onMeasure custom view explanation

Question

I tried to do custom component  I extended View class and do some drawing in onDraw overrided method  Why I need to override onMeasure  If I didn t  everything seen to be right  May someone explain it  How should I write my onMeasure method  I ve seen couple tutorials  but each one is a little bit different than the other  Sometimes they call super onMeasure at the end  sometimes they use setMeasuredDimension and didn t call it  Where is a difference   After all I want to use several exactly the same components  I added those components to my XML file  but I don t know how big they should be  I want to set its position and size later  why I need to set size in onMeasure if in onDraw when I draw it  is working as well  in custom component class  When exactly I need to do that

User · Answer

actually  your answer is not complete as the values also depend on the wrapping container  In case of relative or linear layouts  the values behave like this    EXACTLY match parent is EXACTLY   size of the parent AT MOST wrap content results in an AT MOST MeasureSpec UNSPECIFIED never triggered   In case of an horizontal scroll view  your code will work

User · Answer

If you don t need to change something onMeasure - there s absolutely no need for you to override it    Devunwired code  the selected and most voted answer here  is almost identical to what the SDK implementation already does for you  and I checked - it had done that since 2009     You can check the onMeasure method here    protected void onMeasure int widthMeasureSpec  int heightMeasureSpec        setMeasuredDimension getDefaultSize getSuggestedMinimumWidth    widthMeasureSpec               getDefaultSize getSuggestedMinimumHeight    heightMeasureSpec       public static int getDefaultSize int size  int measureSpec        int result   size      int specMode   MeasureSpec getMode measureSpec       int specSize   MeasureSpec getSize measureSpec        switch  specMode        case MeasureSpec UNSPECIFIED          result   size          break      case MeasureSpec AT MOST      case MeasureSpec EXACTLY          result   specSize          break            return result      Overriding SDK code to be replaced with the exact same code makes no sense    This official doc s piece that claims  the default onMeasure   will always set a size of 100x100  - is wrong

User · Answer

onMeasure   is your opportunity to tell Android how big you want your custom view to be dependent the layout constraints provided by the parent  it is also your custom view s opportunity to learn what those layout constraints are  in case you want to behave differently in a match parent situation than a wrap content situation    These constraints are packaged up into the MeasureSpec values that are passed into the method   Here is a rough correlation of the mode values    EXACTLY means the layout width or layout height value was set to a specific value   You should probably make your view this size   This can also get triggered when match parent is used  to set the size exactly to the parent view  this is layout dependent in the framework   AT MOST typically  means the layout width or layout height value was set to match parent or wrap content where a maximum size is needed  this is layout dependent in the framework   and the size of the parent dimension is the value   You should not be any larger than this size  UNSPECIFIED typically means the layout width or layout height value was set to wrap content with no restrictions   You can be whatever size you would like   Some layouts also use this callback to figure out your desired size before determine what specs to actually pass you again in a second measure request    The contract that exists with onMeasure   is that setMeasuredDimension   MUST be called at the end with the size you would like the view to be   This method is called by all the framework implementations  including the default implementation found in View  which is why it is safe to call super instead if that fits your use case   Granted  because the framework does apply a default implementation  it may not be necessary for you to override this method  but you may see clipping in cases where the view space is smaller than your content if you do not  and if you lay out your custom view with wrap content in both directions  your view may not show up at all because the framework doesn t know how large it is   Generally  if you are overriding View and not another existing widget  it is probably a good idea to provide an implementation  even if it is as simple as something like this    Override protected void onMeasure int widthMeasureSpec  int heightMeasureSpec         int desiredWidth   100      int desiredHeight   100       int widthMode   MeasureSpec getMode widthMeasureSpec       int widthSize   MeasureSpec getSize widthMeasureSpec       int heightMode   MeasureSpec getMode heightMeasureSpec       int heightSize   MeasureSpec getSize heightMeasureSpec        int width      int height         Measure Width     if  widthMode    MeasureSpec EXACTLY              Must be this size         width   widthSize        else if  widthMode    MeasureSpec AT MOST              Can t be bigger than            width   Math min desiredWidth  widthSize         else             Be whatever you want         width   desiredWidth               Measure Height     if  heightMode    MeasureSpec EXACTLY              Must be this size         height   heightSize        else if  heightMode    MeasureSpec AT MOST              Can t be bigger than            height   Math min desiredHeight  heightSize         else             Be whatever you want         height   desiredHeight               MUST CALL THIS     setMeasuredDimension width  height       Hope that Helps

[android] onMeasure custom view explanation

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