Develop Reference

Item drawn in Custom view onDraw() keeps disappearing

I'm drawing 3 things in my custom view in the onDraw() method: a vector drawable, a simple line and a triangle (made from 4 Points and a Path). This custom view is displayed in a tab.
If I swipe to go to another tab I see that the system calls onDraw(). When I return to the tab holding my custom view the vector drawable and simple line are still visible but the triangle has disappeared. If I now swipe to another tab, onDraw() runs again and back in the tab with the custom view, all items (including the triangle) are now visible. This disappearing/appearing continues to happen as I swipe back and forth. Why is my triangle disappearing?
UPDATE 1 (hacky fix):
I've tried experimenting and notice that when I move my triangle Path object creation out of my init() method and put it directly in the onDraw() method - then all works well, nothing disappears. But, I now get the 'Avoid object allocations during draw' warning as I'm creating this object in onDraw();
UPDATE 2 (better fix?):
After more experimenting, it's definitely the Path causing this problem. Another solution to this - which doesn't incur the 'Avoid object allocations during draw' warning is: keep Path creation in init() and remove the line of code 'myPath.setFillType(Path.FillType.EVEN_ODD)'. It solves my problem, but I've no idea why.
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
// Co-ordinates
int width = getWidth();
int halfWidth = width/2;
int left = 0;
int top = 0;
int centreX = left + halfWidth;
int centreY = top + halfWidth;
int baseSize = Math.round((float)(halfWidth * 0.05));
// Vector drawable - always draws fine!
myVectorDrawable.setBounds(left, top, left + width, top + width);
myVectorDrawable.draw(canvas);
// Simple line - always draws fine!
canvas.drawLine(left, top, 20, 20, paint);
// Triangle - sometimes visible, sometimes disappears!
Point myTriangleBottomMiddle = new Point(centreX, centreY);
Point myTriangleBottomLeft = new Point(centreX, centreY + baseSize);
Point myTriangleBottomRight = new Point(centreX, centreY - baseSize);
Point myTriangleTopMiddle = new Point(centreX + halfWidth, centreY);
myPath.moveTo(myTriangleBottomMiddle.x, myTriangleBottomMiddle.y);
myPath.lineTo(myTriangleBottomLeft.x, mTriangleBottomLeft.y);
myPath.lineTo(myTriangleTopMiddle.x, myTriangleTopMiddle.y);
myPath.lineTo(myTriangleBottomRight.x, myTriangleBottomRight.y);
mPath.close();
canvas.drawPath(myPath, myPaint);
}
Below the code where I set up stuff so as not to burden the onDraw() method.
private void init() {
// Vector drawable
myVectorDrawable = r.getDrawable(R.drawable.gauge_dial);
// Triangle path - ** THIS BEING HERE SEEMS TO BE THE PROBLEM **
myPath = new Path();
myPath.setFillType(Path.FillType.EVEN_ODD);
// Triangle Paint
myPaint = new Paint();
myPaint.setColor(r.getColor(R.color.black));
myPaint.setStrokeWidth(2);
myPaint.setAntiAlias(true);
myPaint.setStyle(Paint.Style.FILL);
// Simple line paint
Paint paint = new Paint();
paint.setColor(Color.BLACK);
}

Add a call to reset() on the path.
// Triangle - sometimes visible, sometimes disappears!
Point myTriangleBottomMiddle = new Point(centreX, centreY);
Point myTriangleBottomLeft = new Point(centreX, centreY + baseSize);
Point myTriangleBottomRight = new Point(centreX, centreY - baseSize);
Point myTriangleTopMiddle = new Point(centreX + halfWidth, centreY);
myPath.reset();
myPath.moveTo(myTriangleBottomMiddle.x, myTriangleBottomMiddle.y);
myPath.lineTo(myTriangleBottomLeft.x, mTriangleBottomLeft.y);
myPath.lineTo(myTriangleTopMiddle.x, myTriangleTopMiddle.y);
myPath.lineTo(myTriangleBottomRight.x, myTriangleBottomRight.y);
mPath.close();
canvas.drawPath(myPath, myPaint);
Also, I would recommend putting the vector drawable into a separate view so it's not redrawn everytime you need to animate the triangle (assuming this is going to be an animated guage dial).

How to draw equidistant dashes using DashPathEffect

At the moment I’m using DashPathEffect with hardcoded intervals to draw a circle as next:
float[] intervals = new float[]{ 3, 18 };
DashPathEffect path = new DashPathEffect(intervals, 0);
paint.setPathEffect(path);
… … … …
canvas.drawCircle(x, y, radius, paint);
But this produces a non-equidistant dash where the circle starts and ends, as shown in the image below:
I can of course adjust it manually, but this would only work for one specific device density, and produce again the same problem in a different display density.
What would the formula to calculate equidistant dashes?

You need n dashes plus n gaps to have the same total length as the circumference of the circle. The below code assumes you've correctly determined both the center point and the radius you want to use.
double circumference = 2 * Math.PI * radius;
float dashPlusGapSize = (float) (circumference / NUM_DASHES);
intervals[0] = dashPlusGapSize * DASH_PORTION;
intervals[1] = dashPlusGapSize * GAP_PORTION;
DashPathEffect effect = new DashPathEffect(intervals, 0);
paint.setPathEffect(effect);
canvas.drawCircle(center, center, radius, paint);
For instance, I've used NUM_DASHES = 20, DASH_PORTION = 0.75f, and GAP_PORTION = 0.25f, and I see:
You can use different values for these constants to change how many dashes you chop the cirlce into, or how big the dash/gap are relative to each other (as long as DASH_PORTION + GAP_PORTION adds up to 1).

In case you have a different figure you can use this method to measure your custom path length:
val measure = PathMeasure(path, false)
val length = measure.getLength()

Android: drawTextOnPath multiline

I current have a custom view that I override the onDraw and draw an arc. I want to draw text within this arc. To do this, I use drawTextOnPath and this display curved text at the top of the arc. However, sometimes the text is quite long, so I want to allow it to go on to multiple lines.
I currently use code like this to draw on to multiple lines: -
textView.getPaint().getTextBounds(s, 0,
s.length(), r);
int yOffset=r.height() + textSpacing;
int textStart=0;
int numberOfLines= (int) (r.width()/arcWidth) + 1;
for (int i=0; i < numberOfLines; i ++) {
canvas.drawTextOnPath(s.substring(textStart, textStart + s.length() / numberOfLines),
childHolder.path, 0, yOffset, paint);
yOffset+=r.height() +textSpacing;
textStart=s.length()/numberOfLines;
}
However, this obviously doesn't take into account how wide the text is further down the arc. Is there a way of doing this with using something like staticlayout/dynamiclayout (text does change a lot).
If anyone could point me in either something in android SDK I can use, or the maths to calculate the available width

This bit of code solved my issue: -
PathMeasure pm = new PathMeasure(path, false);
layout = new DynamicLayout(spannableText, spannableText, paint, (int) arcPathMeasure.getLength(), Layout.Alignment.ALIGN_CENTER, 1, 0, true);
Thanks pskink!

What do I do about drawText() and drawRect() drawing in two different locations when given the same coordinates?

I'm trying to draw a text to the screen and then have a rectangle in the same spot to account for knowing when the user clicks within it (and clicks the text). Problem is when I put the text in one spot on the screen and the rectangle in the same spot, it apparently isn't in the same spot. Is there some setting I need to set somewhere?
private final String[] options = {"Start", "High Scores", "Help", "Quit"};
public void draw(final Canvas g)
{
for (int i = 0; i < options.length; i++)
{
width = HORIZONTALOFFSET * 3 + spaceInvadersTitle.getWidth();
height = GamePanel.getScreenHeight() / 4 - (75 * 2 + TEXT_SIZE) / 2 + i * 75;
rect[i] = new Rect(width, height, width + 325, height + TEXT_SIZE);
g.drawRect(rect[i], paint);
g.drawText(options[i], width, height, paint);
}
}
FYI "width" and "height" are more like x- and y-coords. The horizontal alignment is not the problem - just the vertical. If you'll notice from the code, I'm setting the starting x- and y-coordinates for the drawText and drawRect the same exact position, but that's not how they're showing on the screen. Instead it seems the drawText is using that position as a lowerleft anchorpoint or something like that. Is that how it works? If so, how do I change that?
Also, if you have any suggestions on how to approach listening for when the user clicks on a drawn text, I'm all ears. This is the easiest way I could think of, and how I do it in regular desktop Java.

Android DrawArc SweepGradient over 360°

I'm trying to make a wheel of colours which allows the user to select between Solid color or Gradients.
The user can also move those colours around the wheel.
The problem is with the gradient. When the angle is over 360° the colour displayed is going back to the first colour instead of continue the gradient.
I'm using a custom view and the code I will paste is called from the onDraw method.
int start = Color.rgb((int)previous.startRed, (int)previous.startGreen, (int)previous.startBlue);
int end = Color.rgb((int)previous.endRed, (int)previous.endGreen, (int)previous.endBlue);
int[] colors = {start, end, end};
float from = from_angle / 360.0f;
float to = (from_angle + to_angle) / 360.0f;
float[] positions = {from,to, to};
Log.v("Print", "print positions " + positions[0] + " " + positions[1]);
Shader gradient = new SweepGradient (oval.centerX(), oval.centerY(),colors, positions);
paint.setShader(gradient);
canvas.drawArc(oval, from_angle, to_angle + 1, false, paint);
Here is a picture to show the problem I face.
Any hint would be appreciated. Thank you.