Databases
StackOverflow2013
Postsdbo
POMessage forwarding does not work on particular messages
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POMessage forwarding does not work on particular messages
    primarykey
    data
    text
    <p>I've found out that some Obj-C messages do no get properly forwaded when an object does not explicity handle them.</p> <p>Here's an example:</p> <p>Create a new UITableView like:</p> <pre><code>UITableView *v ? [[UITableView alloc] initWithFrame:CGRectMake(0, 0, 0, 0) style:UITableViewStylePlain]; </code></pre> <p>Then create an instance of an NSObject with the following methods:</p> <pre><code>(A) - (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section { NSLog(@"tableView:numberOfRowsInSection:"); return 1; } (B) - (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath { NSLog(@"tableView:cellForRowAtIndexPath:"); return nil; } - (NSMethodSignature *)methodSignatureForSelector:(SEL)aSelector { NSLog(@"methodSignatureForSelector: %@", NSStringFromSelector(aSelector)); return [NSMethodSignature signatureWithObjCTypes:"@@:@"]; } - (void)forwardInvocation:(NSInvocation *)anInvocation { NSLog(@"forwardInvocation: %@", NSStringFromSelector(anInvocation.selector)); } </code></pre> <p>Let's call this object ds and set it as the dataSource for the table view:</p> <pre><code>v.dataSource = ds; </code></pre> <p>And when you run your code you should see that both methods (A) and (B) get called by the table view in the expected order:</p> <pre><code>LOG: tableView:numberOfRowsInSection: LOG: tableView:cellForRowAtIndexPath: </code></pre> <p>Now it will get interesting, try removing (or just change the name of) method (A), and you should get:</p> <pre><code>LOG: methodSignatureForSelector: tableView:numberOfRowsInSection: LOG: forwardInvocation: tableView:numberOfRowsInSection: </code></pre> <p>Which is fine because the table view tries to call the <strong>tableView:numberOfRowsInSection:</strong> and since the method is not there, the message gets forwarded to <strong>forwardInvocation:</strong></p> <p>Apparently, this is the default behavior for all messages that reference a method that is not defined at an object.</p> <p>However try restoring the (A) method and removing (B), and you should get:</p> <pre><code>LOG: tableView:numberOfRowsInSection: </code></pre> <p>... and an error telling you that <strong>tableView:cellForRowAtIndexPath:</strong> was called but didn't return a cell.</p> <p>For some reason <strong>tableView:cellForRowAtIndexPath:</strong> gets called (or not) but without following the expected <strong>forwardInvocation:</strong> pathway.</p> <p>Is this a bug?</p> <p>Am I doing something wrong?</p> <p>Are there some messages that explicitly do not get forwarded?</p> <hr> <p>Even though <strong>respondsToSelector:</strong> should return FALSE to both functions, somewhere in the implementation sends the message for <strong>cellForRowAtIndexPath:</strong> to some other object and breaks the default <em>expected</em> behavior...</p> <p>What I've learned is basically:</p> <p>If you are planning to respond to a message in your object, even through some implementation of <strong>forwardInvocation:</strong>, you should <strong>ALWAYS</strong> overload the <strong>respondsToSelector:</strong> method and return TRUE for all messages you plan to implement in your object.</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. USalmosnow
    1. USalmosnow
    plurals
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POMessage forwarding does not work on particular messages
      1. This table or related slice is empty.
      1. VTDownMod
    2. VO
      singulars
      1. POMessage forwarding does not work on particular messages
      1. This table or related slice is empty.
      1. VTUpMod
    1. COThis is in the documentation: https://developer.apple.com/library/mac/documentation/cocoa/conceptual/ObjCRuntimeGuide/Articles/ocrtForwarding.html
      singulars
      1. POMessage forwarding does not work on particular messages
      1. USErik B
    2. COAlso, you're not supposed to put the answer inside the question. If you want to answer your own question you add an answer.
      singulars
      1. POMessage forwarding does not work on particular messages
      1. USErik B
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload