android - 你能解释一下 TextView + Gravity + SingleLine 和 Canvas 的行为吗？

Question

我有一个自定义 TextView，它的 onDraw 方法被覆盖以在 TextView 周围绘制红色边框。

当我将重力设置为左侧以外的其他值时 AND singleLine=true :不绘制边框。

• 有什么理由吗？
• 有什么解决方法吗？

以下屏幕截图说明了问题:第二个 textView 没有边框

_{TL 表示 TOP|LEFT ； C 表示 CENTER ； SL表示单线； T 表示真实； F 表示错误}

这是代码(可以复制/粘贴并运行 - 不需要布局文件)

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.Gravity;
import android.widget.RelativeLayout;
import android.widget.TextView;


public class MyActivity extends Activity {

    private final int TEXT_VIEW_WIDTH_PX = 400;
    private final int TEXT_VIEW_HEIGHT_PX = 60;
    private Paint borderPaint = new Paint();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        RelativeLayout rootView = new RelativeLayout(this);
        getWindow().setContentView(rootView);

        borderPaint.setColor(Color.RED);

        MyTextView text1 = makeTextView("Grav=TL ; SL=F",100);
        rootView.addView(text1);

        MyTextView text2 = makeTextView("Grav=C ; SL=T",200);
        //combination of the 2 following attributes cause the issue
        text2.setSingleLine(true);
        text2.setGravity(Gravity.CENTER);
        rootView.addView(text2);

        MyTextView text3 = makeTextView("Grav=C ; SL=F",300);
        text3.setGravity(Gravity.CENTER);
        rootView.addView(text3);

        MyTextView text4 = makeTextView("Grav=TL ; SL=T",400);
        text4.setSingleLine(true);
        rootView.addView(text4);
    }

    /**
     * Custom TextView with red border
     */
    private class MyTextView extends TextView {

        private MyTextView(Context context) {
            super(context);
        }

        @Override
        protected void onDraw(Canvas canvas) {
            super.onDraw(canvas);
            canvas.drawLine(1,1,1,TEXT_VIEW_HEIGHT_PX-1,borderPaint);
            canvas.drawLine(1,1,TEXT_VIEW_WIDTH_PX-1,1,borderPaint);
            canvas.drawLine(TEXT_VIEW_WIDTH_PX-1,1,TEXT_VIEW_WIDTH_PX-1,TEXT_VIEW_HEIGHT_PX-1,borderPaint);
            canvas.drawLine(1,TEXT_VIEW_HEIGHT_PX-1,TEXT_VIEW_WIDTH_PX-1,TEXT_VIEW_HEIGHT_PX-1,borderPaint);
        }
    }


    /**
     * create a MyTextView with 'text' context and located at x=50 ; y=marginTop
     * (nothing relevant for the issue here)
     */
    private MyTextView makeTextView(String text, int marginTop){
        MyTextView textView = new MyTextView(this);
        textView.setText(text);
        RelativeLayout.LayoutParams lp = new RelativeLayout.LayoutParams(TEXT_VIEW_WIDTH_PX,TEXT_VIEW_HEIGHT_PX);
        lp.topMargin = marginTop;
        lp.leftMargin = 50;
        textView.setLayoutParams(lp);
        return textView;
    }

}

Answer 1

感谢 pskink 的深入研究(来自对我的另一个答案的评论):

If you want to know why custom onDraw doesn't draw anything, just Log.d the value of canvas.getMatrix().

[It] seems that canvas is translated/horizontally scrolled (at least in my case) 8159 pixels to the left, so calling canvas.translate(8159, 0) fixes the issue, of course 8159 is not a magic number and can vary.

I found it, see VERY_WIDE constant in TextView, it is set to 16384 (2**14), in my case TextView has width of 66 and now (16384-66)/2 == 8159, voila!

...but VERY_WIDE is private so you cannot access it :-(

从这里开始，我想知道是否可以通过编程方式检索偏移量，并且确实可以通过 getScrollX() 轻松检索。这种方法通过取消水平滚动来平移 Canvas 而不是“破解”单行。单线显示更自然。

在自定义 TextView 中:

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

    // translate the canvas before drawing onto it, fixing the position
    canvas.translate(getScrollX(), 0);

    canvas.drawLine(1, 1, 1, TEXT_VIEW_HEIGHT_PX - 1, borderPaint);
    canvas.drawLine(1, 1, TEXT_VIEW_WIDTH_PX - 1, 1, borderPaint);
    canvas.drawLine(TEXT_VIEW_WIDTH_PX - 1, 1, TEXT_VIEW_WIDTH_PX - 1,
        TEXT_VIEW_HEIGHT_PX - 1, borderPaint);
    canvas.drawLine(1, TEXT_VIEW_HEIGHT_PX - 1, TEXT_VIEW_WIDTH_PX - 1,
        TEXT_VIEW_HEIGHT_PX - 1, borderPaint);
}

_{此代码尚未针对所有情况进行彻底测试。我只确认了OP提供的4个案例}