縦(あるいは横)にあわせる形で正方形にするView

attr.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
    
    <declare-styleable name="SquareView">
        <attr name="adjust_width" format="boolean"></attr>
	</declare-styleable>
	
</resources>

SquareView.java

public class SquareView extends View {
    
    private boolean mAdjustWidth;
 
    public SquareView(Context context) {
        super(context, null);
    }
    
    public SquareView(Context context, AttributeSet attrs) {
        this(context, attrs, 0);
    }
    
    public SquareView(Context context, AttributeSet attrs, int defStyle) {
        super(context, attrs, defStyle);
 
        mAdjustWidth = false;
        if (attrs != null) {
            TypedArray attrsArray = context.obtainStyledAttributes(attrs, R.styleable.SquareView);
            mAdjustWidth = attrsArray.getBoolean(R.styleable.SquareView_adjust_width, false);
            attrsArray.recycle();
        }    
        
    }
    
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
 
        int sideLength = 0;
        if (mAdjustWidth) {
            // 横幅に合わせる
            sideLength = getMeasuredWidth();
        } else {
            // 縦幅に合わせる
            sideLength = getMeasuredHeight();
        }
        setMeasuredDimension(sideLength, sideLength);
 
    }
    
    public void setAdjustWidth(boolean adjustWidth) {
        mAdjustWidth = adjustWidth;
    }
 
}