By default, onDraw() isn't called for ViewGroup.
So I have a question: What happen when we set background drawable for the ViewGroup
onDraw() is inherited from View class since ViewGroup is subclass of View. So when you set Background on a ViewGroup, it works the same way as any other view. When you call setBackground on a view, it sets requestLayout flag to true and invalidates the view.
Also, Official documentation for View class states that:
If you set a background drawable for a View, then the View will draw
it before calling back to its onDraw() method.
You can see how it works in the source code of Android's View class
public void setBackgroundDrawable(Drawable d) {
...
{
requestLayout = true;
}
...
computeOpaqueFlags();
if (requestLayout) {
requestLayout();
}
mBackgroundSizeChanged = true;
invalidate(true);
}
If you look at documentation for invalidate(), you will find
public void invalidate () Invalidate the whole view. If the view is
visible, onDraw(android.graphics.Canvas) will be called at some point
in the future.
setBackgroundColor() and setBackgroundResource() use setBackgroundDrawable() internally so they work the same way.
So to sum it up, onDraw is called at some point in future when you do setBackground().
Related
I have this class that extends View (it's in C# because I use Xamarin)
public class ScannerOverlay : View
{
public ScannerOverlay(Context context) : base(context)
{
//Initializing some values here
}
protected override void OnDraw(Canvas canvas)
{
base.OnDraw(canvas);
//Draw some stuff on the canvas
}
}
I create this view using: View v = new ScannerOverlay(context);
Now I want to add a button to this view. How do I do this? The view has no layout whatsoever so AddView() is not going to work. And from what I found it's not possible to draw a button on a canvas.
Fixed it by extending from RelativeLayout instead of View.
If (like in my case) the OnDraw Method is not getting called, set the following in the constructor: this.SetWillNotDraw(false);
Basically, you can't add a View to another View.
Hard Way: You can create a View as if it has many Views inside it, but you should draw them manually using Canvas and Paint, also handle the click based on touch location of that each "fake" View. So it will take you a lot of time. This is called Custom View.
Easy Way: You can add any View to a ViewGroup and any child of ViewGroup like FrameLayout, LinearLayout, and RelativeLayout with ViewGroup.AddView(view) function. This is called Compound View.
ViewGroup extends View so you directly get all features of View event though you extend ViewGroup. In this way you can add new View to CustomView directly.
public class ScannerOverlay : ViewGroup
{
public ScannerOverlay(Context context) : base(context)
{
//Initializing some values here
}
protected override void OnDraw(Canvas canvas)
{
base.OnDraw(canvas);
//Draw some stuff on the canvas
}
}
I have a logo view, which is a full screen fragment containing single ImageView.
I have to perform some operations after the logo image is completely visible.
Following code is used to invoke the special task
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
ImageView logoImageMaster = new ImageView(getContext());
//logoImageMaster.setImageResource(resID); //even after removing this, i am getting the callback twice
try {
// get input stream
InputStream ims = getActivity().getAssets().open("product_logo.png");
// load image as Drawable
Drawable d = Drawable.createFromStream(ims, null);
// set image to ImageView
logoImageMaster.setImageDrawable(d);
}
catch(IOException ex) {
}
logoImageMaster.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
#Override
public void onGlobalLayout() { //FIXME get called twice. Check this out, no info to distinguish first from second
// Log.e("PANEL", "onGlobalLayout of Logo IV ---------------------------------");
activityInterface.doSpecialLogic();
}
});
return logoImageMaster;
}
My exact problem is, onGlobalLayout is called twice for this view hierarchy.
I know that onGlobalLayout is invoked in performTraversal of View.java hence this is expected.
For my use case of Single parent with Single child view, I want to distinguish the view attributes such that doSpecialLogic is called once[onGlobalLayout is called twice] , after the logo image is completely made visible.
Please suggest some ideas.
OnGlobalLayoutListener gets called every time the view layout or visibility changes. Maybe you reset the views in your doSpecialLogic call??
edit
as #Guille89 pointed out, the two set calls cause onGlobalLayout to be called two times
Anyhow, if you want to call OnGlobalLayoutListener just once and don't need it for anything else, how about removing it after doSpecialLogic() call??
if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN) {
//noinspection deprecation
logoImageMaster.getViewTreeObserver().removeGlobalOnLayoutListener(this);
} else {
logoImageMaster.getViewTreeObserver().removeOnGlobalLayoutListener(this);
}
activityInterface.doSpecialLogic();
It seems to be called one time for each set done over the imageView
logoImageMaster.setImageResource(resID);
logoImageMaster.setImageDrawable(d);
You should Try using kotlin plugin in android
This layout listener is usually used to do something after a view is measured, so you typically would need to wait until width and height are greater than 0. And we probably want to do something with the view that called it,in your case
Imageview
So generified the function so that it can be used by any object that extends View and also be able to access to all its specific functions and properties from the function
[kotlin]
inline fun <T: View> T.afterMeasured(crossinline f: T.() -> Unit) {
viewTreeObserver.addOnGlobalLayoutListener(object : ViewTreeObserver.OnGlobalLayoutListener {
override fun onGlobalLayout() {
if (measuredWidth > 0 && measuredHeight > 0) {
viewTreeObserver.removeOnGlobalLayoutListener(this)
f()
}
}
})
}
[/kotlin]
Note:
make sure that ImageView is described properly in the layout. That is its layout_width and layout_height must not be wrap_content. Moreover, other views must not result in this ImageView has 0 size.
I have CustomView and it works ok.
Now i need to change some item color by code.
So here is the code i have:
CustomView cv = new CustomView(mContext);
cv.setItemColor(Color.parseColor("#e77400"));
inside of my customView i add method:
public void setItemColor(int color){
mItemColorDefault = color;
invalidate();
requestLayout();
}
but after this nothing happen and customView does not refresh...
Please, help to fix this.
Thanks!
There is no need to call requestLayout() unless the size of the CustomView is not changed. This method only relates to views positioning updates.
You must override the onDraw() method as it is the place when your view is drawn. The invalidate() method causes onDraw() to invoked.
Use the mItemColorDefault color to draw your view with it in onDraw().
Here is the example of custom view with onDraw() method overridden:
https://github.com/dawidgdanski/TicTacToe/blob/master/game/src/main/java/pl/dawidgdanski/tictactoe/game/view/TicTacToeView.java
Hope this helps somehow.
The event onDraw() is emitted every time the View need to be repainted cause some graphic changes inside.
Unfortunately if the View is hidden (invisible) this event is not emitted since, obviously, there is no need to repaind anything. However I would to know if there is some trick to "cheat" the View to emit the onDraw() event and redraw itself exactly like it would really showed inside the screen.
Basically I need to capture screenshot status of a View component in all its changes but whitout show it (running it in background).
I guess it would be very hard to get such result but, just in case, I'll try to ask.
Thank you
I believe a View's visibility is checked by its parent and not the View itself. You can pass in a Canvas backed by a Bitmap straight in to View#draw(Canvas canvas) and it will draw itself on to the Bitmap. However, the based on the source code of View#setVisibility(), the View's background will still be invisible.
public void setVisibility(int visibility) {
setFlags(visibility, VISIBILITY_MASK);
if (mBGDrawable != null) mBGDrawable.setVisible(visibility == VISIBLE, false);
}
Everything else should appear in the View as is (unless it's children are also set to invisible of course).
EDIT:
Converting a view to Bitmap without displaying it in Android?
There are examples there on how to do that.
public static Bitmap getBitmapFromView(View view) {
Bitmap returnedBitmap = Bitmap.createBitmap(view.getWidth(), view.getHeight(),Bitmap.Config.ARGB_8888);
Canvas canvas = new Canvas(returnedBitmap);
view.draw(canvas);
return returnedBitmap;
}
EDIT 2:
Since setVisibility() is not part of the View, you could override it and simply not set the View to be invisible. Something like:
boolean isInvisible = false;
#Override
public void setVisibility(int visibility) {
if (visibility == View.INVISIBLE) {
invisible = true;
} else {
invisible = false;
super.setVisibility();
}
}
#Override
public void onDraw(Canvas canvas) {
// change state code
if (!invisible) {
// draw code
super.onDraw(canvas);
}
}
#Override
public void draw(Canvas canvas) {
if (!invisible) {
// draw code
super.draw(canvas);
}
}
I have no idea what side-effects this would cause so be extremely careful and weary. Perhaps someone else would have a better solution. Another solution is you can simply call onDraw() on a different canvas whenever you want to draw it. This would require you to create a super class that is the parent layout View of the View you want to draw. Then in the parent's onDraw() method, call the child's onDraw() method separately if it's visibility is set to INVISIBLE.
If someone is interested I managed to solve this problem by override the invalidate() method. Instead of onDraw() that is called by the system only if the view is currently visible the invalidate() function is called "internally" and can be used to check if the view need to be reapinted in the same way.
In my app, I need to set the width of a view based on the width of another view. However, Activity's onCreate() method does not seem to be a good place to do so. The view's width via getWidth() returns 0. Other methods onStart() and onResume() also behave similarly.
I am wondering if there is any method on an Activity that is called after a view has been initialized? Or, is there is another way I can achieve my objective.
Try onGlobalLayoutListener. Get the instance of your main root view, and then just use addOnGlobalLayoutListener() method.
You will receive a callback when your views are already created and measured on Screen:
http://developer.android.com/reference/android/view/ViewTreeObserver.OnGlobalLayoutListener.html
Declare onGlobalLayoutListener in onCreate method:
View firstView = (View) findViewById(R.id.firstView);
View secondView = (View) findViewById(R.id.secondView);
firstView.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
#Override
public void onGlobalLayout() {
// Ensure you call it only once :
firstView.getViewTreeObserver().removeGlobalOnLayoutListener(this);
int width = firstView.getWidth();
int height = firstView.getHeight();
// set dimensions of another view with that dimensions
secondView.setWidth(width);
secondView.setHeight(height);
}
});