for this example，from the Qt documentation , http://qt-project.org/doc/qt-4.8/network-download.html the program first read all data to the buffer ,then all data are written to the disk from the buffer ,right ?
then for this example，from the Qt documentation , http://qt-project.org/doc/qt-4.8/network-downloadmanager.html the program is reading and writing data all the time ,right ?
and which way is more proper ？
I dont think that 'more proper way' applicable in this case, they both have pro and cons and obviously depends on what you trying to build.
Depends what functionality you need. If you need only 'completed' items to be saved and their are size relatively small to be kept in memory you can first fetch and them save. If you dont know upfront which items size you are working with and want to have things like 'continue interrupted download' then you should probably save things as they are coming from the network.
Related
I'm trying to understand how I can generate a waveform from an audio (or video) file to display to the user.
I've been googling around for quite a while now and can't determine if this is even possible in Qt without using something like FFmpeg. I've seen all of these classes: QMediaPlayer, QMediaContent, QMediaResource, QAudioProbe and experimented with the Qt Media Player Example but am just not seeing where I can access the actual audio buffer.
So I have 2 questions:
Is what I want to do even possible without 3rd party libraries?
If it is possible, can some kind soul outline what I need to read and understand in order to access the audio data
I have tried the suggestions from this question (Audio visualization with QMediaPlayer) but the result of audioProbe->setSource(player) is always false and the method processBuffer never gets called.
audioProbe = new QAudioProbe(this);
bool success = audioProbe->setSource(player);
qDebug() << success;
connect(audioProbe, SIGNAL(audioBufferProbed(QAudioBuffer)), this, SLOT(processBuffer(QAudioBuffer)));
Update: Adding some additional detail in the hope of clarifying things.
For testing/learning I am using the Media Player Example which ships with Qt, so it is set up correctly with Q_OBJECT etc.
For audio, I tested with both .mp3 and .wav files. FWIW, the player example won't play video for some reason (.mp4, .avi were tested)
The player in the code is QMediaPlayer – which inherits from QMediaObject. The example code for the Player class is here. I added my code (in original comment above) right after the player is instantiated. I also tried adding it once media is loaded.
I tried declaring my slot first as private, then as public – either way, it is never called.
Frustrating that such a simple thing is so hard.
Going the "no external library" route will likely just lead to more of a headache and more work than is necessary. The other advantage of going with an established library is you won't be bound to one file format, as not all formats store their data the same way. If the audio format is uncompressed (wav or other) you can read the header until you get to the data chunk. An answer to this question here details this in C. You should be able to get an idea for the file format from this to apply it to another language.
You will want to understand how many channels are in the wav file, bit depth, and also the sampling rate before you can do anything worthwhile with the data. All this info can be grabbed from the header.
It turns out that QAudioProbe is not supported on OSX – the platform I am working on. Took quite a while (a "Qt while. . .") to ferret that info out so I am posting it here explicitly.
See this document for full details: Qt 5.5.0 Multimedia Backends
I wrote a tool which was originally thought for analyzing hard disc images. Now I'm trying to use this tool for live analyzis of computer systems, means my tool tries to access the physical drive.
I implemented my tool in QT accessing the images using the QFile class. Instead of images I hand over the physical drive, under windows it is \.\PHYSICALDRIVE0.
I was wondering first I didnt get any errors, I can open the device, I can seek, get the position, almost everything. The only thing I have problems with is retrieving the drive size with size().
Some code example:
QFile file( "\\.\PHYSICALDRIVE0" );
file.open( QIODevice::ReadOnly );
file.size(); //returns 0
I'm not too deep into QT, probably this is some easy thing. I would like to thank everybody who has an idea what is the reason.
thanks in advance!
QFileInfo may be able to help you out. It sounds like opening a read only file at that part of windows partition is allowed maybe even if it doesn't exist. There might be a chance that the call of GetLastError() may give more information why a file size of zero was returned.
With QFileInfo, you can check to make sure it exists before it opens.
You may end up needing some platform specific calls to be able to work with Physical Drives:
Volume to physical drive
It looks like there may be some example code of looking at partitions with PartMod on SourceForge.
As a side note of querying sizes of file folders, I thought it had to be cached somewhere by the operating system, or had to be calculated at the time of the query in many cases. I know it seems like that happens when looking at folder properties in Windows or Get Info on OSX.
Also, looking at the Volume to physical drive answers, there is a field there for the extent length. I think this is what you are looking for.
Hope that helps.
I have an app written in Qt that has many widgets, including lists, tables, trees, line edits, etc. As the user progresses through the app, they may need to save the current state, leave, and come back later to pick up where they left off. I do not have a the data separate from the GUI (all data is stored in the widgets themselves). I have found a way to loop through all the widgets and save the current state, but it takes a significant amount of time. Is there a faster or built in way to save the current state of the GUI? I am trying to avoid re-writing the code to have a data model that is only represented by the GUI due to limited time available.
Is Qt's session management what you're looking for?
You can save every param on its actual change, or when user leaves the page, not on exit. And don't try to save every parameter of every widget, use smarter algorithm
As a good crossplatform and flexible way to save state of your GUI is to use Qt's mechanism -- QSettings (official documentation).
But, really, there are re-writing moments, which you can't avoid.
My first question here :)
I have a report generating website. When the user clicks a button the report is generated in a different sub as a html-file and is written to a txt-file. The html-file is later converted to a PDF in a different sub.
When the report is long (200 pages), I get out of memory exception when the PDF is generated. Memory seams to be allocated by the html generation, since when I convert the html to PDF in a different webform it works perfect.
I have tried to use analysis program like ANTS, but I dont have the knowledge to sort it out.
How can I release the html generation from memory?
Thanks!
/Georg
Your memory from a good component should hopefully get cleared out - however in this case since its a fairly large document it may by OK design but max the memory out. You can
1. Try to increase the memory in IIS available to your worker process
2. http://support.microsoft.com/kb/911716
3. (you didnt specify server version so this is dependent on that) http://support.microsoft.com/kb/820108
With ANTS - there are tutorials on RedGates site discussing its usage. If its a third party component there may not be much you can do except increase the available memory or contact the vendor.
Having had a quick look at the Flex docs I can't seem to find any reference to providing audio content to be played from a custom (possibly encrypted - don't worry, it's not that evil) container format. Is this possible and if so, could someone point me in the right direction.
Or if that's not possible, some way to hook into the disk/network (disk is much more important in this case) I/O of the sound playing mechanism to provide a supported container in memory from a custom wrapper.
Since Flash Player 10, it's posible to write PCM / raw audio data to a Sound Object.
Basically, you call play on an "empty" Sound Object and it will start dispatching periodically a SampleDataEvent, requesting data. You then can write to the audio stream through the data ByteArray exposed by the event object.
http://help.adobe.com/en_US/FlashPlatform//reference/actionscript/3/flash/events/SampleDataEvent.html?filter_flex=4
http://www.adobe.com/devnet/flash/articles/dynamic_sound_generation/index.html
Also, if you're interested in good articles and reference for audio programming in Actionscript, you might want to check out Andre Michelle's stuf:
http://blog.andre-michelle.com/
http://lab.andre-michelle.com/
A flash.media.Sound must either be:
constructed/loaded with a URLRequest,
inherit its data through embedding
There currently is no provision for directly piping mp3 (or aac, or video) data to a any "media" object, such as Sound. You can only get the Sound object to download the data for itself. There are people who are upset about this, including myself; you are not alone!
I say "at this stage" because it's not unthinkable that Adobe will update the API to make this possible in a future version. For the now, you're best to go with the decoding-to-a-dynamic-sound workaround mentioned by Juan, if you really need to be able to do this.
And post a feature request at Adobe's bug tracker, or vote on an existing one!