Difference of setValue() & postValue() in MutableLiveData

问题

There are two ways that make change value of MutableLiveData. But what is difference between setValue() & postValue() in MutableLiveData.

I could not find documentation for same.

Here is class MutableLiveData of Android.

package android.arch.lifecycle;

/**
 * {@link LiveData} which publicly exposes {@link #setValue(T)} and {@link #postValue(T)} method.
 *
 * @param <T> The type of data hold by this instance
 */
@SuppressWarnings("WeakerAccess")
public class MutableLiveData<T> extends LiveData<T> {
    @Override
    public void postValue(T value) {
        super.postValue(value);
    }

    @Override
    public void setValue(T value) {
        super.setValue(value);
    }
}

回答1:

Based on the documentation:

setValue():

Sets the value. If there are active observers, the value will be dispatched to them. This method must be called from the main thread.

postValue():

Posts a task to a main thread to set the given value. If you called this method multiple times before a main thread executed a posted task, only the last value would be dispatched.

To summarize, the key difference would be:

setValue() method must be called from the main thread. But if you need set a value from a background thread, postValue() should be used.

回答2:

All above answers are correct. But one more important difference. If you call postValue() on field that has no observers and after that you call getValue(), you don't receive the value that you set in postValue(). So be careful if you work in background threads without observers.

回答3:

setValue()

Sets the value. If there are active observers, the value will be dispatched to them.

This method must be called from the main thread.

postValue

If you need set a value from a background thread, you can use postValue(Object)

Posts a task to a main thread to set the given value.

If you called this method multiple times before a main thread executed a posted task, only the last value would be dispatched.

回答4:

setValue() is called directly from caller thread, and synchronously notifies observers. Also it can be called only from MainThread.
postValue() uses inside something like this new Handler(Looper.mainLooper()).post(() -> setValue()), so it runs setValue via Handler in MainThread. Also it can be called from any thread.

回答5:

setValue() method must be called from the main thread. If you need set a value from a background thread, you can use postValue().

More here .

回答6:

This is not a direct answer to the above problem. The answers from Sagar and w201 are awesome. But a simple rule of thumb I use in ViewModels for MutableLiveData is:

private boolean isMainThread() {
    return Looper.myLooper() == Looper.getMainLooper();
}

private MutableLiveData<Boolean> mutVal = new MutableLiveData<>(false);
public LiveData<Boolean> getMutVal() { return this.mutVal;  }
public void setMutVal(boolean val) {
    if (isMainThread()) mutVal.setValue(val);
    else mutVal.postValue(val);
}

Replace mutVal with your desired value.

来源：https://stackoverflow.com/questions/51299641/difference-of-setvalue-postvalue-in-mutablelivedata

标签

android

android-livedata

mutablelivedata