I have a complex custom view - photo collage.
What is observed is whenever any UI interaction happens, the view is redrawn.
How can I avoid complete redrawing (for example, use a cached UI) of the view specially when I click the "back" button to go back to previous activity because that also causes redrawing of the view.
While exploring the API and web, I found a method - getDrawingCache() - but don't know how to use it effectively.
How do I use it effectively?
I've had other issues with Custom Views that I outline here.
I found a better way than using getDrawingCache.
In the method onDraw, apart from drawing in the natural canvas, I also draw on an memory-only canvas.
Bitmap cacheBmp = Bitmap.Create(....);
Canvas cacheCanvas = new Canvas(cacheBmp);
void onDraw(Canvas c)
{
if(updateDueToInteraction)
{
c.drawXXX(...);
cacheCanvas.drawXXX(...);
} else
{
c.drawBitmap(cacheBmp, 0, 0);
}
}
First of all you will have to use the setDrawingCacheEnabled(true) method, so that you're View is cache-enabled. Then, you can use the getDrawingCache(boolean) method which returns a Bitmap representing the View. Then, you can draw that bitmap manually.
If you don't enable caching by calling the setDrawingCacheEnabled(true) method, you will have to call buildDrawingCache() before (and call destroyDrawingCache() when you're done).
Bye!
Related
I have a headache in current Android project. I want to detect the changing of the current page. For example, there is a TextView to display device time, which is updated per second. How to detect this change? I searched a lot on SO (thanks SO), but none works for me.
More information: I don't use standard Activity to create page. My way is:
All widgets are created into a View object which is then used to create a container object. After that, I just handle this container to draw on a canvas with a VSYNC callback Choreographer.FrameCallback periodically.
Indeed, it works to draw. All are ok. Except: I want to draw canvas only when the page's content changed. So back to my beginning question, how to detect this "changing" event? I am sure there is some kind of callback to handle this problem. I used the following solution, but onGlobalLayout is not called when textview's text changed.
CanvasAppViewContainer container;//CanvasAppViewContainer extends AbsoluteLayout
LayoutInflater li =(LayoutInflater)getService().getSystemService(Context.LAYOUT_INFLATER_SERVICE);
View view = li.inflate(resourceId, null);//passed a correct the layout id
container = new CanvasAppViewContainer(getService(), view, getWidth(), getHeight(), getSurface());
rootView = (LinearLayout) findViewById(R.id.rootView); //root element of layout
rootView.getViewTreeObserver().addOnGlobalLayoutListener(
new ViewTreeObserver.OnGlobalLayoutListener() {
#SuppressLint("NewApi")
#Override
public void onGlobalLayout() {
Log.d("test", "onGlobalLayout");
}
});
BTW: Even if I register the view tree observer for the textview, onGlobalLayout is still not called.
Thanks
onGlobalLayout() is only called when the layout changes. When you want to detect a change in the text use TextView.addTextChangedListener()
My project is based on surfaceView and up until now I've had all of my rendering in onDraw which I am overriding. All seemed to be OK.
However, I've just updated my SDK and now it gives me an error telling me:
Suspicious method call; should probably call "draw" rather than "onDraw"
Could someone please explain the difference between these two?
I've read some similar questions around the net but I've not found an explanation that I understand.
Thanks
I tried cleaning my project and it did solve the problem. Try it.
SurfaceView.draw() basically calls View.draw(); If you want to implement your drawing, you should do it in View.onDraw() which is for you to implement which even says in the source code comments.
This method is called by ViewGroup.drawChild() to have each child view
draw itself. This draw() method is an implementation detail and is not
intended to be overridden or to be called from anywhere else other
than ViewGroup.drawChild().
As for difference between them:
draw():
13416 /*
13417 * Draw traversal performs several drawing steps which must be executed
13418 * in the appropriate order:
13419 *
13420 * 1. Draw the background
13421 * 2. If necessary, save the canvas' layers to prepare for fading
13422 * 3. Draw view's content
13423 * 4. Draw children
13424 * 5. If necessary, draw the fading edges and restore layers
13425 * 6. Draw decorations (scrollbars for instance)
13426 */
onDraw() is empty. Its for you to implement.
I have the problem since ever.
I handle it like this:
1) Declare a method like the following.
#SuppressLint("WrongCall")
public void drawTheView() {
theCanvas = null;
try{
theCanvas = getHolder().lockCanvas();
if(theCanvas != null) {
onDraw(theCanvas);
}
} finally {
getHolder().unlockCanvasAndPost(theCanvas);
}
}
2) Now you can modify the onDraw() Method:
#Override
public void onDraw(Canvas canvas) {
//Do some drawing
}
You can call the drawTheView() method from everywhere you want and call the onDraw() method this way without getting the error...
I think this is a practical way.
Note that in the case of drawing, overriding draw() and calling super.draw is often used when a ViewGroup wants to draw content over its child views. Content drawn in onDraw will appear under children.
As friiky said, #SuppressLint("WrongCall") fixed my problem. However it must be in front of the method name, not the above.
What I did is put mouse over the error code, right click and select Add #SuppressLint("WrongCall")
onDraw gives you a canvas to draw to the screen.
draw() allows you to manually draw a canvas to the screen (you have to make the canvas yourself).
I am developing an app which instantiates a bunch of bitmap objects (e.g. buttons, which have cache bitmaps, so they don't have to get rendered again and again)
Now, I realised that when I run and start the app repeatedly on my huawei mobile device, I get an OutOfMemoryException at a point where the app tries to allocate some memory for the bitmaps.
So I guess it's the bitmaps which make trouble. I do know that there is a bitmap.recycle() method though.
Now my question: what is best practice to clean up memory?
Why isn't there some View method like View::onDestroy() which can be implemented for cleanup purpose?
EDIT: example
my "CirclyButton" (extends Button) class always draws a cached bitmap onDraw:
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
canvas.drawBitmap(this.getDefaultBitmap(), 0, 0, paint);
}
private Bitmap getDefaultBitmap(){
if(mBitmapDefault == null){
mBitmapDefault = Bitmap.createBitmap(8*radius, 8*radius, Config.ARGB_8888);
Canvas canvas = new Canvas(mBitmapDefault);
this.drawDefault(canvas);
return mBitmapDefault;
}
return mBitmapDefault;
}
So I guess this allocated data should be recycled somewhere...?
Views don't have an onDestroy method because views usually don't get destroyed, activities do. A view won't just be destroyed if nothing happens to its activity (Unless you inflate a different layout... That's not the case, right?), and if something happens to its activity, you do have a callback getting called.
If there is a recycle() method, make sure you call it. And remove all reference to memory taking objects in the onDestroy, i.e:
#Override
public void onDestroy() {
object1 = null;
object2 = null;
//...
}
So the GC can do its job. I had the same problem with the AdView of AdMob, although they did have a destroy method it didn't really help. But deleting my references of the view fixed the problem.
Provide more information about where are you using your bitmaps, i have some serious experience of working with images and saving memory.
For example in my app i have a list of some data, which display some bitmap in each row. I store my list in a fragment(for fragment support i use compatibility library), and i recycled my bitmaps on this fragment onDestroy method.
Later i decided to optimize my list, so i added scroll listener to my list and started recycling bitmaps, when they are scrolled off the screen.
I have some methods in my View that modify some of the shapes that are drawn when called. In Java in order to make sure the component is updated I would call repaint(). Is there something that will make sure my view is updated correctly?
I had read somewhere that calling invalidate() in the onDraw() method would keep things up to date and therefore I wouldn't need to have something like repaint() in my methods that modify that shapes that are drawn.
Is this correct, or is there something else I have to do?
EDIT
To add in an example, a method I call in my view is:
public void setLineThickness(int thickness) {
aLineThickness = thickness;
if(aLineThicness > 1)
//repaint(); - Okay in Java but not in Android
}
Calling invalidate() will tell the view it needs to redraw itself (call onDraw) sometime in the future. So if you change something in your view, like the line thickness, call invalidate() after it. That way you know your view will eventually be updated.
All your drawing code should be implemented in onDraw() and your other methods should just change the state of your view, which will then be used to draw it, after you call invalidate().
I'm trying to move a ball on canvas. a and b are similar to x,y coordinate positions. Any way from my code im trying to get different values dynamically. The a,b are global variables. But it seems that "invalidate()" or the refreshing of screen only happens afer end of the whole loop. Do you know why?. And if i have to build this on another thread please suggest me with some simple codes.
private void shootBall(){
while (a>b){
a = getPositionX();
b = getPositionY();
invalidate();
}
}
}
I think it's more correct to say that you can call invalidate() from within a loop, but that that invalidation will not be handled (the canvas won't be redrawn) until after your loop is complete. The problem is that you are calling invalidate on the same thread (the UI toolkit thread) as the one that would call your onDraw() method. So unless/until you hand control back to the toolkit, it cannot possibly do the rendering. So your invalidate() call does actually invalidate the view ... but the view won't be redrawn until after your loop completes and your function returns.
It is more correct to change the position in some function that is called via some timer (which is essentially what the animation classes do). In that function, you would change the values, invalidate() appropriately, and return. Then the toolkit re-renders the scene and your function will get future callbacks and update the position accordingly.
do it like this, and use postInvalidate() instead:
private void shootBall(){
new Thread(new Runnable() {
public void run() {
while (a>b){
a = getPositionX();
b = getPositionY();
postInvalidate();
}
}
}).start();
}
edit: but as mentioned before, don't assume that the invalidate redraws the screen, it marks it as to-be-redrawn and the UI thread will get around to it.
You can put invalidate() at the end of onDraw() like in this example: How can I use the animation framework inside the canvas?
However this works well on some devices while bottlenecks and slows down on other.
To use a thread and SurfaceView go through all of these tutorials: http://www.droidnova.com/playing-with-graphics-in-android-part-i,147.html
UI cant be modified form any new thread..you should use invalidate() in the same thread where your view