In a custom View, I need to perform some additional work inside onDraw() if and only if the View was invalidated by the application; that is, my own code called invalidate() in the UI thread or postInvalidate() in a non-UI thread. If on the other hand onDraw() is being called because the system invalidated the View, I don't wish that additional work to be performed.
What's the best way to achieve this? My immediate thought is to simply override invalidate() and postInvalidate() and set a flag in both of those, but it would be nicer if there was a single UI-thread method I could override.
Any thoughts please?
Thanks, Trev
postInvalidate() ends up calling invalidate() so you don't need to override both. But if you override invalidate(), the system will call the overridden version.
There is a way to do that without extending view class.
view.getViewTreeObserver().addOnDrawListener(new ViewTreeObserver.OnDrawListener() {
#Override
public void onDraw() {
//View was invalidated
}
});
Related
I am programming a simple game and has to draw animation on the screen and currently calling invalidate() method from within onDraw(). I simply wish to periodically update my canvas.
I would like to move the invalidate() call somewhere else as calling within onDraw() is rather bad. However, my target platform is android gingerbread and I probably can't utilize those methods which involve multithreading.
Is there anyway to setup a timer outside of onDraw() which invalidate() if the time lapse interval meets my desire without involving multithreading?
I probably can't utilize those methods which involve multithreading.
That's not true. It is not what you need though. If you need to call invalidate periodically on your view, you can use the View's internal handler and its postDelayed method, to invoke invalidate(). The runnable runs on the UI Thread.
Wouldn't it be an idea to call invalidate() after an user input?
If you want a timer look to this:
https://developer.android.com/training/scheduling/alarms.html
This method should be viable with your android version.
I'd like to be notified after the view finishes redrawing after I ask it to invalidate. As said in this answer, the invalidate() method doesn't call a View's onDraw() the UI immediately, but schedules the repaint in a message queue which is executed after when the main thread is idle.
I'd like to show a progress dialog, do some UI modifications and then dismiss the dialog when the view is drawn properly. Is there some trick that I can do to know when the View was drawn? Maybe by subclassing the view, overriding the onDraw() method?
I think you answered your question yourself. Why not:
public class DrawListenerView extends View{
private Callback callback;
public DrawListenerView(Callback callback){
this.callback = callback;
}
#Override
protected void onDraw (Canvas canvas){
super.onDraw(canvas);
//add your method here you want to call
//Or use a Callback-pattern
callback.finish();
}
}
public interface Callback(){
public void finish();
}
If you look at the Source:
http://grepcode.com/file/repository.grepcode.com/java/ext/com.google.android/android/2.1_r2/android/view/View.java#View.invalidate%28%29
The comment above says
Invalidate the whole view. If the view is visible, onDraw(android.graphics.Canvas) will >be called at some point in the future. This must be called from a UI thread. To call from >a non-UI thread, call postInvalidate().
Try it :)
Edit: probably you want to use a Callback to handle this.
You're finding it strange to try to do this because you're going about it all wrong. ;)
You're talking about wanting to wait to dismiss a dialog until something is finished drawing. That implies that you have a drawing operation that is long enough that you want to wait for it.
Drawing a frame in onDraw should be fast. You have 16 milliseconds per frame to do any sort of input processing and drawing if you want to hit 60fps and have a smooth UI. Drawing should never take long enough that you would want to show a progress dialog while it's finishing. (Aside from that, drawing as a result of invalidating part of your UI blocks your UI thread, and your progress dialog wouldn't illustrate any progress until it's done anyway.)
If you need to do some complex off-screen rendering to show later, you should do it in an AsyncTask or similar off of your UI thread, not in a view's actual onDraw method. Once you get the finished callback from that, you can quickly draw the prerendered image you just created and dismiss your progress dialog.
I subclassed View to get an View on which the user can "draw" with his fingers. I implemented the Interface View.OnTouchListener.
How can I trigger within the onTouch method the redraw of the View? Do I need to implement a Thread / Runnable? I thought that invalidate() triggers the redraw, but this doesn't work.
Just call this.invalidate in the onTouchEvent method of your view, it really should work unless you're not doing the proper thing in your onDraw method. Make sure you're referencing to the right canvas an draw the thing in your overridden onDraw method instead of for example the constructor.
#Override
public boolean onTouchEvent(MotionEvent event) {
this.invalidate();
return true;
}
What is the difference between Android's invalidate() and postInvalidate() methods? When does each one get called? Must the methods be called only in classes which extend View?
If you want to re-draw your view from the UI thread you can call invalidate() method.
If you want to re-draw your view from a non-UI thread you can call postInvalidate() method.
Each class which is derived from the View class has the invalidate and the postInvalidate method. If invalidate gets called it tells the system that the current view has changed and it should be redrawn as soon as possible. As this method can only be called from your UI thread another method is needed for when you are not in the UI thread and still want to notify the system that your View has been changed. The postInvalidate method notifies the system from a non-UI thread and the view gets redrawn in the next event loop on the UI thread as soon as possible. It is also shortly explained in the SDK documentation:
CLICK HERE
UPDATE:
There are some problems that arise when using postInvalidate from other threads (like not having the UI updated right-away), this will be more efficient:
runOnUiThread(new Runnable() {
public void run() {
myImageView.setImageBitmap(image);
imageView.invalidate();
}
});
I'm new to programming androids but I have quite a bit of experience programming blackberries.
I created an app that has an activity class (main.java) and a view class (game.java).
Inside the view class I have some bitmaps being drawn to the screen. I created a thread and I'm moving the images around in the thread. However when I call invalidate() inside the thread it never redraws the screen.
Are you not able to invalidate() the screen from a thread? I know the thread is running and the invalidate is being called, it just never makes the changes on the screen.
You have to use View.postInvalidate() if you call it from a non-UI thread.
According to docs:
public void postInvalidate ()
Since: API Level 1
Cause an invalidate to happen on a subsequent cycle through the event loop. Use this to invalidate the View from a non-UI thread.