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Android UI Not Crashing When Modifying View off UI Thread

Scenario:
I ran into a strange issue while testing out threads in my fragment.
I have a fragment written in Kotlin with the following snippet in onResume():
override fun onResume() {
super.onResume()
val handlerThread = HandlerThread("Stuff")
handlerThread.start()
val handler = Handler(handlerThread.looper)
handler.post {
Thread.sleep(2000)
tv_name.setText("Something something : " + isMainThread())
}
}
is MainThread() is a function that checks if the current thread is the main thread like so:
private fun isMainThread(): Boolean = Looper.myLooper() == Looper.getMainLooper()
I am seeing my TextView get updated after 2 seconds with the text "Something something : false"
Seeing false tells me that this thread is currently not the UI/Main thread.
I thought this was strange so I created the same fragment but written in Java instead with the following snippet from onResume():
#Override
public void onResume() {
super.onResume();
HandlerThread handlerThread = new HandlerThread("stuff");
handlerThread.start();
new Handler(handlerThread.getLooper()).post(new Runnable() {
#Override
public void run() {
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
e.printStackTrace();
}
textView.setText("Something something...");
}
});
}
The app crashes with the following exception as expected:
android.view.ViewRootImpl$CalledFromWrongThreadException: Only the original thread that created a view hierarchy can touch its views.
at android.view.ViewRootImpl.checkThread(ViewRootImpl.java:7313)
at android.view.ViewRootImpl.requestLayout(ViewRootImpl.java:1161)
I did some research but I couldn't really find something that explains this. Also, please assume that my views are all inflated correctly.
Question:
Why does my app not crash when I modify my TextView in the runnable that's running off my UI thread in the Fragment written in Kotlin?
If there's something in some documentation somewhere that explains this, can someone please refer me to this?
I am not actually trying to modify my UI off the UI thread, I am just curious why this is happening.
Please let me know if you guys need any more information. Thanks a lot!
Update:
As per what #Hong Duan mentioned, requestLayout() was not getting called. This has nothing to do with Kotlin/Java but with the TextView itself.
I goofed and didn't realize that the TextView in my Kotlin fragment has a layout_width of "match_parent." Whereas the TextView in my Java fragment has a layout_width of "wrap_content."
TLDR: User error + requestLayout(), where thread checking doesn't always occur.

The CalledFromWrongThreadException only throws when necessary, but not always. In your cases, it throws when the ViewRootImpl.checkThread() is called during ViewRootImpl.requestLayout(), here is the code from ViewRootImpl.java:
#Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
checkThread();
mLayoutRequested = true;
scheduleTraversals();
}
}
void checkThread() {
if (mThread != Thread.currentThread()) {
throw new CalledFromWrongThreadException(
"Only the original thread that created a view hierarchy can touch its views.");
}
}
And for TextView, it's not always necessary to relayout when we update it's text, we can see the logic in the source code:
/**
* Check whether entirely new text requires a new view layout
* or merely a new text layout.
*/
private void checkForRelayout() {
// If we have a fixed width, we can just swap in a new text layout
// if the text height stays the same or if the view height is fixed.
if ((mLayoutParams.width != LayoutParams.WRAP_CONTENT
|| (mMaxWidthMode == mMinWidthMode && mMaxWidth == mMinWidth))
&& (mHint == null || mHintLayout != null)
&& (mRight - mLeft - getCompoundPaddingLeft() - getCompoundPaddingRight() > 0)) {
// Static width, so try making a new text layout.
int oldht = mLayout.getHeight();
int want = mLayout.getWidth();
int hintWant = mHintLayout == null ? 0 : mHintLayout.getWidth();
/*
* No need to bring the text into view, since the size is not
* changing (unless we do the requestLayout(), in which case it
* will happen at measure).
*/
makeNewLayout(want, hintWant, UNKNOWN_BORING, UNKNOWN_BORING,
mRight - mLeft - getCompoundPaddingLeft() - getCompoundPaddingRight(),
false);
if (mEllipsize != TextUtils.TruncateAt.MARQUEE) {
// In a fixed-height view, so use our new text layout.
if (mLayoutParams.height != LayoutParams.WRAP_CONTENT
&& mLayoutParams.height != LayoutParams.MATCH_PARENT) {
autoSizeText();
invalidate();
return; // return with out relayout
}
// Dynamic height, but height has stayed the same,
// so use our new text layout.
if (mLayout.getHeight() == oldht
&& (mHintLayout == null || mHintLayout.getHeight() == oldht)) {
autoSizeText();
invalidate();
return; // return with out relayout
}
}
// We lose: the height has changed and we have a dynamic height.
// Request a new view layout using our new text layout.
requestLayout();
invalidate();
} else {
// Dynamic width, so we have no choice but to request a new
// view layout with a new text layout.
nullLayouts();
requestLayout();
invalidate();
}
}
As you can see, in some cases, the requestLayout() is not called, so the main thread check is not introduced.
So I think the key point is not about Kotlin or Java, it's about the TextViews' layout params which determined whether requestLayout() is called or not.

Most likely, in Kotlin case, there is some overhead in setText() which assures that it runs in UI thread.

How to get the exact touch position of a image?

I have an idea for implementing of a login with password as the image touch positions when the member touch the same position I have to login the user.Can anyone help me how to implement it I can get the position of screen only. and the x and y coordinated are changing even when I touch the same position.

I would say create a custom View that extends ImageView:
public class PasswordView extends ImageView {
public PasswordView(Context context) {
super(context);
// Any additional PasswordView setup
}
#Override
public boolean onTouchEvent(MotionEvent event) {
float xTouchPos = event.getX();
float yTouchPos = event.getY();
// If the PasswordView was clicked
if (event.getAction() == MotionEvent.ACTION_DOWN) {
// Check if they touched in the secret spot
}
return super.onTouchEvent(event);
}
}
By subclassing ImageView, you will still have access to all the standard features of an ImageView (setting an image source, etc.) and you will also be able to add the additional functionality for checking a user's touch position.
Here's an example project that uses onTouchEvent.

LikeView (android view) in libgdx

I'm trying to place a Like button in my app. After much searching I found that it is impossible to use me own custom button, so I am left only with implementing the default like button from the facebook sdk.
Since this LikeView seems to be a native android like view I don't really know how to put this into my libGDX app.
I would like to use this button only in a specific Screen and set its bounds so that it fits with the rest of my UI. Does anyone have an example of how to create this like button without using the XML (as it is done in all the documentation I have found so far).

Adding the following functions to my application makes it show in the correct position. Unfortunately the LikeView does not get the correct size, but is centered inside the view, which means changing the width/height just moves it.
public void GenerateLikeButton()
{
application.runOnUiThread(new Runnable(){
#Override
public void run() {
float x = 560 * game.global_scale;
int width = (int) (440 * game.global_scale);
int height = (int) (152* game.global_scale);
float y_from_bottom = game.screen_height - ((56+152+70) * game.global_scale + game.ad_height);
Gdx.app.log("like", "from bottom: "+ y_from_bottom);
likeButton = new LikeView(application);
likeButton.setLikeViewStyle(LikeView.Style.BUTTON);
likeButton.setX(x);
likeButton.setY(y_from_bottom-height);
likeButton.setObjectId(LIKE_URL);
likeButton.setVisibility(View.GONE);
application.layout.addView(likeButton,width,height);
likeButton.invalidate();
}
});
}
#Override
public void ShowLikeButton(final boolean visible)
{
application.runOnUiThread(new Runnable()
{
#Override
public void run()
{
if(visible)
likeButton.setVisibility(View.VISIBLE);
else
likeButton.setVisibility(View.GONE);
}
});
}

How to find all pixels touched on an image view?

On my app I have an ImageView which I turned into a Bitmap for editing. I need to detect which pixels on the ImageView were touched by the user. In addition, if the user draws a line with his finger, I need to know all the pixels that were touched in order to change them. How do I detect which pixels were touched?

Ok Jonah, here are some directions for you.
I guess you want that blending effect to react quickly to user input so first thing you'd better go for a custom SurfaceView instead of a ImageView because it is more suitable for drawing high frame rate animations required in 2D action games and animations. I strongly recommend you to read this guide; giving special attention to the part about the use of SurfaceView, before going any further. You will basically need to create a class that extends SurfaceView and implements SurfaceHolder.Callback. This view will then be responsible to listen for user touch events and to render the frames to animate the blending effect.
Take a look at following code as a reference:
public class MainView extends SurfaceView implements SurfaceHolder.Callback {
public MainView(Context context, AttributeSet attrs) {
super(context, attrs);
SurfaceHolder holder = getHolder();
holder.addCallback(this); // Register this view as a surface change listener
setFocusable(true); // make sure we get key events
}
#Override
public boolean onTouchEvent(MotionEvent event) {
super.onTouchEvent(event);
// Check if the touch pointer is the one you want
if (event.getPointerId(event.getActionIndex()) == 0) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
// User touched screen...
case MotionEvent.ACTION_CANCEL:
// User dragged his finger out of the view bounds...
case MotionEvent.ACTION_UP:
// User raised his finger...
case MotionEvent.ACTION_MOVE:
// User dragged his finger...
// Update the blending effect bitmap here and trigger a frame redraw,
// if you don't already have an animation thread to do it for you.
return true;
}
return false;
}
/*
* Callback invoked when the Surface has been created and is ready to be
* used.
*/
public void surfaceCreated(SurfaceHolder holder) {
// You need to wait for this call back before attempting to draw
}
/*
* Callback invoked when the Surface has been destroyed and must no longer
* be touched. WARNING: after this method returns, the Surface/Canvas must
* never be touched again!
*/
public void surfaceDestroyed(SurfaceHolder holder) {
// You shouldn't draw to this surface after this method has been called
}
}
Then use it on the layout of your "drawing" activity like this:
<com.xxx.yyy.MainView
android:id="#+id/main_view"
android:layout_width="match_parent"
android:layout_height="match_parent"/>
To draw to this surface you need the following code:
Canvas c = null;
try {
c = mSurfaceHolder.lockCanvas(null);
synchronized (mSurfaceHolder) {
if (c != null)
c.drawBitmap(blendingImage, 0, 0, null); // Render blending effect
}
} catch (Exception e) {
Log.e("SurfaceView", "Error drawing frame", e);
} finally {
// do this in a finally so that if an exception is thrown
// during the above, we don't leave the Surface in an
// inconsistent state
if (c != null) {
mSurfaceHolder.unlockCanvasAndPost(c);
}
}
A fully functional example would be impractical to put in an answer so I recommend you to download the Lunar Lander sample game from Google for a full working example. Note however, that you won't need a game animation thread (although it won't hurt having one), like the one coded in the Lunar Lander sample, if all you need is the blending effect. The purpose of that thread is to create a game loop in which game frames are constantly generated to animate objects that may or may not depend on user input. In your case, all you need is to trigger a frame redraw after processing each touch event.
EDIT: The following code are fixes to get the code you've provided in the comments, working.
Here are the changes to MainActivity:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// put pics from drawables to Bitmaps
Resources res = getResources();
BitmapDrawable bd1 = (BitmapDrawable) res.getDrawable(R.drawable.pic1);
// FIX: This block makes `operation` a mutable bitmap version of the loaded resource
// This is required because immutable bitmaps can't be changed
Bitmap tmp = bd1.getBitmap();
operation = Bitmap.createBitmap(tmp.getWidth(), tmp.getHeight(), Bitmap.Config.ARGB_8888);
Canvas c = new Canvas(operation);
Paint paint = new Paint();
c.drawBitmap(tmp, 0f, 0f, paint);
BitmapDrawable bd2 = (BitmapDrawable) res.getDrawable(R.drawable.pic2);
bmp = bd2.getBitmap();
myView = new MainView(this, operation, bmp);
FrameLayout preview = (FrameLayout) findViewById(R.id.preview);
preview.addView(myView);
}
...
Here are the changes to the MainView class:
public class MainView extends SurfaceView implements Callback {
private SurfaceHolder holder;
private Bitmap operation;
private Bitmap bmp2;
private boolean surfaceReady;
// took out AttributeSet attrs
public MainView(Context context, Bitmap operation, Bitmap bmp2) {
super(context);
this.operation = operation;
this.bmp2 = bmp2;
holder = getHolder(); // Fix: proper reference the instance variable
holder.addCallback(this); // Register this view as a surface change
// listener
setFocusable(true); // make sure we get key events
}
// Added so the blending operation is made in one place so it can be more easily upgraded
private void blend(int x, int y) {
if (x >= 0 && y >= 0 && x < bmp2.getWidth() && x < operation.getWidth() && y < bmp2.getHeight() && y < operation.getHeight())
operation.setPixel(x, y, bmp2.getPixel(x, y));
}
// Added so the drawing is now made in one place
private void drawOverlays() {
Canvas c = null;
try {
c = holder.lockCanvas(null);
synchronized (holder) {
if (c != null)
c.drawBitmap(operation, 0, 0, null); // Render blending
// effect
}
} catch (Exception e) {
Log.e("SurfaceView", "Error drawing frame", e);
} finally {
// do this in a finally so that if an exception is thrown
// during the above, we don't leave the Surface in an
// inconsistent state
if (c != null) {
holder.unlockCanvasAndPost(c);
}
}
}
#Override
public boolean onTouchEvent(MotionEvent event) {
super.onTouchEvent(event);
if (!surfaceReady) // No attempt to blend or draw while surface isn't ready
return false;
// Check if the touch pointer is the one you want
if (event.getPointerId(event.getActionIndex()) == 0) {
switch (event.getAction()) {
case MotionEvent.ACTION_DOWN:
// User touched screen. Falls through ACTION_MOVE once there is no break
case MotionEvent.ACTION_MOVE:
// User dragged his finger...
blend((int) event.getX(), (int) event.getY());
}
// Update the blending effect bitmap here and trigger a frame
// redraw,
// if you don't already have an animation thread to do it for you.
drawOverlays();
return true;
}
return false;
}
/*
* Callback invoked when the Surface has been created and is ready to be
* used.
*/
public void surfaceCreated(SurfaceHolder holder) {
surfaceReady = true;
drawOverlays();
}
/*
* Callback invoked when the Surface has been destroyed and must no longer
* be touched. WARNING: after this method returns, the Surface/Canvas must
* never be touched again!
*/
public void surfaceDestroyed(SurfaceHolder holder) {
// You shouldn't draw to this surface after this method has been called
surfaceReady = false;
}
#Override
public void surfaceChanged(SurfaceHolder holder, int format, int width,
int height) {
// TODO Auto-generated method stub
}
}
This code works for me. I just hope I didn't forget anything =:)
Let me know if you still have trouble, ok?

So the answer to this is that you're going to have to be a little clever, but it really shouldn't be so bad. Instead of posting all the code to do what you want to do, I'll give you a link here and an explanation.
So by managing the touch events of an application, you can figure out the average coordinates of a touch event. Using that information you can determine the center of all the pixels touched and continue to track that as a line is drawn with the finger. To track a straight line use the
case MotionEvent.ACTION_DOWN:
case MotionEvent.ACTION_UP:
clauses to determine the start and end of the line. If you want to track a line that is not straight and drawn, you're going to need a little more tracking using
case MotionEvent.ACTION_MOVE:
and that should get you fairly started. You may need a little bit of logic to deal with the fact that you will be drawing a very thin line and I suspect that's not quite what you're going for. Maybe it is though. Either way this should be a good place to get started.
EDIT
In regards to your first comment, here is a link you can use for an example. I have to make a small disclaimer though. To get all of the pieces to work together correctly, it may not seem that simple at first. I assure you that this is one of the simplest examples and breaks the tutorial into sections.
For what you would like to do, I think you'll want to pay particular attention to section 2 (no to be confused with step 2):
2. Facilitate Drawing
I suggest this because it shows different ways to use information form the TouchEvent. The things included in section 1 will explain a little bit about the environment to setup displaying a TouchEvent's captured data whereas section 3 is mostly about aesthetics. This may not directly answer your question, but I suspect it will get you where you need to be.
Happy coding! Leave a comment if you have any questions.

Android: Make screen "flash" white

I want to make my android screen "flash" white temporarily (only once). It needs to be long enough that the user will be able to tell that the screen did in fact turn white.
The user will land on a screen, which will flash white after 5 seconds, then display the original screen again (possibly fading from white).
What would be the best way to do this? Right now I am creating a rectangle that takes up the entire screen and programatically showing and hiding it.

Add a foreground by creating an overlaying (match_parent size) ImageView and set android:src to a white drawable.
Set the visibility of this view to android:visibility="gone".
When you want to show the white flash, I recommend an animation to fade the white flash in and display it.
This is a basic animation which will fade in an ImageView's drawable. The fade out will be the reverse of this.
public static void FadeIn(final ImageView v,
final int begin_alpha, final int end_alpha, int time,
final boolean toggleVisibility) {
if (Integer.valueOf(android.os.Build.VERSION.SDK_INT) >= android.os.Build.VERSION_CODES.JELLY_BEAN)
v.setImageAlpha(begin_alpha);
else
v.setAlpha(begin_alpha);
if (toggleVisibility) {
if (v.getVisibility() == View.GONE)
v.setVisibility(View.VISIBLE);
else
v.setVisibility(View.GONE);
}
Animation a = new Animation() {
#Override
protected void applyTransformation(float interpolatedTime,
Transformation t) {
if (interpolatedTime == 1) {
if (Integer.valueOf(android.os.Build.VERSION.SDK_INT) >= android.os.Build.VERSION_CODES.JELLY_BEAN)
v.setImageAlpha(end_alpha);
else
v.setAlpha(end_alpha);
if (toggleVisibility) {
if (v.getVisibility() == View.GONE)
v.setVisibility(View.VISIBLE);
else
v.setVisibility(View.GONE);
}
} else {
int new_alpha = (int) (begin_alpha + (interpolatedTime * (end_alpha - begin_alpha)));
if (Integer.valueOf(android.os.Build.VERSION.SDK_INT) >= android.os.Build.VERSION_CODES.JELLY_BEAN)
v.setImageAlpha(new_alpha);
else
v.setAlpha(new_alpha);
v.requestLayout();
}
}
#Override
public boolean willChangeBounds() {
return true;
}
};
a.setDuration(time);
v.startAnimation(a);
}
You could, instead of creating a new view, use the android:foreground property if you're using a framelayout and toggle the opacity from 0 (transparent) to 1 (opaque). (NB. For the first method, when the flash view is visible, you won't be able to interact with any elements under it, which makes this a better option than a FrameLayout with a foreground property). This is harder to implement, however.