Kotlin extensions to commit Fragments safely

Since fragments appeared in Android, many developers have phê bình this component. The main reasons to not recommend fragments in android are; lifecycle phức tạp, hard to debug and easy to lose the fragment state.

Và lý do cuối cùng, how many of you have seen this exception in your implementations?

java.lang.IllegalStateException: Can not perform this action after onSaveInstanceState
    at android.support.v4.app.FragmentManagerImpl.checkStateLoss(FragmentManager.java:1341)
    at android.support.v4.app.FragmentManagerImpl.enqueueAction(FragmentManager.java:1352)
    at android.support.v4.app.BackStackRecord.commitInternal(BackStackRecord.java:595)
    at android.support.v4.app.BackStackRecord.commit(BackStackRecord.java:574)

Mục tiêu của bài viết này là chỉ cho bạn cách để ngăn chặn the state loss fragment exception using kotlin extensions and android support library (version up to 26.0.0).

Why is this exception được xảy ra???

If you don’t know why this exception is thrown, I strongly recommend you to read the Fragment Transactions & Activity State Loss article from @alexjlockwood, in which he explains it in great detail.

Android Support Library

Since version 26.0.0-beta 1 we can check in your app if fragments can be committed to allow the state loss or not.

FragmentManager and Fragment have an isStateSaved() method to allow truy vấn của a transaction mà không có mất state. This is especially useful to check when handling an onClick() event before executing any transaction.

As you can see, the isStateSaved() method is really useful in order to avoid tình huống bất ngờ.

Kotlin extensions

Kotlin supports extension functions and extension properties.

So, How to combine both?

In the following example you can see how an activity extension can commit a fragment in a safe way using isStateSaved() method from the android support library:

/**
 * Method to replace the fragment. The [fragment] is added to the container view with id
 * [containerViewId] and a [tag]. The operation is performed by the supportFragmentManager.
 */
fun AppCompatActivity.replaceFragmentSafely(fragment: Fragment,
                                            tag: String,
                                            allowStateLoss: Boolean = false,
                                            @IdRes containerViewId: Int,
                                            @AnimRes enterAnimation: Int = 0,
                                            @AnimRes exitAnimation: Int = 0,
                                            @AnimRes popEnterAnimation: Int = 0,
                                            @AnimRes popExitAnimation: Int = 0) {
    val ft = supportFragmentManager
            .beginTransaction()
            .setCustomAnimations(enterAnimation, exitAnimation, popEnterAnimation, popExitAnimation)
            .replace(containerViewId, fragment, tag)
    if (!supportFragmentManager.isStateSaved) {
        ft.commit()
    } else if (allowStateLoss) {
        ft.commitAllowingStateLoss()
    }
}

Then, it can be invoked directly from any activity that extends from AppCompatActivity:

    private fun replaceFragment() {
        replaceFragmentSafely(
                fragment = Fragment2(),
                tag = TAG_FRAGMENT_TWO,
                containerViewId = R.id.content,
                allowStateLoss = true
        )
    }

Thank full and cover by article