ProgressBar 不会在 Android Studio 中从代码中可见

Question

So I have made an app, and I'm trying to show the progress bar at the time when a button is clicked.

It shows only if I put the View.VISIBLE outside the handler.

Kotlin code:

button.setOnClickListener {

progressBar.visibility = View.VISIBLE
    runBlocking {

        try {
            //HTTP request removed because of confidentiality, assume I'm setting delay

        } catch (e: Exception) {
            Log.e(tag, "Error: " + e.message)
            Toast.makeText(this@MainActivity, "Could not find page", Toast.LENGTH_LONG)
                .show()
        }

    }
    progressBar.visibility = View.GONE
}

Button XML:

<ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyle"
        android:alpha="1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerInParent="true"
        android:visibility= "invisible"/>

I've been searching for hours now and couldn't find something relevant to my case.

Output wanted: Get the progress bar at the time the button is clicked.

Answer 1

Executing a command like progressBar.visibility = View.VISIBLE may take a few nanoseconds(144100 approx).

The main thread might ask some other threads to perform some tasks and instead of waiting for the response of those threads, it resumes its normal execution(next lines of code). Your next line of code(runBlocking: Runs a new coroutine and blocks the current thread interruptibly until its completion.) blocks the current until the coroutine started from the runBlocking method is finished. This simply means that the Main thread can't execute other commands or process responses received from the other threads until the coroutine is finished.

Once the coroutine is finished. Main thread resumes its execution, showing and hiding progress bar simultaneously, which is why you don't see the progress bar spinning.

Main Thread should only be used for UI operations. Instead of blocking your main thread, you should perform the network operations in coroutine by switching to IO context.

Check here on types of Dispatchers. https://medium.com/androiddevelopers/coroutines-on-android-part-i-getting-the-background-3e0e54d20bb

bt.setOnClickListener {
    progressBar.visibility = View.VISIBLE
    GlobalScope.launch {
        networkCall()
    }

}

private suspend fun networkCall() {
    withContext(Dispatchers.Default) {
        for (i in 1..50000) {
            Log.v("Network call", "$i")
        }
        runOnUiThread {
            progressBar.visibility = View.INVISIBLE
        }
    }
}

Answer 2

从代码中删除该行：

progressBar.visibility = View.GONE

Answer 3

Becuase your style is ( style="?android:attr/progressBarStyle" ), and your width/height are (wrap_content). for Bar style. you should give it value (for example 100dp to WIDTH, and 20dp to HEIGHT.