Home » Android » android – Usage of forceLayout(), requestLayout() and invalidate()

android – Usage of forceLayout(), requestLayout() and invalidate()

Posted by: admin March 10, 2020 Leave a comment

Questions:

I’m a bit confused about the roles of forceLayout(), requestLayout() and invalidate() methods of the View class.

When shall they be called?

How to&Answers:

To better understand answers provided by François BOURLIEUX and Dalvik I suggest you take a look at this awesome view lifecycle diagram by Arpit Mathur:
enter image description here

Answer:

invalidate()

Calling invalidate() is done when you want to schedule a redraw of the view. It will result in onDraw being called eventually (soon, but not immediately). An example of when a custom view would call it is when a text or background color property has changed.

The view will be redrawn but the size will not change.

requestLayout()

If something about your view changes that will affect the size, then you should call requestLayout(). This will trigger onMeasure and onLayout not only for this view but all the way up the line for the parent views.

Calling requestLayout() is not guaranteed to result in an onDraw (contrary to what the diagram in the accepted answer implies), so it is usually combined with invalidate().

invalidate();
requestLayout();

An example of this is when a custom label has its text property changed. The label would change size and thus need to be remeasured and redrawn.

forceLayout()

When there is a requestLayout() that is called on a parent view group, it does not necessary need to remeasure and relayout its child views. However, if a child should be included in the remeasure and relayout, then you can call forceLayout() on the child. forceLayout() only works on a child if it occurs in conjunction with a requestLayout() on its direct parent. Calling forceLayout() by itself will have no effect since it does not trigger a requestLayout() up the view tree.

Read this Q&A for a more detailed description of forceLayout().

Further study

Answer:

Here you can find some response:
http://developer.android.com/guide/topics/ui/how-android-draws.html

For me a call to invalidate() only refreshes the view and a call to requestLayout() refreshes the view and compute the size of the view on the screen.

Answer:

you use invalidate() on a view that you want to redraw, it’ll make its onDraw(Canvas c) to invoked, and requestLayout() will make the whole layout rendering ( measurement phase and positioning phase) run again. You should use it if you are changing child view’s size on runtime but only in particular cases like constraints from the parent view(by that I mean that the parent height or width are WRAP_CONTENT and so match measure the children before they can wrap them again)

Answer:

This answer is not correct about forceLayout().

As you can see in the code of forceLayout() it merely marks the view as “needs a relayout” but it does neither schedule nor trigger that relayout. The relayout will not happen until at some point in the future the view’s parent was laid out for some other reason.

There is also a much bigger issue when using forceLayout() and requestLayout():

Let’s say you’ve called forceLayout() on a view. Now when calling requestLayout() on a descendent of that view, Android will recursively call requestLayout() on that descendent’s ancestors. The problem is that it will stop the recursion at the view on which you’ve called forceLayout(). So the requestLayout() call will never reach the view root and thus never schedule a layout pass. An entire subtree of the view hierarchy is waiting for a layout and calling requestLayout() on any view of that subtree will not cause a layout. Only calling requestLayout() on any view outside that subtree will break the spell.

I’d consider the implementation of forceLayout() (and how it affects requestLayout() to be broken and you should never use that function in your code.