How does ImageView just redraw part of its content when invalidate(Rect) is called?

How does ImageView just redraw part of its content when invalidate(Rect) is called?

By : user3862313
Date : November 22 2020, 07:01 PM
it fixes the issue As far as I can gather there are two optimizations going one. For one, if the child is a viewgroup, only those children of this group are redrawn that intersect the invalidated area. Also, the canvas is clipped to the bounds of this rect. This means that less pixels have to be shuffled through the bus to the framebuffer, and that draw operations that are clipped completely can be skipped.
This is a semi-educated guess. I browsed the source a few months back and am fairly certain of the ViewGroup thing. The second one can be tested by overriding clipping in onDraw and checking if it redraws everything.
code :

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C#: Windows Forms: What could cause Invalidate() to not redraw?

C#: Windows Forms: What could cause Invalidate() to not redraw?

By : Mugnaioni Lorenzo
Date : March 29 2020, 07:55 AM
like below fixes the issue To understand this you have to have some understanding of the way this works at the OS level.
Windows controls are drawn in response to a WM_PAINT message. When they receive this message, they draw whichever part of themselves has been invalidated. Specific controls can be invalidated, and specific regions of controls can be invalidated, this is all done to minimize the amount of repainting that's done.
code :
Partial redraw -> invalidate (Rect rect)

Partial redraw -> invalidate (Rect rect)

By : Emil Tejeda
Date : March 29 2020, 07:55 AM
I wish did fix the issue. In my onDraw I have all code needed to build my entire view but how can I detect if I want to perform only a partial redraw. I guess a partial redraw should be triggered by calling canvas.invalidate(Rect rect); Right? In developer settings of my device I enabled “Show screen updates” but this always tells me that my entire screen is redrawn… , you must be doing something wrong, see this custom View:
code :
class V extends View {

    Rect clip = new Rect();
    private int cnt = 20;

    public V(Context context) {

    public boolean onTouchEvent(MotionEvent event) {
        Log.d(TAG, "calling invalidate " + cnt);
        invalidate(10, 10, cnt, cnt);
        return false;

    protected void onDraw(Canvas canvas) {
        Log.d(TAG, "onDraw clip " + clip);
class V extends View {

    Rect clip = new Rect();

    public V(Context context) {
        int[] colors = {0xff000000, 0xffff0000, 0xffffffff};
        Drawable d = new GradientDrawable(GradientDrawable.Orientation.TOP_BOTTOM, colors);

    public boolean onTouchEvent(MotionEvent event) {
        int x = (int) event.getX();
        int y = (int) event.getY();
        invalidate(x, y, x + 10, y + 10);
        return true;

    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        int w = MeasureSpec.getSize(widthMeasureSpec);
        setMeasuredDimension(w, w * 4);

    protected void onDraw(Canvas canvas) {
        Log.d(TAG, "onDraw clip height: " + clip.height());
    ScrollView sv = new ScrollView(this);
    V v = new V(this);
How to redraw ctx.rect?

How to redraw ctx.rect?

By : Emerson Paduan
Date : March 29 2020, 07:55 AM
With these it helps I've managed to achieve a goal I need. Basically, I've used an example from fabricJS demo and changed a logic a bit.
code :
var canvas = this.__canvas = new fabric.Canvas('canvas');

fabric.util.object.extend(fabric.Object.prototype, {
    redrawAlignmentLines: function(){
    const ctx = this.canvas.getContext("2d");
    const bb = this.getBoundingRect();

    canvas.contextContainer.strokeStyle = 'magenta';

    if (this.alignmentLines.left) {
        ctx.moveTo(bb.left, 0);
        ctx.lineTo(bb.left, 600);

var red = new fabric.Rect({
  id: 1,
  left: 200,
  top: 200,
  width: 100,
  height: 100,
  fill: 'red',
  angle: 30,
  originX: 'center',
  originY: 'center',
  alignmentLines: {
    top: true,
    left: true

canvas.on('after:render', function() {
    canvas.forEachObject(function(obj) {
How to redraw SurfaceView using invalidate() metod?

How to redraw SurfaceView using invalidate() metod?

By : Andexpress
Date : March 29 2020, 07:55 AM
This might help you I have a class GameBoard which extends SurfaceView. Inside I have an OnDraw method: , Override surfaceCreated(). Then add this line:
Invalidate() doesn't actually redraw children

Invalidate() doesn't actually redraw children

By : jungmin
Date : March 29 2020, 07:55 AM
This might help you I ran into this same issue with invalidate(), and since there isn't a good answer elsewhere on SO, I figured I'd post my solution.
This is a recursive function that will loop through all of the children views of a ViewGroup. If the child is a ViewGroup, it will recurse and search the child. Otherwise, it will call invalidate() on the child (this is the termination case).
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