I have the following problem:
My app has a thread that updates the game state. The app needs to make a change to the state of the View object, triggered by this thread.
The documentation for View states that it should only be modified from the UI thread, and that a Handler should be used to place and handle events.
Yet, there is a function post() in View where I can post a Runnable object that will execute in the UI thread without involvement of Handler. Can I not call this function from threads other than the UI thread??
I'm confused!
Yes you can use View.post(). Internally it uses a handler to post the Runnable.
If you want to run some arbitrary code on the UI thread from a background thread, you can use Activity's runOnUiThread():
runOnUiThread(new Runnable() {
#Override
public void run() {
}
});
Related
This question already has answers here:
How do we use runOnUiThread in Android?
(13 answers)
Closed 4 years ago.
In following code
The way we are using runOnUiThread
shouldn't this create issue with the existing UI Thread
hence creating an issue with the application , hence shouldnt be used
Thread thread = new Thread(new Runnable(){
#Override
public void run(){
//what is meant by the inside code of this run(), how is this updating the UI
runOnUiThread(new Runnable(){
#Override
public void run(){
}
})
}
})
In android,for long running task you should use separate thread such as AsyncTask() or service.Suppose you want to update your UI like you want to show any Toast to user then you should write runOnUiThread(),because only UI thread will allow to touch UI components.
getActivity().runOnUiThread(new Runnable()
{
#Override
public void run() {
Toast.makeText(getContext(), "API calling done!!", Toast.LENGTH_LONG).show();
}
});
There are two types of thread in Android.
1 is UI or Main thread on which your UI elements (layouts) are rendered.
2 is Worker Thread in which long task should be executed (like AsyncTask & Networking).
If you write some task in new Thread, that mean that task will be executed in worker thread.
Now you will use runOnUiThread or new Handler(Looper.getMainLooper()) because you can not touch UI elements in worker thread.
So basically when you are updating UI like setText(), or Toast or any UI operations, you will have to UI thread and you should use worker thread when you are doing some long executions.
Edit
Generally we don't have to manage threading in Android. Because all libraries we use are smart. Although in some cases we have to manage threading as well.
Example
Assume you are calling an web-service(api) in a new Thread, now when response comes you want show a Toast. If you just write Toast.show... directly in response inside worker Thread you will get exception.
Only the original thread that created a view hierarchy can touch its views.
Now to overcome this issue you have to use runOnUiThread, so that you can show Toast.
Whenever we have some Long running tasks we switch to some worker threads and avoid Main Thread and allow a smoother user experience and avoid ANR.
But, when the time comes to update the UI we must “return” to the
Main Thread, as only Main Thread is allowed to touch and update the application
UI.
we can achieve this by making a call to the Activity’s runOnUiThread() method:
Basically what runOnUiThread() will do is - Runs the specified action
on the UI thread. It will check the current thread and if it finds its
the MainThread it will execute that task immediately , otherwise first
it will switch you to app MainThread and then it will execute the
given task.
When a new Thread is created and executed, it does the task in the background thread. But the method runOnUiThread() is used for running the code on the main UI thread.
In your code, runOnUiThread() method is called and hence you are able to update the UI thread from the other thread.
when thread is running in the method runOnUiThread () will update the UI Components (textview .. etc..)
by calling runOnUiThread, you can update the status too
shouldn't this create issue with the existing UI Thread
Not at all. You said "existing UI Thread". There is only one UI thread. The runOnUiThread() will only add the runnable to a queue of tasks which the UI thread executed one by one. You can check the doc.
No worries! This is one of the standard way for updating UI components from a separate thread or even on UI thread itself on Android platform.
Also, runOnUiThread is an method of Activity class, it runs the specified action on the UI thread. If the current thread is the UI thread, then the action is executed immediately. If the current thread is not the UI thread, the action is posted to the event queue of the UI thread.
Another standard way is using Handler and Message as officially documented by Android Developer here https://developer.android.com/training/multiple-threads/communicate-ui
I've got this piece of code:
public void updateOptionLists() {
Log.d("UI", "Called update");
if (updating){
return;
}
updating = true;
runOnUiThread(
new Runnable() {
#Override
public void run() {
updating = false;
updateOptionList();
scrollToLastTapped();
Log.d("UI","Updating");
}
});
Log.d("UI", "Posted update");
}
What I'd expect from logcat would be something like this:
Called update
Posted update
Updating
As far as I know runOnUi should be asynchronous, right? And considering that the functions called alter the views, which takes a while, this should be running asynchronous. Right?
So I look at my logcat:
Called update
Updating
Posted update
Why does this happen? And how do I make sure this runs asynchronous?
runOnUiThread will execute your code on the main thread, which (in this example) also appears to be where it's called from. This explains the ordering of the log statements you see - all code is executing on a single thread, so is synchronous per the documentation (my emphasis):
Runs the specified action on the UI thread. If the current thread is the UI thread, then the action is executed immediately. If the current thread is not the UI thread, the action is posted to the event queue of the UI thread.
runOnUiThread is typically used to execute code on the main thread from a different (i.e. background thread). The use of this separate background thread is what will make a task asynchronous. Calling back to the UI thread at the end of that task is required if you want to modify UI with the results of your background thread calculations.
Android provides several mechanisms for doing work on a background thread and then posting back to the main thread (not all use runOnUiThread explicitly for the latter operation). Good things to read up on include Thread, Handler, AsyncTask, Service, etc.
As far as I know runOnUi should be asynchronous, right?
runOnUiThread, as the name states, runs on UI thread, which is a main thread of an application. It runs synchronously with other code running within that thread and asynchronously with code in other threads.
The word 'asynchronous' has no meaning without a context: some code can run asynchronously with other parts of the code, which means these parts of the code run in different threads. Saying that something 'should be asynchronous' makes no sense without this kind of context.
is updateOptionLists running on the UI Thread?
If this is the case, i would expect this behavior to be ok.
In a normal case you use runOnUiThread from a background thread to come again to the Ui Thread..
Because you call upadetOptionLists() on ui prosess, upadetOptionLists() and runUiThread() both run in the same thread.
To separte theam you need run content in other new thread as following
public void updateOptionLists() {
new Thread(new Runnable() {
#Override
public void run() {
Log.d("UI", "Called update");
if (updating){
return;
}
updating = true;
runOnUiThread(
new Runnable() {
#Override
public void run() {
updating = false;
updateOptionList();
scrollToLastTapped();
Log.d("UI","Updating");
}
});
Log.d("UI", "Posted update");
}
}).start();
}
For accessing the view, you must be in the UI Thread (the main one). So, runOnUiThread will be executed on it.
You must do the long work in an other thread and only call runOnUiThread for little thing like changing a color or a text but not to calculating it.
From the documentation:
Runs the specified action on the UI thread. If the current thread is the UI thread, then the action is executed immediately. If the current thread is not the UI thread, the action is posted to the event queue of the UI thread.
http://developer.android.com/reference/android/app/Activity.html#runOnUiThread(java.lang.Runnable)
Multi Threaded Programming: Theory vs Actual
Puppy Photos Explain is the Best pic.twitter.com/adFy17MTTI— Cian Ó Maidín (#Cianomaidin) 6. april 2015
But joke aside.
If you want to test how the threads work in correlation, try:
runOnUiThread(
new Runnable() {
#Override
public void run() {
updating = false;
updateOptionList();
scrollToLastTapped();
Thread.sleep(100);
Log.d("UI","Updating");
}
});
How would I use a Looper with an asyncTask?
The doImBackground method is called to programmatically create a TableLayout called MyResultTable. The MyResultTable, in turn, has a handler that is called by onTouch during MotionEvent.ACTION_MOVE.
After getting complaints about using handle inside asynchTask, I decide to run handler on UIThread. But that's causing my onTouch method to have slow response. So my question is, how do I use a looper with an asyncTask?
UIThread code:
activity.runOnUiThread(new Runnable() {
public void run() {
handler.post(mResizeViews);
}
});
My Looper attempt: (not working: blank screen)
protected MyResultTable doInBackground(Void... params) {
Looper.prepare();
MyResultTable table = new MyResultTable(context, other);
Looper.loop();
return tabl;
}
How would I use a Looper with an asyncTask?
You wouldn't.
The doImBackground method is called to programmatically create a TableLayout called MyResultTable.
That is not an appropriate role for doInBackground(). Use doInBackground() for slow things: network I/O, disk I/O, etc. Use onPostExecute() for generating the UI ("a TableLayout called MyResultTable") based upon the data retrieved by doInBackground().
The MyResultTable, in turn, has a handler that is called by onTouch during MotionEvent.ACTION_MOVE.
That does not make much sense. You use a Handler to forward events to the main application thread from a background thread. Your onTouchEvent() is called on the main application thread, and therefore it does not need a Handler.
I have a WebView which I initialize with loadUrl in AsyncTask.doInBackground. I initialize it like below :
webView.post(new Runnable() {
#Override
public void run() {
webView.loadUrl(authURL);
}
});
AsyncTask is executed as last in Activity.onCreate(), the problem is that most of the time webpage does not get loaded, I see white screen. If I replace webView with handler then all is ok. What am I missing here?
Why are you doing this in doInBackground() if it needs to run on the UI thread anyway?
The difference between Hander.post() and View.post() is that Handler will run your code
on the thread the Handler instance was created on (which is not necessarily the UI thread), while View will always run it on the UI thread (because views are bound to it).
In my Android app, I am extracting the code to update UI elements into a separate utility package for reuse. I would like my code to be proactive and update the UI differently if the current execution context is from a UI thread versus a non-UI thread.
Is it possible to programmatically determine whether the current execution is happening on the UI thread or not?
A trivial example of what I am looking to achieve is this - my app updates a lot of TextViews all the time. So, I would like to have a static utility like this:
public static void setTextOnTextView(TextView tv, CharSequence text){
tv.setText(text);
}
This obviously won't work if called from a non-UI thread. In that case I would like to force the client code to pass in a Handler as well, and post the UI operation to the handler.
Why don't you use the runOnUiThread method in Activity
It takes a runnable and either runs it straight away (if called from the UI thread), or will post it to the event queue of the UI thread.
That way you don't have to worry about if your method has been called from the UI thread or not.
When you're not sure the code is executed on the UI thread, you should do:
activity.runOnUiThread(new Runnable() {
#Override
public void run() {
// your code here
}
});
This way, whether you're on the UI thread or not, it will be executed there.
You can use the View post method.
Your code would be:
tv.post(new Runnable() {
public void run() {
tv.setText(text);
}
});