Caching part of canvas in custom view. Android

Question

I have extended EditText view with overridden onDraw method:

@Override
protected void onDraw(Canvas canvas) {
    int count = getLineCount();
    drawLineNumbers(canvas, count); // here I draw line numbers on canvas,
                                    // like on picture
    super.onDraw(canvas);

}

As we know, onDraw method of EditText calls very often, e.g. it calls for each indicator flashing - that's why redrawing all line numbers consume a lot of hardware resources.

Is there efficient method to cache part of canvas and do not redraw it every time? In my case I want to redraw it only when line's count changed. I have already tried save it in Bitmap, but on files with big amount of lines it throwing OutOFMemmory exception.

Any thoughts are welcome.

Answer 1

make a compound custom view:

class NumEditText extends LinearLayout

and add two views: left margin view with line numbers and an. associated EditText

EDIT:

try this skeleton:

public class NumberedEditText extends LinearLayout {

private LineNumbers lineNumbers;
private ET et;

public NumberedEditText(Context context) {
    super(context);
    setOrientation(HORIZONTAL);
    lineNumbers = new LineNumbers(context);
    et = new ET(context);
    addView(lineNumbers);
    addView(et, new LayoutParams(LayoutParams.MATCH_PARENT, LayoutParams.MATCH_PARENT));
}

class LineNumbers extends View {
    private final static String TAG = "NumberedEditText.LineNumbers";
    public LineNumbers(Context context) {
        super(context);
    }

    @Override
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        super.onScrollChanged(l, t, oldl, oldt);
        invalidate();
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int fixed = 50;
        widthMeasureSpec = MeasureSpec.makeMeasureSpec(fixed, MeasureSpec.EXACTLY);
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        Log.d(TAG, "onDraw ");
    }
}

class ET extends EditText implements TextWatcher {
    private final static String TAG = "NumberedEditText.ET";
    private Layout layout;
    private int cnt;

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

    @Override
    protected void onSizeChanged(int w, int h, int oldw, int oldh) {
        super.onSizeChanged(w, h, oldw, oldh);
        addTextChangedListener(this);
        layout = getLayout();
        cnt = layout.getLineCount();
    }

    @Override
    protected void onScrollChanged(int l, int t, int oldl, int oldt) {
        super.onScrollChanged(l, t, oldl, oldt);
        lineNumbers.scrollTo(l, t);
    }

    @Override
    public void onTextChanged(CharSequence text, int start, int before, int after) {
    }

    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {
    }

    @Override
    public void afterTextChanged(Editable s) {
        if (layout.getLineCount() != cnt) {
            cnt = layout.getLineCount();
            lineNumbers.invalidate();
        }
    }
}
}