I'm writing an app with the sole purpose of trying to understand how the view hierarchy in Android works. I am having some really harsh problems in it right now. I'll try to be concise in my explanation here.
Setup:
Currently I have three Views. 2 are ViewGroups and 1 is just a View. Let's say they're in this order:
TestA extends ViewGroup
TestB extends ViewGroup
TestC extends View
TestA->TestB->TestC
Where TestC is in TestB and TestB is in TestA.
In my Activity I simply display the views like so:
TestA myView = new TestA(context);
myView.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,ViewGroup.LayoutParams.MATCH_PARENT));
setContentView(myView);
Problems:
The onDraw(Canvas canvas) method of TestA is never called. I've seen a couple solutions to this saying that my view doesn't have any dimensions (height/width = 0), however, this is not the case. When I override onLayout(), I get the dimensions of my layout and they are correct. Also, getHeight()/Width() are exactly as they should be. I can also override dispatchDraw() and get my base views to draw themselves.
I want to animate an object in TestB. Traditionally, I would override the onDraw() method on call invalidate() on itself until the object finish the animation it was supposed to do. However, in TestB, when I call invalidate() the view never gets redrawn. I'm under the impression that it's the job of my parent view to call the onDraw() method again, but my parent view does not call the dispatchDraw() again.
I guess my questions are, why would my onDraw() method of my parent view never get called to begin with? What methods in my parent view are supposed to be called when one of it's children invalidate itself? Is the parent the one responsible for ensure it's children get drawn or does Android take care of that? If Android responds to invalidate(), why does my TestB never get drawn again?
Ok, after some research and a lot of trying, I've found out I was doing three things wrong in regards to problem #2. A lot of it was simple answers but not very obvious.
You need to override onMeasure() for every view. Within onMeasure(), you need to call measure() for every child contained in the ViewGroup passing in the MeasureSpec that the child needs.
You need to call addView() for every child you want to include. Originally, I was simply created a view object and using it directly. This allowed me to draw it once, but the view was not include in the view tree, thus it when I called invalidate() it wasn't invalidating the view tree and not redrawing. For example:
In testA:
TestB childView;
TestA(Context context){
****** setup code *******
LayoutParams params = new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
childView = new TestB(context);
childView.setLayoutParams(params);
}
#Override
protected void dispatchDraw(Canvas canvas){
childView.draw(canvas);
}
This will draw the child view once. However, if that view needs updating for animations or whatever, that was it. I put addView(childView) in the constructor of TestA to add it to the view tree. Final code is as such:
TestB childView;
TestA(Context context){
****** setup code *******
LayoutParams params = new LayoutParams(LayoutParams.WRAP_CONTENT, LayoutParams.WRAP_CONTENT);
childView = new TestB(context);
childView.setLayoutParams(params);
addView(childView);
}
#Override
protected void dispatchDraw(Canvas canvas){
childView.draw(canvas);
}
Alternatively, I could override dispatchDraw(Canvas canvas) like so if I had many more children to draw, but I need some custom element between each drawing like grid lines or something.
#Override
protectd void dispatchDraw(Canvas canvas){
int childCount = getChildCount();
for(int i = 0; i < size; i++)
{
drawCustomElement();
getChildAt(i).draw(canvas);
}
}
You must override onLayout() (it's abstract in ViewGroup anyway, so it's required anyway). Within the this method, you must call layout for every child. Even after doing the first two things, my views wouldn't invalidate. As soon as I did this, everything worked just perfectly.
UPDATE:
Problem #1 has been solved. Another extremely simply but not-so-obvious solution.
When I create an instance of TestA, I have to call setWillNotDraw(false) or else Android will not draw it for optimization reasons. So the full setup is:
TestA myView = new TestA(context);
myView.setLayoutParams(new ViewGroup.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,ViewGroup.LayoutParams.MATCH_PARENT));
myView.setWillNotDraw(false);
setContentView(myView);
This isn't a direct answer, but here is a fantastic tutorial on how to draw custom views and gives a great crash cours in how to animate a view. The code is very simple and clean too.
Hope this helps!
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()
I am trying to do calculate view's x,y positions after completion of loading of activity. What I did is view.postDelayed(runnable, 2000) which is working fine. code reviewer is not happy with this and suggested to use OnGlobalLayoutListener to know about the completion of activity loading. Somehow I don't like OnGlobalLayoutListener because it is associated with entire view tree which is not required for my solution. I am trying to understand pros and cons of these approaches. Thanks!
If all you are trying to do is read the view's x and y coordinates, I recommend using view.post(Runnable) with no delay (unless there is a good reason to include a delay). This will add the Runnable to a message queue to in the UI thread. The Runnable will wait to execute until after your View is inflated and attached to the window. Since View position property values depend on the view's layout context, posting a Runnable will give you the timing that you are looking for.
As you mentioned in your question description, an OnGlobalLayoutListener will apply to the entire View's layout as the class name suggests. An OnGlobalLayoutListener should only be considered if you are concerned with the layout state or visibility of any or all views within the view tree. I.e. anything that causes the view tree to be re-laid out.
Code reviewer is not happy because you wait 2s and guess that the loading of the activity is finished by then. This may be the case with your emulator or device but on older and slower devices the activity may not have finished loading. To be 100% safe that the activity has finished loading you should use the listener to inform the 'listener' when loading is completed.
I think that the correct way of doing this is by adding an onPreDrawListener to the view. This is the listener that is called when the view is about to be drawn, and where you already have all the information about it's size (width and height) and position (X and Y).
Example:
final View v = new View(getContext());
v.getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
#Override
public boolean onPreDraw() {
v.getViewTreeObserver().removeOnPreDrawListener(this);
float myX = v.getX();
float myY = v.getY();
return true;
}
});
Don't forget to make the method onPreDraw return true, I don't know why but Android Studio makes it return false when you create the listener.
Also don't forget to remove the listener from the view, otherwise it might be uselessly called again.
I have a fragment and I need to measure location/width/height of its views on screen and pass to some other class.
So what I have is a function which does it, something like this :
private void measureTest(){
v = ourView.findViewById(R.id.someTextField);
v.getLocationOnScreen(loc);
int w = v.getWidth();
...
SomeClass.passLocation(loc,w);
...
The problem is that the location/width/height of views is not ready within fragment lifecycle.
So if I run that function within these lifecycle methods :
onCreateView
onViewCreated
onStart
onResume
I either get wrong location and width/height measurments or 0 values.
The only solution I found is to add a GlobalLayoutListener like this to mainView
mainView.getViewTreeObserver().addOnGlobalLayoutListener(new ViewTreeObserver.OnGlobalLayoutListener() {
public void onGlobalLayout() {
if(alreadyMeasured)
mainView.getViewTreeObserver().removeGlobalOnLayoutListener(this);
else
measureTest();
}
});
this gets the job done.. but its just Yack! IMO.
Is there a better way of doing this? seems like such a basic thing to do
inside onActivityCreated of your fragment retrieve the currentView (with getView()) and post a runnable to its queue. Inside the runnable invoke measureTest()
There is no better way. That code isn't that bad! It's fired as soon as the view is layed out (my terminology might be a bit weird there) which happens right after measuring. That is how it is done in the BitmapFun sample (see ImageGridFragment, line 120) in Google's Android docs. There is a comment on that particular piece of code stating:
// This listener is used to get the final width of the GridView and then calculate the
// number of columns and the width of each column. The width of each column is variable
// as the GridView has stretchMode=columnWidth. The column width is used to set the height
// of each view so we get nice square thumbnails.
I have a Class called LineGraphView which is a subclass of View. I want to be able to pass some data to this view so that it can then draw accordingly in its onDraw() method.
Analysis Fragment (Where the view is referenced and method is called from)
LineGraphView altitudeGraph;
LineGraphView speedGraph;
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup group, Bundle saved)
{
super.onCreateView(inflater, group, saved);
View view = inflater.inflate(R.layout.analysisfrag, group, false);
this.altitudeGraph = (LineGraphView) view.findViewById(R.id.linegraph_Altitude);
this.speedGraph = (LineGraphView) view.findViewById(R.id.linegraph_Speed);
return view;
}
My question is, if I then want to be able to access that views width and height attributes using getWidth() and getHeight(), when am I able to do so because so far all thats being returned is 0.
Do I need to create a listener which lets the LineGraphView's parent know its ready to be accessed?
Thanks in advance.
If you're already creating your own custom view, I would just add some private variables with getters/setters for the information you need to pass. From there, just handle the drawing in your onDraw() method -- from there, you can get your View's width and height (or just the Canvas width and height, which might be the same values).
The answer is here: https://stackoverflow.com/a/3594216/969325
Basically you are calling getWidth/height too early (during onCreate). I believe you could use a handler to get it after onCreate. You could also try this: https://stackoverflow.com/a/10118459/969325 (using onWindowFocusChanged).
In my application I have an infinite loop on one of my View's onMeasure() overrides. Debugging the source code starting from a break point in my onMeasure, I am able to trace myself all the way up the stack trace up to the PhoneWindow$DecorView's measure() (top most class in my View Hierarchy), which gets called by ViewRoot.performTraversals(). Now from here if I keep stepping over, I eventually get the PhoneWindow$DecorView's measure() called again by a message in the Looper.loop() class. I'm guessing something has queued up a message that it needs to remeasure, like an invalidate.
My question is, what triggers that a measure call needs to occur on a View?
From my understanding of the layout/measure/draw process, this will only occur when the invalidate() method is called on a specific view, and that will trickle down and perform a layout/measure/draw pass for that view invalidated and all of its children. I would assume that somehow my top most View in my View Hierarchy is getting invalidated.
However, I've explicitly put a break point on every single invalidate call I have, and am not calling invalidate in some infinite manner. So I do not think that is the case. Is there another way to trigger a measure pass? Could something internally be triggering this? I'm kind of out of ideas after seeing that nothing is infinity invalidating.
In order to trigger a measure pass for a custom View you must call the requestLayout() method.
For example, if you are implementing a custom view that extends View and it will behave like a TextView, you could write a setText method like this:
/**
* Sets the string value of the view.
* #param text the string to write
*/
public void setText(String text) {
this.text = text;
//calculates the new text width
textWidth = mTextPaint.measureText(text);
//force re-calculating the layout dimension and the redraw of the view
requestLayout();
invalidate();
}
Well, If you are changing a View's content, it will eventually have to call invalidate(). For example, you have a TextView with a text called "Text 1". Now, you change the text of the same TextView to "Text 2". Here aslo, invalidate will be called.
So basically, when something changes on the view, more often than not, you would expect the invalidate method to be called, and a corresponding call to the measure().
Look at the source code for TextView, for example.
http://www.google.com/codesearch#uX1GffpyOZk/core/java/android/widget/TextView.java&q=TextView%20package:android&type=cs
Count the number of invalidate calls. There are quite a few.