Category: Programming

Hello guys!

I am working on an application that needs to display certain amount of items in a a scrollable view that have headers on top of every one of them, so as the title says, RecyclerView.

However, my data that is coming from SQLite backed database didn’t fit too well on the mould of just using different kind of view types, because the data was coming from SQLite on demand and if I were to use view types to support headers I would have to process the whole database before showing it and add some kind of layer that provides me with the headers.

After couple of quick googles I found couple of promising libraries , but unfortunately for my need, they didn’t have support for GridLayout or required same kind of view type mapping as regular headers, but I wouldn’t give up now, as I knew RecyclerView provides lovely way of adding custom decorations, which were used to implement timeshops sticky headers, and after studying the code of other implementations I was ready to go!

But first I needed to make my own SpanSizeLookUp implementation to separate different sections of the grid.

So the basic idea is that last item of each section is given with a span that makes it fill the rest of the row, so that the next sections items are dropped to the next row.

First I needed the information of which item belongs to which section, which I made my StickyGridHeadersAdapter in similar fashion as in timeshops implementation.

So the primary function of the adapter is to provide the header id for each position and its corresponding view.

So now that I got the information I am ready to extend the SpanSizeLookUp!

SpanSizeLookUp has one important function of determining each items span size, which is done by implementing getSpanSize method that takes the position of them item as parameter.

My implementation of getSpanSize first determines whether the item in question is the last item before next header, and if so, it calculates the remaining amount of columns in that row, that it needs to fill in order to bump the following item to next row.

So first we implement the last item before next header check, which is quite simple, take current and next item, and if their ids are different, they belong to different categories, meaning that the item in question is actually the last item before next header! Pretty simple huh?

    /**
     * checks if item is last before next header
     * <p/>
     * aka if next items id is different than current
     *
     * @param itemPosition
     * @return
     */
    private boolean isLastBeforeNextHeader(int itemPosition) {
        //header id of item in question
        long headerId = adapter.getHeaderId(itemPosition);

        //next header id, -1 if out of bounds, aka the rest of the row must be filled!
        long nextHeaderId = -1;

        //next item position, aka next item in question
        int nextItemPosition = itemPosition + 1;

        //checking if is within the bounds of the adapter
        if (nextItemPosition >= 0 && nextItemPosition < adapter.getItemCount()) {
            nextHeaderId = adapter.getHeaderId(nextItemPosition);
        }

        //checking if the ids different
        return headerId != nextHeaderId;
    }

Then we need to implement the actual span size lookup, it needs a little bit more logic!

    @Override
    public int getSpanSize(int position) {
        if (isLastBeforeNextHeader(position)) {

            //gets the number of items before this particular position in range of 0..spancount - 1
            int categoryOffset = getNumberOfItemsBeforePositionInCategory(position);

            //gets column index in range of 0..spancount - 1
            int columnIndex = (categoryOffset % spanCount);

            //gets number of extra columns in range of 0..spancount - 1
            int extraColumns = spanCount - (columnIndex + 1);

            return 1 + extraColumns;
        } else {
            //is just any ordinary item, takes one column width..
            return 1;
        }
    }

So first we use our lastly implemented method and check if it actually is the last item within the section, and if not, just return one.

But if the case is different, aka if it is the last item, we first calculate the offset of this item in question within the section, which is needed to calculate the remaining columns within the row, but for that we need to implement a way to calculate those preceding items.

Fortunately for us, its quite simple chore, given the fact that we can query the header id of each item, so basically what we do is we loop through each item preceding the current item til we find a change in header id!

    private int getNumberOfItemsBeforePositionInCategory(int position) {
        long categoryId = adapter.getHeaderId(position);

        for(int i = 1; i < position; i++) {
            if(adapter.getHeaderId(position - i) != categoryId) {
                return i - 1;
            }
        }

        return position;
    }

And to account for the case that this happens to be within the first category, we just return the given position, as it is the offset to the start of the list.

So with that info we can now calculate the column index..

//gets column index in range of 0..spancount - 1
int columnIndex = (categoryOffset % spanCount);

which is just taking modulus of the offset within category and the span count

and to get the amount of items there are left on this row after the particular item, we just substract the column index + 1 from the span count, accounting for the the range of 0.. span count – 1 of the columnIndex instead of 1 – span count

//gets number of extra columns in range of 0..spancount - 1
int extraColumns = spanCount - (columnIndex + 1);

so then we can just return the amount of columns that this is supposed to fill, meaning one plus the amount of extra columns

return 1 + extraColumns;

So now if we run the code, what we can see, is that each category is withing its own boundaries, cool!

This may not be obvious from the preview above, but if you look closely the last item within each section(read: color) is spanning to fill the rest of the row.

Now we only need to add the headers for each section, let the fun begin!

So we need to implement RecyclerView.ItemDecoration, which has two important methods, onDrawOver to draw over the RecyclerView, which handles the drawing of the header view, and getItemOffsets, which provides the offsets to the views, giving enough space for the header in this case, but the possibilities are limitless!

So first lets give enough space for the headers by implementing getItemOffsets!

    @Override
    public void getItemOffsets(Rect outRect, View view, RecyclerView parent, RecyclerView.State state) {
        super.getItemOffsets(outRect, view, parent, state);

        int itemPosition = parent.getChildAdapterPosition(view);

        if (itemPosition == RecyclerView.NO_POSITION) {
            return;
        }

        boolean underHeader = isUnderHeader(itemPosition);

        if (underHeader) {

            View header = getHeaderView(parent, itemPosition);

            outRect.top = header.getHeight();
        }
    }

This method in my case only checks if the item is under header, and if so, gives it top padding from the header height

To determine if item is under header we use isUnderHeader method which is implemented as follows:

    private boolean isUnderHeader(int itemPosition) {
        return isUnderHeader(itemPosition, spanCount);
    }

    /**
     * checks if item is "under header"
     * <p/>
     * Items is under header if any of the following conditions are true:
     * <p/>
     * a) within spanCount the header id has changed once
     *
     * @param itemPosition
     * @return
     */
    private boolean isUnderHeader(int itemPosition, int spanCount) {
        if (itemPosition == 0) {
            return true;
        }

        //get current items header id
        long headerId = adapter.getHeaderId(itemPosition);

        //loop through each item within spancount
        for (int i = 1; i < spanCount + 1; i++) {
            long previousHeaderId = -1;

            int previousItemPosition = itemPosition - i;

            //gets previous items headerId
            if (previousItemPosition >= 0 && previousItemPosition < adapter.getItemCount()) {
                previousHeaderId = adapter.getHeaderId(previousItemPosition);
            }

            //checks if header id at given position is different from previous header id and if so, returns true to indicate this item belongs under the header
            if (headerId != previousHeaderId) {
                return true;
            }
        }

        return false;
    }

We just loop the range of span count and check if there’s any item that has different header id than the given position, and if so, we know that this item belongs under header and needs to be given space!

now we can implement the drawing of the header, this function is a bit more exotic to handle the stickiness of headers, meaning moving the topmost header upwards when it is pushed by the following header.

    @Override
    public void onDrawOver(Canvas c, RecyclerView parent, RecyclerView.State state) {
        super.onDrawOver(c, parent, state);

        final int childCount = parent.getChildCount();

        //checks if there's any childs, aka can we even have any header?
        if (childCount <= 0 || adapter.getItemCount() <= 0) {
            return;
        }

        //stores the "highest" seen top value of any header to perform the pusheroo of topmost header
        int highestTop = Integer.MAX_VALUE;

        //loops through childs in the recyclerview on reverse order to perform the pushing of uppermost header faster, because before it, there is the next headers top stored to highestTop
        for (int i = childCount - 1; i >= 0; i--) {
            View itemView = parent.getChildAt(i);

            //fetches the position within adapter
            int position = parent.getChildAdapterPosition(itemView);

            if (position == RecyclerView.NO_POSITION) {
                continue;
            }

            //only draw if is the first withing recyclerview, aka is the first view in whole tree or if the item in question is the first under its category(or header..)
            if (i == 0 || isFirstUnderHeader(position)) {
                //fetches the header from header provider, which is basically just call to adapters getHeader/bindHeader
                View header = headerProvider.getHeader(parent, position);

                //calculates the translations of the header within view, which is on top of the give item
                int translationX = parent.getLeft();
                int translationY = Math.max(itemView.getTop() - header.getHeight(), 0);

                tempRect.set(translationX, translationY, translationX + header.getWidth(),
                        translationY + header.getHeight());

                //moves the header so it is pushed by the following header upwards
                if(tempRect.bottom > highestTop) {
                    tempRect.offset(0, highestTop - tempRect.bottom);
                }

                //draws the actual header
                drawHeader(parent, c, header, tempRect);

                //stores top of the header to help with the pushing of topmost header
                highestTop = tempRect.top;
            }
        }
    }

so first we perform couple of sanity checks to avoid useless drawing, we check if there actually is any items within the RecyclerView and if not, we just return to avoid useless work

        final int childCount = parent.getChildCount();

        //checks if there's any childs, aka can we even have any header?
        if (childCount <= 0 || adapter.getItemCount() <= 0) {
            return;
        }

then we initialize the value to store the preceeding headers top value to perform the pusheroo of topmost header, as commented

//stores the "highest" seen top value of any header to perform the pusheroo of topmost header
int highestTop = Integer.MAX_VALUE;

then for each item in backward order we do the following:

check if header should be drawn, by checking if is first item, which always should show header, making the sticky header effect, or if is first under any header

if (i == 0 || isFirstUnderHeader(position)) {

and the checking if the item is last header is quite simple, we just check if the preceding item has different header than current

    private boolean isFirstUnderHeader(int position) {
        return position == 0 || adapter.getHeaderId(position) != adapter.getHeaderId(position - 1);
    }

and the item is first in the whole RecyclerView or first within its category, we now know that we can draw the header for this particular item, so we do that!

first we get the header item

//fetches the header from header provider, which is basically just call to adapters getHeader/bindHeader
View header = headerProvider.getHeader(parent, position);

which works in similar fashion as in timeshops implementation, we just call adapters get/bind header methods to obtain the header bound for this header id

then we just calculate the translations for this header, to draw it on top of the item in question

//calculates the translations of the header within view, which is on top of the give item
int translationX = parent.getLeft();
int translationY = Math.max(itemView.getTop() - header.getHeight(), 0);

then we set these values for a rect to pass them around and to more easily apply moving when translation topmost header to give room for the next

tempRect.set(translationX, translationY, translationX + header.getWidth(),
                        translationY + header.getHeight());

Then we adjust the rect position if it is pushed by the following header, aka if the current headers bottom is smaller than highest top, meaning the previously processed headers top

//moves the header so it is pushed by the following header upwards
if(tempRect.bottom > highestTop) {
    tempRect.offset(0, highestTop - tempRect.bottom);
}

then we actually draw the header

//draws the actual header
drawHeader(parent, c, header, tempRect);

and what this method call does is just draws the view on the given canvas within the rect

    public void drawHeader(RecyclerView recyclerView, Canvas canvas, View header, Rect offset) {
        canvas.save();

        canvas.translate(offset.left, offset.top);

        header.draw(canvas);

        canvas.restore();
    }

and then just set the highestTop to current headers top

//stores top of the header to help with the pushing of topmost header
highestTop = tempRect.top;

and I think we’re about to be done here!

After a quick run we can see it works as expected!

However this is just a simple and quick implementation and we might be missing something here, but for my simple use case it works, and if I find out anything, I’ll update here!

Should of probably used thicker header on the preview but… meh!

Thanks for reading anyway!

Today I was implementing an autofilling login form for Android Application which had the following requirements:

• When user logs in he can check “Remember credentials” box to make the application remember the previous credentials
• When user would log in next time if he had saved single username before he would be straightly offered with the previously saved credentials
• If the user had previously entered multiple credentials a drop down will be shown that will let the user decide which of the credentials to use

An obvious choice to perform this task is to use AutoCompleteTextView, but unfortunately it has couple of limitations, which I posted about before, when I created InstantAutoCompleteTextView.

InstantAutoCompleteTextView solved some of these problems, but after that I still had couple of them left to overcome, and most important of them, the AutoCompleteTextView wouldn’t offer auto completion if you haven’t inputted any characters, or that after filling field completely, it would still show the user as hint.

I was using ArrayAdapter as AutoCompleteTextViews username adapter, to list matching usernames, but ArrayAdapters filter uses start of each word for matching prefixes, but in my case, as I was matching for usernames, I would need to match whole username, so basically I had to extend the ArrayAdapter to perform this operation.

Unfortunately ArrayAdapter stores the objects that it has as private field, so my extending adapter wouldn’t have access to them, without reimplementing most of its logic, so what I did, was open the ArrayAdapters source and reimplement most of its business, and reimplement the Filter it provides.

How I changed the Filter is that instead of matching for each word, I matched the whole string, and if the inputted text was exact to the adapters object, it wouldn’t be included, as when user had an exact match, it wouldn’t be needed to be shown anymore.

    private class ExactArrayFilter extends Filter {


        @Override
        protected FilterResults performFiltering(CharSequence prefix) {
            FilterResults results = new FilterResults();

            if(originalObjects == null) {
                originalObjects = Collections.synchronizedList(objects);
            }

            if (prefix == null || prefix.length() == 0) {
                List<T> list = new ArrayList<>(originalObjects);

                results.values = list;
                results.count = list.size();
            } else {
                String prefixString = prefix.toString().toLowerCase();

                final List<T> values = Collections.synchronizedList(originalObjects);

                final int count = values.size();
                final ArrayList<T> newValues = new ArrayList<>();

                for (int i = 0; i < count; i++) {
                    final T value = values.get(i);
                    final String valueText = value.toString().toLowerCase();

                    // Match against the whole with prefix and disinclude exact matches
                    if (valueText.startsWith(prefixString) && !valueText.equals(prefixString)) {
                        newValues.add(value);
                    }
                }

                results.values = newValues;
                results.count = newValues.size();
            }

            return results;
        }

        @Override
        protected void publishResults(CharSequence constraint, FilterResults results) {
            objects = (List<T>) results.values;
            if (results.count > 0) {
                notifyDataSetChanged();
            } else {
                notifyDataSetInvalidated();
            }
        }
    }

Now I have my Filter that makes the popup disappear after matching input is typed into.

Then I just have to implement the piece of code that handles the password autofill, and how I did that, is I stored set of user credentials as plain text(very secure) in JSON string to preferences, and now I could just when the AutoCompleteTextViews value changes, look up if there’s a already saved password and use that.

So what that basically meant is to add an TextWatcher and on afterTextChanged perform this check and set a fake password on the password field and store the selected password, or “override password” on other field to be used on login, so the potentially blackhat user can’t use the number of characters in the password to gain access to the password more easily.

Also now that I knew if the password was “fake” and “overriden” I could add a TextWatcher and another afterTextChanged callback that would reset the password if override was present, to imitate usual autofill behaviour.

Below you can see the final result

https://youtube.com/watch?v=tng9motar1E

Thanks for reading!

Hello guys!

Today I was stuck with yet another Android related problem, I was creating a edit text with auto completion that always shows the drop down to help the user remember the last choices.

So the ultimate requirement is to on activity open show list of all items in the auto completion adapter in a drop-down, which should be simple right?

The obvious choice is to use AutoCompleteTextView, which provides the feature of listing possible choices after inputting certain amount of characters, however I knew that it wouldn’t show the listing before inputting certain amount of characters.

First thing to do was of course to open the documentation which didn’t appear to contain any helpful information, however I threw together the activity with one of these view with a dummy adapter and tested how it works normally.

Normally it takes two matching characters to get the drop down to appear, and there is actually a option to set the threshold, so I decided to set it to 0 and checked what would happen if I focused the field

Judging from the documentation of setThreshold I know that if the threshold is set under one, the threshold is actually being set to minimum of one.

When threshold is less than or equals 0, a threshold of 1 is applied.

source: developer.android.com

No luck.. You have to input at least one character to make the popup appear. I think its time to browse through the source..

First thing I noticed was that the AutoCompleteTextView has reference to the ListPopupWindow which is ultimately used to show the actual popup that lists the possible options

I looked for the conditions which needs to be fulfilled in order for the popup to be shown and found out that there is actually a flag on the ListPopupWindow, isDropDownAlwaysVisible, which also has a setter, setDropDownAlwaysVisible, so I was sure I was onto something!

Unfortunately the setDropDownAlwaysVisible setter had a @hide javadoc annotation, which removes the method from public with the following comment

Only used by AutoCompleteTextView under special conditions.

So I decided to find out those conditions, but there was only single method call to this setter within AutoCompleteTextView which was just a delegate setter which too had a @hide javadoc annotation, but this time the comment declared that it was pending API council approval, which meant this was a dead end.

This basically meant that I needed to hook on the onFocusChanged by either subclassing or setting a listener and within that hook make the popup visible.

Luckily there was public showDropDown method within AutoCompleteTextView which meant that subclassing wouldn’t be necessary and this could be a matter of a simple View.OnFocusChangeListener, the day was saved!

So I created a View.OnFocusChangeListener and implemented its onFocusChange callback that calls showDropDown on focus

Unfortunately it turns out that the onFocusChange may be called even if the window containing isn’t necessarily visible, so this is fixed with matter of checking the fact, right?

So I added a check if getWindowVisiblity of the view that the onFocusChange listener passes as parameter is visible, but as it turns out after orientation change the View.OnFocusChangeListener is triggered before the window is visible, okay.. we have a problem here..

I thought to myself, okay.. so if we add a callback to the view so I’ll know when the containing view is visible, right? Unfortunately the View doesn’t provide an API or a mechanism to do attach a callback to the onAttachedToWindow method, so I had to scrape that idea..

After swallowing my pride that this couldn’t be implemented using a listener, I decided to give it a try to subclass the AutoCompleteTextView and override nessaccary methods such as onFocusChanged and onAttachedToWindow

I ended up creating a little helper method showDropDownIfFocused that checks whether enoughToFilter, isFocused and getWindowVisibility matches the conditions and if so, call the showDropDown method of AutoCompleteTextView and calling this from onFocusChanged  and onAttachedToWindow

I also override enoughToFilter method to make sure that I could set minimum threshold to zero using a special flag of my new InstantAutoCompleteTextView

Below you can see the final result of the progress.

 

The final source code can be obtained as gist here

If you have better ideas of how the problem could be solved, feel free to comment below.

Thanks for reading