CODEWITHSUNDEEP
javaandroidcanvasondraw
Bob
Bob

Reputation: 1455

Why does my drawing disappear from a canvas?

I perform some drawing on a canvas every 10 sec. Unfortunately it disappears before it will be redrawn, so we have 10 sec with a blank screen. I tired to save a canvas before drawing and restore it, did not help. This bug was appeared after I had shifted a line canvas.drawPath(linePath, linePaint); from outside of a loop into the loop.

CODE:

private void drawLine(Canvas canvas) {
        yStep = (yHeight) / fullChargeLevel;
        xStep = (xWidth) / timeStampBarsQuantity;

        boolean check = false;
        float time;
        float chrg;

        while (batteryUsageHistory != null && batteryUsageHistory.moveToNext()) {
            int charge = batteryUsageHistory.getInt(1);
            int time_stamp = batteryUsageHistory.getInt(2);

            if (charge < 1) {

                if(check){
                    canvas.drawPath(linePath, linePaint); //This line I shifted into here
                    linePath.reset();
                }

                check = false;

                continue;
            }

            time = xPos + time_stamp * xStep;
            chrg = yPos - (charge * yStep);

            if (!check) {
                linePath.moveTo(time, chrg);
                check = true;

                continue;
            }
            linePath.lineTo(time, chrg);
        }
        //canvas.drawPath(linePath, linePaint); //This line I shifted from here
    }

Upvotes: 0

Views: 1450

Answers (2)

Bob
Bob

Reputation: 1455

I just moved back that line canvas.drawPath(linePath, linePaint); I was previously moved. And it works!.

Upvotes: 1

Krzysztof Cichocki
Krzysztof Cichocki

Reputation: 6414

You should implement your drawing logic in onDraw by extending some View class:

protected void onDraw(Canvas canvas) {
   // here goes your custom drawing logic
}

as stated in: https://developer.android.com/training/custom-views/custom-drawing.html

it is because android redraws the component when it needs to. It is always like that you need to implement the drawing method and the GUI framework will call this method when neccessary, it is not the opposite, that the GUI framework displays your painting, it is just calling your painting method when needed.

Upvotes: 1

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