas.安卓.com/tools"
安卓:id="@+id/linearLayout"
安卓:layout_width="match_parent"
安卓:layout_height="match_parent"
安卓:orientation="vertical"
tools:context=".MainActivity" >

<FrameLayout
    安卓:id="@+id/frameLayout"
    安卓:layout_width="match_parent"
    安卓:layout_height="0dp"
    app:layout_constraintBottom_toTopOf="@id/button"
    app:layout_constraintTop_toTopOf="parent"/>

<Button
    安卓:id="@+id/button"
    安卓:layout_width="match_parent"
    安卓:layout_height="wrap_content"
    安卓:text="Button"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintEnd_toEndOf="parent"/>

</安卓.support.constraint.ConstraintLayout>

主要活动。爪哇

public class MainActivity extends AppCompatActivity {

FrameLayout frameLayout;

MyView myView;

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

    frameLayout = findViewById(R.id.frameLayout);

    myView = new MyView(this,200);

    frameLayout.addView(myView);

    findViewById(R.id.button).setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            frameLayout.removeView(myView);
            myView = new MyView(MainActivity.this,600);

            frameLayout.addView(myView);
        }
    });
}
}

我的观点。爪哇

public class MyView extends View {

Paint mPaint;
int sideLength;

public MyView(Context context){
    super(context);
}

public MyView(Context context,int sideLength) {
    super(context);

    this.sideLength = sideLength;

    mPaint = new Paint();
    mPaint.setAntiAlias(true);

    mPaint.setStyle(Paint.Style.FILL);
    mPaint.setColor(Color.BLUE);

}

@Override
protected void onDraw(Canvas canvas) {
    super.onDraw(canvas);

    canvas.drawRect(getWidth()/2, getHeight()/2, sideLength, sideLength, mPaint);
}

}

Answer 1

getWidth()/2和getheight()/2将为您提供容器的中心坐标。 假设您的容器是一个尺寸为(2000, 2000) in pixels的完美正方形，那么您的中心坐标将是（10001000）。记住这一点，您正试图在坐标1000处绘制正方形的左侧，在坐标1000处绘制正方形的顶部

现在下一部分就是问题所在。您正试图在坐标600处绘制右侧，在坐标600处绘制底部

绘制矩形和正方形时，应注意以下事项：

1. 右坐标应大于左坐标
2. 底部坐标应大于顶部坐标

下面是在FrameLayout的中心绘制正方形的工作代码

MyView.java

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

public class MyView extends View {

    Paint mPaint;
    int sideLength;
    PointF center;

    public MyView(Context context) {
        super(context);
    }

    public MyView(Context context, int sideLength) {
        super(context);

        this.sideLength = sideLength;

        mPaint = new Paint();
        mPaint.setAntiAlias(true);

        mPaint.setStyle(Paint.Style.FILL);
        mPaint.setColor(Color.BLUE);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawRect(center.x - sideLength, center.y - sideLength, center.x + sideLength, center.y + sideLength, mPaint);
    }

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        int y = getHeight() / 2;
        int x = getWidth() / 2;

        center = new PointF(x, y);
    }
}

activity_main.xml

<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:id="@+id/linearLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".MainActivity" >

    <FrameLayout
        android:id="@+id/frameLayout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="#dedede"
        app:layout_constraintBottom_toTopOf="@id/button"
        app:layout_constraintTop_toTopOf="parent"/>

    <Button
        android:id="@+id/button"
        android:layout_width="match_parent"
        android:layout_height="56dp"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"/>

</androidx.constraintlayout.widget.ConstraintLayout>