We're trying to write our own MapView, and I'm trying to figure out how adding overlays to the mapview causes them to get drawn in other mapping APIs
I've got a MapView that extends ViewGroup. I think I've figured out that calling MapView.invalidate() causes the mapview's dispatchDraw method to be called. Does that sound correct?
If that's true, when is the onDraw and draw method of the mapview called?
More importantly is all this view and which methods called when stuff documented well anywhere?
Thanks!
EDIT
This SO post explained that for classes that extend ViewGroup, the onDraw method is not called automatically. You have to force it if you need it. But as ebarrenchea pointed out, the order is draw, onDraw, dispatchDraw IF all the methods are called
Calling invalidate on your viewgroup will force draw to run which will in turn call onDraw and then dispatchDraw. You should have a look at the view documentation here and the View source code here for more information.
invalidate() has to be called from UI thread to cause onDraw(). Try to use postInvalidate() which should have the same effect as invalidate(), but works from non UI threads.
Related
I override View.onDraw() method and draw some lines on it, but sometimes it shows nothing until touch it.
A View's onDraw() is called when:
-> The view is initially drawn
-> Whenever invalidate() is called on the view
Check if you need to call invalidate(). or please give the full code with xml
I'm learning about custom views and wanted to learn about invalidate() and requestLayout().
Please refer to this answer and its diagram:
https://stackoverflow.com/a/25846243/4243687
invalidate() tells Android that the state of the view has changed and needs to be re-drawn.
requestLayout() means the size of the view may have changed and needs to be remeasured, and then re-drawn.
invalidate() will invoke dispatchDraw(), draw(), and onDraw() hence it re-renders the view.
requestLayout() on the other hand does pretty much everything from measuring to re-rendering again.
Why do so many of the examples out there (even the TextView source code) call invalidate() and then requestLayout() right on the next line?
invalidate() is used specifically for redrawing the content of your view. The redraw does not happen synchronously. Instead, it flags the region of your view as invalid so that it will be redrawn during the next render cycle.
requestLayout() should be used when something within it has possibly changed its dimensions. In this case, the parent view and all other parents up the view hierarchy will need to readjust themselves via a layout pass.
If you are not doing anything to your view that would change its size, then you do not have to call requestLayout().
If you go back and look at the places in the code for TextView where requestLayout() is being called, it will be on methods where the view's bounds will be affected. For example, setPadding(), setTypeface(), setCompoundDrawables(), etc.
So, when requestLayout() is called, it should be paired with a call to invalidate to ensure that the entire view is redrawn.
After seeing the following diagram, I was under the impression that calling requestLayout() would eventually result in an onDraw.
Therefore, there would be no need to call these together because it would be redundant.
invalidate();
requestLayout();
However, it turns out that that diagram is misleading. Some views might in fact invalidate themselves when there is a layout change, but this is not a certainty. Calling requestLayout() is not guaranteed to result in onDraw being called.
My source (thanks to this comment) is the Romain Guy (who is an Android engineer at Google):
requestLayout() itself does not lead to a draw pass but some views
might react to a Layout change by calling invalidate.
Therefore, to be certain a relayout will result in a redraw, then you should pair an invalidate() with the requestLayout(). (The opposite is not true, though. If you only need a redraw, then there is no need to call requestLayout(). A single invalidate() will do.)
Relevant excerpt from the book Expert Android that answers the question:
Because the onClick event has caused the dimensions to change, our
view needs to become bigger and take more space. How do we express
that need to Android, Well, we request Layout(). This method goes up
the chain, marking every view parent that it needs to be remeasured.
When the final parent gets this request (the view root), the parent
schedules a layout traversal. A layout traversal may or may not result
in onDraw, although in this case it should. As a good programming
practice, we also call invalidate() to ensure the drawing phase as
well.
Android docs: Creating a View class
public boolean isShowText() {
return mShowText;
}
public void setShowText(boolean showText) {
mShowText = showText;
invalidate();
requestLayout();
}
Notice that setShowText calls invalidate() and requestLayout(). These
calls are crucial to ensure that the view behaves reliably. You have
to invalidate the view after any change to its properties that might
change its appearance, so that the system knows that it needs to be
redrawn. Likewise, you need to request a new layout if a property
changes that might affect the size or shape of the view. Forgetting
these method calls can cause hard-to-find bugs.
I put a Log.d() call into the onDraw() of my extended View, so I could see how often and when it's getting called. It gets called upon instantiation of the view, which is not surprising. But then I notice, it gets called on every tap that is handled by onTouchEvent(), even though my code there isn't doing anything remotely related to graphics. However, in the documentation for Views, I can't seem to find anything about when onDraw() is actually called. I'm not really concerned about my particular project here (this doesn't cause a problem for me), I would just like to know if there is a list somewhere or something that shows the order of operations for a View, particularly what causes onDraw() to get called.
AFAIK, a View's onDraw() is called when:
The view is initially drawn
Whenever invalidate() is called on the view
Invalidate can be called by you or the system whenever needed. For example, a lot of Views change how they look onTouch, like an EditText getting an outline and cursor, or a button being in the pressed state. Due to this, Views are redrawn on touch.
I agree that it would be nice to have a document that detailed the working of Views, and if one exists and somebody knows where to find it, please let us know.
onDraw() is called when invalidate() is called.
But you should know for ViewGroups: onDraw() will not be called like you expect. Rather, onDispatchDraw().
However, in a ViewGroup you can call setWillNotDraw(false) in the constructor to make onDraw() to be called on invalidate().
Take a look at this answer
If you set a background drawable for a View, then the View will draw
it for you before calling back to its onDraw() method.
onAttachedToWindow () is called when the view is attached to a window.
At this point it has a Surface and will start drawing. Note that this
function is guaranteed to be called before
onDraw(android.graphics.Canvas), however it may be called any time
before the first onDraw -- including before or after onMeasure(int,
int).
invalidate() mark the area defined by dirty as needing to be drawn. If
the view is visible, onDraw(android.graphics.Canvas) will be called at
some point in the future.
One important thing to keep in mind is that try to minimize calling of invalidate() function with no arguments.
Instead try to maximize the invalidate() function with four arguments.As drawing whole view is very expensive.The second variant refreshes only the part of view.
Additional to the above: The soft keyboard causes a View.invalidate()-->View.onDraw() sequence after resizing the Window to sensibly accommodate the 'keyboard'. A custom View.onDraw() must leave itself in a state that anticipates this possibility.
Such phenomenum explains why the app you developed and tested on a tablet with a bluetooth keyboard went to the dogs once it reached the real world (-:
When is onDraw called (check this for more details)
The onDraw method is called whenever android thinks that your view
should be redrawn. This can be tha case when your view is animated, in
which case onDraw is called for every frame in the animation. It is
also called when the layout changes and your view is re-positioned on
the screen.
But what if some data inside your view has changed and you want to
make sure that the view is redrawn. You can’t call onDraw directly.
Instead you should call invalidate to tell the view that it needs to
redraw itself.
I am really confused with this three functions: removeAllViewsInLayout(), postInvalidate() and refreshDrawableState().
removeAllViewsInLayout()
When I use removeAllViewsInLayout() in my program, all views were gone. But when trigger postInvalidate() to refresh, nothing. I think removeAllViewsInLayout() deletes all of my views. Is there a way to clear all things in my view but not delete it?
postInvalidate()
I want to refresh my view. But with refreshDrawableState(), I can do only once, why?
If you call postInvalidate() (or just call invalidate() if on the UI thread), the system will schedule a redraw of your view. Your onDraw() method can then do whatever it wants, including just drawing a blank canvas.
If you call removeAllViewsInLayout() it will do just that -- remove every view, including your custom view. You probably don't want to do that. (EDIT: removeAllViewsInLayout() is intended to be called only as part of layout calculations for a ViewGroup. A typical use would be by a ViewGroup that "owns" a large number of views but during onLayout() processing decides to display only those children that actually fits on the screen. If you are not in the process of computing the view layout, you should call removeAllViews() instead.)
Calling refreshDrawableState() is only useful if your view is using a Drawable object that is sensitive to the state of the view. For example, a Button will use this so that the background drawable changes color when the button is pressed. For what you are doing, you don't need to bother with this method, either.
Now, I have a Framelayout, it includes two views. when I call the top view's invalidate() method, I found the another view's onDraw() also be called. I suppose the another view's onDraw() should not be called, Is there a way to stop the onDraw be called?
Is there someone tell the reason why onDraw() be called?
You could try adding willNotDraw="true" (see here) to your view, but I'm not sure it helps.