Invocation of Observer.onChanged() doesn't mean that content actually becomes different
Observation
The documentation of method onChanged(T)
But, I found that trying to post the same data to a certain LiveData, onChanged will still be called.
Experiment
// member variable of an activity
val liveString = MutableLiveData<String>()
// onCreate
liveString.observe(this/*an activity*/, Observer Log.d(TAG, it) )
// when a button is clicked
liveString.postValue("abc")
Then, click the button many times
Result
Even the posted string is always the same, the observer will be triggered every time.
android-architecture-components observers android-livedata
asked Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
add a comment |
Observation
The documentation of method onChanged(T)
But, I found that trying to post the same data to a certain LiveData, onChanged will still be called.
Experiment
// member variable of an activity
val liveString = MutableLiveData<String>()
// onCreate
liveString.observe(this/*an activity*/, Observer Log.d(TAG, it) )
// when a button is clicked
liveString.postValue("abc")
Then, click the button many times
Result
Even the posted string is always the same, the observer will be triggered every time.
android-architecture-components observers android-livedata
asked Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
add a comment |
Observation
The documentation of method onChanged(T)
But, I found that trying to post the same data to a certain LiveData, onChanged will still be called.
Experiment
// member variable of an activity
val liveString = MutableLiveData<String>()
// onCreate
liveString.observe(this/*an activity*/, Observer Log.d(TAG, it) )
// when a button is clicked
liveString.postValue("abc")
Then, click the button many times
Result
Even the posted string is always the same, the observer will be triggered every time.
android-architecture-components observers android-livedata
asked Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
Observation
The documentation of method onChanged(T)
But, I found that trying to post the same data to a certain LiveData, onChanged will still be called.
Experiment
// member variable of an activity
val liveString = MutableLiveData<String>()
// onCreate
liveString.observe(this/*an activity*/, Observer Log.d(TAG, it) )
// when a button is clicked
liveString.postValue("abc")
Then, click the button many times
Result
Even the posted string is always the same, the observer will be triggered every time.
android-architecture-components observers android-livedata
android-architecture-components observers android-livedata
asked Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
asked Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
asked Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
asked Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
asked Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
1,11386
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
Source code tracing
LiveData.setValue
→LiveData.dispatchingValue
→LiveData.considerNotify
→Observer.onChanged
There is no Object.equals(Object) checking in codeflow from setValue to onChanged
Conclusion
onChanged is called just because LiveData<T>.setValue(T) is ever called.
Therefore, even onChanged(T) is called, the underlying data may still be the same.
To achieve real "onChanged", you may use a MediatorLiveData to observe the original LiveData, and decide whether to forward the onChanged based on object equality check.
answered Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
add a comment |
Your Answer
StackExchange.ifUsing("editor", function ()
StackExchange.using("externalEditor", function ()
StackExchange.using("snippets", function ()
StackExchange.snippets.init();
);
);
, "code-snippets");
StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "1"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);
else
createEditor();
);
function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);
);
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53274547%2finvocation-of-observer-onchanged-doesnt-mean-that-content-actually-becomes-di%23new-answer', 'question_page');
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
Source code tracing
LiveData.setValue
→LiveData.dispatchingValue
→LiveData.considerNotify
→Observer.onChanged
There is no Object.equals(Object) checking in codeflow from setValue to onChanged
Conclusion
onChanged is called just because LiveData<T>.setValue(T) is ever called.
Therefore, even onChanged(T) is called, the underlying data may still be the same.
To achieve real "onChanged", you may use a MediatorLiveData to observe the original LiveData, and decide whether to forward the onChanged based on object equality check.
answered Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
add a comment |
Source code tracing
LiveData.setValue
→LiveData.dispatchingValue
→LiveData.considerNotify
→Observer.onChanged
There is no Object.equals(Object) checking in codeflow from setValue to onChanged
Conclusion
onChanged is called just because LiveData<T>.setValue(T) is ever called.
Therefore, even onChanged(T) is called, the underlying data may still be the same.
To achieve real "onChanged", you may use a MediatorLiveData to observe the original LiveData, and decide whether to forward the onChanged based on object equality check.
answered Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
add a comment |
Source code tracing
LiveData.setValue
→LiveData.dispatchingValue
→LiveData.considerNotify
→Observer.onChanged
There is no Object.equals(Object) checking in codeflow from setValue to onChanged
Conclusion
onChanged is called just because LiveData<T>.setValue(T) is ever called.
Therefore, even onChanged(T) is called, the underlying data may still be the same.
To achieve real "onChanged", you may use a MediatorLiveData to observe the original LiveData, and decide whether to forward the onChanged based on object equality check.
answered Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
Source code tracing
LiveData.setValue
→LiveData.dispatchingValue
→LiveData.considerNotify
→Observer.onChanged
There is no Object.equals(Object) checking in codeflow from setValue to onChanged
Conclusion
onChanged is called just because LiveData<T>.setValue(T) is ever called.
Therefore, even onChanged(T) is called, the underlying data may still be the same.
To achieve real "onChanged", you may use a MediatorLiveData to observe the original LiveData, and decide whether to forward the onChanged based on object equality check.
answered Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
answered Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
answered Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
answered Nov 13 '18 at 5:43
Owen ChenOwen Chen
1,11386
1,11386
add a comment |
add a comment |
Thanks for contributing an answer to Stack Overflow!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Some of your past answers have not been well-received, and you're in danger of being blocked from answering.
Please pay close attention to the following guidance:
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f53274547%2finvocation-of-observer-onchanged-doesnt-mean-that-content-actually-becomes-di%23new-answer', 'question_page');
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function ()
StackExchange.helpers.onClickDraftSave('#login-link');
);
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
